Assessment of Phase Change Materials Incorporation into Construction Commodities for Sustainable and Energy-Efficient Building Applications

Abstract

The significant energy consumption and contribution to greenhouse gas emissions by the construction sector need careful attention to explore innovative sustainable solutions for improving the energy efficiency and thermal comfort of building envelopes. The integration of phase-change materials (PCMs) into building commodities is a favorable technology for minimizing energy consumption and enhancing thermal performance. This review paper covers the impact of PCM incorporation into construction materials, such as walls, roofs, and glazing units. Additionally, it examines different embedding techniques like direct incorporation, immersion, macro and micro-encapsulation, and form and shape-stable PCM. Factors affecting the thermal performance of PCM-integrated buildings, including melting temperature, thickness, position, volumetric change, vapor pressure, density, optical properties, latent heat, thermal conductivity, chemical stability, and climate conditions, are elaborated. Furthermore, the latest experimental and numerical simulations, as well as modeling techniques, evident from case studies, are investigated. Ultimately, the advantages of PCM integration, including energy savings, peak load reduction, improvement in interior comfort, and reduced heating, ventilation, and air-conditioning dependence, are explained alongside the limitations. Finally, the recent progress and future potential of PCM-integrated construction materials are discussed, focusing on innovations in this field, addressing the status of policies in line with the United Nations Sustainable Development Goals, and outlining research potential for the future.

1. Introduction

The improvement in worldwide energy demand, combined with increasing environmental issues, has prioritized the need for the advancement of energy-efficient buildings [1,2,3,4]. The construction sector is one of the main energy consumers, utilizing a significant amount of energy and thus contributing to large-scale GHG emissions [5,6]. Building construction and development account for 36% of the total worldwide energy utilization and 39% of the energy-related CO₂ emissions [7,8]. According to an International Energy Agency (IEA) report, building envelopes account for 50% of direct heating and cooling [9]. However, in some areas, the building sector utilizes 80% of the total energy [4]. Moreover, it is predicted that by 2035, the building sector will be the fourth largest source of GHG emissions [10]. In addition, the IEA reports that building operations consume 30% of global energy and contribute to 26% of worldwide energy-related emissions, of which 8% accounts for direct emissions and 18% is caused by energy production for building energy use. In 2022, a 1% increase in building energy consumption was observed [11,12]. Hence, it is essential to move towards energy-efficient buildings to decrease operational costs, promote sustainable development [13,14,15,16], and contribute to reducing climate change challenges [17]. Improvements in building envelopes can lead to enhanced thermal performance [18]. Insufficient heating, ventilation, and air conditioning (HVAC) systems can further increase thermal management issues and energy requirements [19].

In the current scenario, the application of thermal energy storage (TES) is extremely beneficial for attaining sustainable construction while enhancing the energy efficiency of construction [15,16,20,21]. Generally, TES systems absorb thermal energy for later utilization, which reduces the dependency on conventional cooling and heating systems and therefore shifts the peak energy demands to off-peak times [22,23,24,25,26]. Hence, by storing energy during low-demand periods or from renewable sources, TES adjusts energy usage and enhances overall building efficiency [27]. TES is a recent development in thermal management techniques that allows the storage of thermal energy without wasting it. This technique is executed through different mechanisms, such as energy storage by thermochemical process or sensible and latent heat storage [28]. The integration of thermal mass into building structures also positively affects their efficiency [29,30]. For this reason, phase change materials (PCMs) are utilized in building envelopes employing latent heat storage (LHS) [31].

PCMs have attracted substantial attention because of their ability to integrate into building applications [13,32,33,34,35]. PCMs absorb, store, and release heat energy during the phase change process, i.e., from solid to liquid and vice versa [8,36]. The potential of LHS allows PCMs to absorb and release large amounts of energy while maintaining a reasonably steady temperature [37]. Further, the basic feature of PCM is its capability to store huge amounts of energy due to its latent heat as an alternative to sensible heat in a fixed volume [25]. Incorporation of PCMs into building commodities like walls, roofs, floors, and glazing units improves thermal inertia, diminishes fluctuations in temperature, and lags in heat peaks, which helps reduce the necessity for active cooling and heating systems [38,39]. Various techniques are used for the incorporation of PCMs into construction materials, such as direct incorporation, encapsulation, and impregnation methods [39,40]. This makes PCMs a versatile choice for enhancing the thermal performance of various construction commodities [3,28]. PCMs are utilized due to their superior energy storage capabilities in the melting and solidification phases, helping to reduce the heating and cooling loads through the building and thus maintaining better thermal comfort [41,42,43,44]. Moreover, PCMs are utilized in solar air and water heaters and photovoltaic (PV) systems to improve their thermal performance [8]. Conversely, PCMs are highly applicable in heating, ventilation, and air conditioning (HVAC) systems, offering a favorable solution to minimize building energy consumption and enhance indoor thermal comfort by efficiently mastering heating and cooling loads [45]. These materials store and release large amounts of thermal energy during the phase-change process, assisting in effective thermal energy storage and load shifting within HVAC operations [28]. Recent research has specifically highlighted their utility in ice storage for air conditioning. For instance, Teng et al. [46] investigated an air-conditioning system integrating ice storage with Al₂O₃ nanoparticle distribution in deionized water, resulting in the facilitation of heterogeneous nucleation, energy transmission, and temperature gradient eradication during the phase change. This incorporation has proven to have an energy-saving potential of nearly 6–9% [47]. Generally, PCMs can improve the energy efficiency of air-cooling systems, resulting in electrical energy savings of 7–41% and a decrease in carbon impact [48]. An integrated latent heat thermal energy storage (ILHTES) system, which couples a PCM-to-air heat exchanger (PAHX) with an air conditioning unit, was optimized to produce an Energy Saving Ratio (ESR) ranging from 16% to 44.7% over the cooling season, depending on the PCM type and climatic conditions. The optimal PCM for such systems usually has a melting temperature close to the HVAC system’s internal set point, with RT25 (23–25 °C) exhibiting exceptional performance across several European climates [49]. The deliberate integration of PCMs within building components, such as walls, ceilings, and floors, or directly into HVAC systems, benefits in mitigates peak load demands and stabilizes interior temperatures.

However, the incorporation of PCMs into building components, while offering significant potential, is not without its challenges and limitations that require careful consideration. A major drawback reported in building applications is the poor thermal conductivity of PCMs [50]. This can lead to partial charging and discharging cycles during the phase transition, which diminishes the heat storage capacity and overall effectiveness of the PCM [8]. Furthermore, although direct incorporation methods are simple and cost-effective, they pose a notable risk of PCM leakage during the melting process, potentially causing incompatibility with other construction materials and increasing the fire hazard for flammable PCMs [51]. Such direct integration can also negatively impact the mechanical properties and strength of building materials, such as decreasing the compressive strength of walls or affecting the durability of concrete [52]. Another inherent issue that limits the widespread applicability of PCMs is their tendency toward supercooling and phase separation [39]. Optimal performance of PCMs is highly sensitive to their type, quantity, thickness, and precise position within the building envelope, as an inappropriate design can lead to inefficient thermal behavior [53]. Lastly, limited long-term research currently restricts a clear understanding of PCM performance over an extended service life in buildings, hindering broader technology commercialization.

Previous reviews have often provided broad knowledge on thermal properties and incorporation methods, yet they have frequently overlooked practical challenges, large-scale applications, and real-world scenarios related to the integration of PCMs with other energy-saving technologies. For example, Liu et al. [53] discussed the thermal performance of macro-encapsulated PCMs, particularly concerning material selection, melting processes at a component level, and optimal locations at a system level within building envelopes, along with a fragmentation of research lacking comprehensive comparison insights into design and applications. Moreover, Malode et al. [54] discussed TES using bio-based PCMs, contributing to green technology. On the other hand, Caggiano et al. [55] conducted a review on numerical and theoretical approaches for PCM-based cementitious components, where multiple numerical and simulation tools are also elaborated. Despite the growing body of literature on PCMs in building applications, the present review comprehensively addresses multiple areas by exploring both experimental and numerical approaches while considering multiple construction commodities.

Highlighting the use of PCMs in sustainable and energy-efficient building practices, this review article provides a comprehensive overview of the applicability of PCMs in building envelopes. The main objectives of this review are to investigate the basic concepts of PCM-integrated construction commodities and their relevance in improving TES in buildings, and their evaluation parameters. Furthermore, this review focuses on assessing PCM-integrated construction commodities through the evaluation of experimental, numerical, and simulation methods with the support of case studies from the literature.

This review provides a comprehensive and organized analysis of the integration of PCMs into building components. Relevant literature was surveyed across multiple platforms using keywords such as “Building thermal management, PCMs integration, environmental sustainability, energy-efficient and carbon-neutral constructions, and building thermal comfort”. Articles were chosen based on their relevance to the current review scope. Furthermore, the selected research articles were classified based on: (I) fundamentals of PCMs in terms of their TES applicability and PCM impact on construction materials, various embedding techniques, and environmental considerations, (II) PCM integration into bricks, plasters and coatings, glasses and windows, roofs and walls, (III) factors that affect PCMs thermal performance, (IV) Assessment of PCM addition into building commodities through experimental and numerical techniques, (V) merits and demerits of PCM incorporation, and (VI) recent progression and future potential. This approach allows for a structured understanding of current advancements and challenges in PCM-based energy-efficient building solutions. This review comprehensively explores the advantages of PCM-aided buildings for energy savings, enhancing occupant comfort, and shifting the peak load to contribute to the SDGs.

2. Fundamentals of Phase Change Materials

PCMs are substances that can absorb and release large quantities of thermal energy through phase transitions, specifically solid to liquid and vice versa [5,23,56]. The transition occurs at an approximately constant temperature, which makes PCMs ideal for TES [8]. Generally, upon heating, PCMs liquefy by absorbing heat and thus store heat energy in the form of latent heat [57,58]. However, upon cooling, it releases thermal energy and solidifies. These transition temperatures indicate the suitability of PCMs for specific applications [5].

PCMs are categorized into three classes: organic, inorganic, and eutectic, each with its own thermophysical properties [59,60]. Moreover, the choice of a PCM depends on different factors like chemical, thermo-physical, economic, and environmental properties [61,62,63,64].

2.1. Thermal Energy Storage in Buildings

Considering TES in buildings, PCMs play a crucial role in utilizing the potential of latent heat, which occurs at nearly constant temperatures. This makes PCMs very effective for heat flow control while reducing energy consumption [5,59,65]. Conversely, traditional building materials rely on sensible heat thermal energy storage (SHTES), while PCMs utilize latent heat thermal energy storage (LHTES), which allows them to store and release thermal energy at nearly a steady temperature [66,67,68,69]. Comparing SHTES with LHTES, 1st is dependent on temperature rise to retain energy, while the latter permits the PCM to store a large amount of energy even in small spaces [70].

The integration of PCM into construction materials allows solar radiant energy to be stored during the daytime and then released at night, resulting in a reduction in dependency on active heating and cooling systems and ensuring the provision of a thermally comfortable interior environment [3,5,71]. Moreover, PCMs have a high energy storage density and occupy less volume than conventional materials like concrete, which relies on the storage of sensible heat [23,40,56,72]. PCMs also limit temperature fluctuations, which result in a more comfortable indoor environment and avoid overheating [19,59,73,74,75]. Another advantage of PCM is that it shifts the peak loads because it can accumulate an excess amount of thermal energy during high peak loads and then release it in off-peak load times, which improves the thermal efficiency of the building and reduces pressure on electricity [5,25,76,77].

In conclusion, considering sustainability in buildings, PCM integration can reduce energy consumption, contribute to the reduction of carbon emissions, and promote environmentally feasible building practices. In addition, it can balance energy supply and demand facilities by integrating renewable sources into power generation systems. Furthermore, the intrinsic properties of PCM, such as LHTES, represent a more sustainable solution for thermal energy storage in buildings. PCM’s high energy storage density and their capability to maintain a stable interior temperature for longer hours permit a substantial decrease in energy demand while enhancing the overall thermal comfort of buildings.

2.2. Evaluation Criteria for Phase Change Materials Selection

The incorporation of PCM into building materials requires a critical assessment of various basic parameters. These parameters are important for optimizing the thermal performance of PCMs [63,78,79], decreasing energy demand, and enhancing indoor thermal comfort [5].

Several parameters are described in

Table 1. Summary of the key parameters for selection of PCM for integration into building materials.

Key Parameter	Description	Specific Desired Values/Ranges	Vitality
Phase transition temperature [62,63,67,71,76,85,86,87]	The temperature at which PCM changes its state from solid to liquid or vice versa	For common building applications, the optimal PCM melting temperature is 20–30 °C. For human thermal comfort 20–28 °C. For cooling up to 21 °C. For water heating 29–60 °C.	This parameter must match the operating temperature of the building to confirm efficient heat storage and release. It determines the PCM activation for absorbing and releasing heat to impact the system’s efficiency.
Latent heat capacity [5,37,61,62,71,81,88,89,90,91,92,93,94]	The amount of thermal energy a PCM can store or release during phase change, such as fusion, at a constant temperature. It is measured in kJ/kg or J/g	This must be as high as possible. The values are based on the PCM type, such as: Paraffin-based PCM 200 kJ/kg. Bio-based PCM 220 kJ/kg. InfiniteR 200 J/g. WinCo Enerciel PCM 175 J/g.	A larger latent heat capacity ensures that PCM can absorb more energy in smaller volumes, reducing the required material and space.
Thermal Stability [8,20,67,87,95,96,97,98,99]	The capability of PCM to maintain its thermo-physical properties, such as phase transition behavior, even after facing several melting and solidification cycles	Must have the ability to be stable over long-term use or at many cycles. Where it is found that: A few PCMs can sustain up to 3000 thermal cycles, showing negligible degradation. A few have shown stability over 1500 thermal cycles.	Better thermal stability ensures long-term performance, robustness, and steady behavior of the PCM for steadfast energy storage and release over the building lifecycle.
Compatibility with construction materials [5,8,39,87,100]	The potential of PCM to not chemically react adversely with nearby materials, such as corrosion	The PCM must not be: Corrosive. Reactive. Create segregation. React with encapsulation materials.	Better compatibility of PCM with construction materials can prevent degradation of the PCM or building structure, resulting in guaranteeing the integrity and safety of the system. It further ensures that the PCM can be perfectly integrated without damaging the structure or any degradation of the materials.

. One of the key factors is the thermal conductivity of the PCMs; intrinsically, PCMs have a minimal thermal conductivity, which can benefit from better insulation [80], whereas a higher thermal conductivity is required to accelerate the energy absorption and release rates [81]. High thermal conductivity can be achieved by adding metal foams and nanoparticles [20,82]. Other thermophysical parameters include a higher specific heat and density, low vapor pressure, and volume changes to ensure steadiness in the phase-change process [8,81,83]. Moreover, kinetic properties also play a significant role; for example, a high nucleation rate can avoid supercooling, while rapid crystal development permits efficient heat recovery [60,61,62]. Safety and environmental considerations are also important to ensure that PCM are non-toxic, non-flammable, and recyclable to maintain an eco-friendly nature [5,8,84]. Cost and availability are two decisive factors for the economic feasibility and accessibility of materials for practical use.

In summary, choosing a PCM for incorporation into construction materials requires a thorough analysis to maintain a balance between the defined factors. A better choice is the selection of a PCM based on the application and environmental conditions. Therefore, it is necessary to select a PCM that ensures the required thermal properties, is compatible with the application, is cost-effective, and is eco-friendly.

2.3. PCM Interaction with Construction Commodities

The incorporation of PCMs into building materials has a major impact on the mechanical strength, durability, and overall structural integrity of buildings. Therefore, for better execution of PCM technology in building envelopes, it is important to understand how these materials interact with each other [52]. The following subsections further elaborate on the effects of the interactions between these materials and construction commodities.

2.3.1. Impact on Mechanical Properties

One of the most prominent issues with PCM-integrated buildings is the reduction in compressive strength [3,101,102]. The reduction is caused by several parameters, such as interaction with the cement matrix, enhanced porosities, and weak interfacial bonds [101,103,104,105]. The literature elaborates that even the addition of small percentages of PCMs can result in a substantial decrease in endurance [106]. The addition of 3.2% and 2.7% microencapsulated PCMs in Portland cement and geo-polymer concrete caused a decrease of 42% and 51%, respectively, in the compressive strength [107]. Another study explored the integration of varying percentages (1%, 3%, and 5%) of mPCM into self-compacting concrete, proving that while mPCM substantially enhanced the thermal performance of concrete, it simultaneously led to a considerable decrease in its mechanical strength. Particularly, the incorporation of 5% PCM resulted in a decrease in compressive strength of up to 70%. This observed decrease in the compressive strength of concrete comprising microcapsules is primarily due to two factors: firstly, a significant disparity between the intrinsic strength of the microcapsules and the inherent strength of the concrete’s other constituents; and secondly, the damage inflicted upon the microcapsules during the mixing process, which can cause the PCM to leak and subsequently interfere with the surrounding concrete matrix [108,109]. The loss in strength depends on the mode of incorporation, where direct incorporation causes the most hostile effects [110]. Moreover, PCM microencapsulation also causes a reduction in stiffness and mechanical strength because of the increase in the porosity of the materials [111], as shown in Figure 1, which shows a preparation technique as well as the impact on mechanical strength by adding PCM into concrete.

The impact of PCM on the bending strength is slightly more complex. A low amount of microencapsulated PCM can lead to better bending performance by employing enhanced interfacial bonding, whereas a higher concentration can cause cracks and a decrease in overall strength [38]. Furthermore, direct integration of PCM reduces the adhesion strength between the binder paste and the aggregates [114]. The addition of PCM in a liquid state can decrease the water content of the concrete mixture, which affects the mechanical properties at higher temperatures [115,116]. In contrast, the literature shows that the addition of PCM to construction materials can reduce thermal stresses and crack formation and improve long-term performance [117].

2.3.2. Impacts on Durability

In terms of durability, the critical issue is the leakage of PCMs during the phase transition process, specifically when PCMs are directly mixed or immersed [115,118]. These leaks lead to incompatibility between materials, and if the PCM is flammable, it can increase the risk of fire [116]. Even when microencapsulated PCMs are used, leaks can occur, compromising the physical properties of concrete [111]. Another concern is the potential for corrosion [119], specifically with the use of inorganic PCMs, as they could increase the deterioration of reinforced concrete and further damage the steel reinforcement [120,121]. Furthermore, PCM-integrated mortars have a greater porosity, which makes them more prone to environmental degradation [122,123]. This high porosity decreases the resistance to exterior factors, including humidity and aggressive means [124].

The long-term stability of PCMs under multiple thermal cycling situations remains a challenge, which requires further research to ensure the durability and reliability of PCM-integrated building commodities.

2.3.3. Building Materials Response

The addition of PCM to concrete can disrupt the cement hydration process and reduce the crystallization of calcium silica hydrate, which weakens the cement bonding [117,125]. Moreover, the alkaline nature of concrete restricts the number of PCMs that can be utilized efficiently [118]. As discussed, the addition of PCMs can also deteriorate mechanical strength and durability [114]. In mortars, the integration of PCM usually leads to a decrease in mechanical performance due to increased porosity and higher water content [122,123]. Integrating PCMs into bricks can enhance thermal efficiency while concurrently reducing material use; however, it requires sufficient encapsulation to avoid leakage [126]. Generally, bricks integrated with PCM exhibit lower flexural strengths [127]. Furthermore, PCM-integrated plaster can help improve the thermal mass to regulate fluctuations in temperature and optimize indoor thermal comfort [60].

2.4. Embedding Techniques

Numerous techniques exist for incorporating PCM into building envelopes, as shown in Figure 2, each with specific advantages and disadvantages. These techniques focus on avoiding damage to PCMs, which is the main requirement for ensuring the effectiveness and durability of construction materials [128,129]. The next subsections provide an overview of different embedding methods described in the literature.

2.4.1. Direct Incorporation

In this technique, PCMs, either in liquid or powder state, are directly mixed with construction materials such as gypsum mortar, cement, or concrete mortar during the initial stages [8,116]. Among all techniques, this is the easiest and most cost-effective method, which does not require any specific skill [71]. Figure 3 shows a simple preparation step. However, these techniques confront some critical issues, such as PCM leakage during the melting phase, which results in a reduction in compatibility and increases the fire hazard if a flammable PCM is used [115,116]. Moreover, it compromises the concentration of water [116], which further affects the mechanical strength of the construction elements, particularly at higher temperatures. Direct mixing can also change the hydration products and thus reduce the bond strength and durability of the materials [114].

Despite these issues, some literature suggests that the proper application of direct incorporation can be effective without causing damage to PCM in terms of loss [131].

2.4.2. Immersion

The immersion technique involves laying porous building materials into PCM in the liquid state, where absorption occurs via capillary action [59,115,120,132]. This method is particularly effective for drywall and porous concrete [133]. However, this approach poses some critical issues, like direct incorporation. The main issue is leakage, which enhances the incompatibility between materials, resulting in compromising their effectiveness [40,120]. Another limitation is that it can promote the corrosion of steel reinforcement, which further reduces its durability [120]. Moreover, this method is only applicable to porous construction materials, which enhances the uneven distribution of the PCM and deteriorates the overall performance of the building [71].

2.4.3. Encapsulation

In encapsulation, PCMs are enclosed inside a protective casing, preventing leakage and enhancing their compatibility with building structures [134,135]. This technique enhances the heat transfer area and optimizes the PCMs’ conductivity [8]. Encapsulation is subdivided into microencapsulation and macroencapsulation.

In microencapsulation, PCM is integrated into microscopic polymeric capsules, which can be directly incorporated into construction commodities without causing leakage [61,62,71,99,136,137]. Generally, these microcapsules are either spherical or have an irregular shape, with a diameter of less than 1 cm, ideally between 1 and 60 μm, which promotes better heat transfer. This mechanism can be achieved through chemical, mechanical, or both treatments [62]. However, microencapsulation has some limitations, such as high cost, reduction in thermal conductivity due to the presence of polymer-based capsules [8,138], and the possibility that the microcapsules may break during mixing, resulting in damage [126,139,140,141].

In contrast, in macroencapsulation, the PCM is sealed in larger containers like tubes, panels, or tanks greater than 1 cm in diameter [59,142,143], some of which are shown in Figure 4. This shaped macroencapsulated PCM can then be integrated into walls, roofs, and ceilings [6,61,71]. This method permits the storage of huge amounts of PCMs in a single unit, which further simplifies their handling while maintaining better compatibility with construction materials [6,61]. Metals such as copper, aluminum, and stainless steel are typically used for macroencapsulation because of their exceptional thermal conductivity, PCM compatibility, and mechanical strength [144]. Moreover, in this technique, PCMs are integrated into pre-manufactured elements without direct mixing with the base materials [8,106]. However, this technique exhibits some concerning issues, including the possibility of PCMs solidification alongside the edges and limited thermal conductivity, which can contribute to a reduction in the heat transfer [62,145].

Other factors that must be considered during the application of these techniques include the careful selection of materials for encapsulation to prevent leakage, maintain the thermal properties of PCMs, and ensure compatibility with PCM and building materials. The encapsulation materials must guarantee structural stability, volumetric change management during phase transition, and protection of the PCM from deterioration caused by environmental factors [144,149]. It is also beneficial to select encapsulation materials like shells that possess good thermal conductivity and mechanical strength [144]. In the literature, it has been found that the most commonly used materials for encapsulation are metal tubes made of copper and aluminum, and stainless steel panels and sheets [149]. In microencapsulation, materials used include polyethylene, white polyester film, high-strength nylon, polyurea, polyurethane, polymethyl methacrylate, polyvinyl acetate, polystyrene, urea formaldehyde, melamine-formaldehyde resins, and gelatine-formaldehyde [40,62,150,151].

2.4.4. Shape-Stable PCM

In this approach, PCM is enclosed inside a polymer porous material carrier matrix to avoid leaks, even if the PCM is in the liquid state [20,40,152,153,154,155]. Generally, the carrier matrix maintains the shape of the PCM during the phase-change process, ensuring steady thermal performance [156], as shown in Figure 5. The benefits of these techniques include better thermal conductivity, higher specific heat, and sustained shape through multiple phase transition cycles [144,153]. A few materials, like graphene, carbon nanotubes, and expanded graphite, have been utilized to retain liquid PCM through capillary forces [157,158]. These composites are made by mixing PCM with support materials at a temperature above the melting point of PCM, followed by a cooling and shaping mechanism [159]. However, this technique is not cost-effective but is considered highly reliable and offers high performance over longer periods [160]. These composites exhibit high apparent specific heat during phase transition, maintain their shape during phase change, and do not require any container [156]. Further, the lower thermal conductivity of shape-stabilized PCMs may restrict their diffusion [161]. This limitation can be overcome by the addition of nanomaterials [20,162].

2.4.5. Form-Stable PCM

In form-stable PCM is combined with support materials, which increases the mechanical strength and avoids leakage, resulting in composite materials. These composites can then be further molded into panels, bricks, or other building commodities [15,160]. Shape-stable PCMs have the potential to retain a substantial amount of one or more types of PCMs without losing them during melting [8]. This is a better option, but it is not cost-effective [160].

2.4.6. Comparative Analysis of Embedding Techniques

PCMs can be integrated into construction commodities through various methods, each with its own characteristics. Direct incorporation involves mixing the PCM with building materials, while immersion involves absorbing the liquid PCM into porous materials [8,40]. On the other hand, encapsulation seals the PCM inside a protective cover, where microencapsulation is suitable for applications at a small scale and macroencapsulation for larger integration usages [149,164]. Shape-stable PCMs are achieved by impregnating a carrier matrix with the PCM, and form-stable PCMs are obtained by mixing the PCM with a structural material [154,155]. The aforementioned techniques vary considerably in terms of effectiveness, cost, and impact on the thermal and structural performance of buildings. This requires a thorough comparative assessment to identify an adequate technique [40]. To analyze the aforementioned techniques, a comparative analysis was performed (

Table 2. PCM incorporation techniques for energy-efficient buildings.

Technique	Description	Merits	De-Merits
Direct incorporation [8,40,59,62,115,118,119,131,153,165,166]	PCM in nano-form or liquid form is blended with building materials like concrete, mortar, and Plaster during production phases	One of the easiest and most economical techniques. Requires no special expertise and can be easily implemented. It can increase the thermal storage capacity of the construction materials.	During the melting phase, there is a high risk of PCM leakage. It may cause a reduction in the mechanical strength of the construction materials. This may affect the hydration process. There is a risk of discordancy, corrosion, and fire vulnerability with a few PCMs.
Immersion [19,40,115,167,168,169]	Building materials are immersed in liquid PCMs, which are absorbed through capillary action	Simple technique to integrate PCM into porous materials. This technique is practical and cost-effective.	Has potential for leakage and incompatibility. May disrupt mechanical properties and durability. It can be affected by the hydration products of the building materials. Can cause a non-uniform distribution of PCM.
Encapsulation [6,40,112,126,137,144,149,164,170,171,172,173]	PCM is entrained in a shielding case or ampule before incorporation	Contribute to avoiding liquid PCM leakage in a phase change process. Ensures the provision of thermal stability, compatibility, and reliability. Provides a greater heat transfer rate per unit volume. Microcapsules can be utilized as a partial substitution for aggregates. Macro-capsules can be integrated into walls, floors, and ceilings without mixing with the base materials.	Not as cost-effective as direct incorporation and immersion methods. A few encapsulating materials can cause a decline in the thermal conductivity of the PCM. The capability to cause breakage of the microcapsules during the mixing phase. In macroencapsulation, there is a possibility of solidifying PCMs at the edges. Microencapsulation can lead to a reduction in the mechanical properties of the building materials.
Shape-stabilized PCMs [128,152,153,164,174,175,176,177]	PCMs are enclosed in a carrier matrix made of a polymer of porous materials	Ensures to maintain shape and avoid leakage even if PCMs are in the liquid phase. This consequently removes the need for container requirements. Provides good thermal conductivity and large specific heat. The end product is highly reliable and efficient.	Much more expensive to execute it. Some poor thermal properties limit its adoptability at a large scale.
Form-stable PCMs [160]	PCMs are mixed with compatible materials to develop a composite material having better structural integrity	Ensures the retention of a higher amount of PCM having no leakage at melting temperatures. Comparatively, a reliable technique.	Not cost-effective to implement it.

).

2.5. Environmental Considerations

PCMs are crucial for the passive heat treatment of buildings to improve their energy efficiency and thermal comfort; however, environmental considerations and lifecycle sustainability must be considered [8,35,37,97,170,173,178,179].

Generally, the climate has a huge impact on the selection, amount, location, and encapsulation method of PCM [8]. Particularly, this technology is much more efficient in extreme weather [180], where the temperature variations on a daily basis can be up to 15 °C [181]. For better TES, the PCM melting temperature must be compatible with the local environmental conditions [182], where for water heating, the temperature range is 29–60 °C, 22–28 °C for the thermal comfort of humans, and for cooling applications, the temperature is below 21 °C [67,76]. According to the literature on optimization, the melting temperature of a PCM changes according to the climate [3,180,183]. For Mediterranean climates, the ideal temperature range is 21–24 °C [3], while higher-melting-point PCMs are required in warmer regions [35]. Moreover, the location of the PCM is also important according to the climate; for example, in hot regions, the best practice is to place it near the outer side of the envelope to work as a thermal barrier, while the opposite is true for cold climates to ensure the provision of heat [8]. Night ventilation plays a crucial role in the solidification of PCM, which requires further adjustment of air temperature, flow rate, and ventilation duration [184,185]. Furthermore, for efficient thermal cycles in extreme climates, higher-melting-point PCMs are suitable [71,186].

On the other hand, life cycle sustainability is another important aspect of PCM-integrated buildings, as this technique can aid to a reduction in energy consumption and CO₂ emission while improving thermal comfort [71]. Therefore, it is important to conduct a life cycle assessment (LCA) and an Environmental Impact Assessment (EIA) to ensure long-term sustainability [187]. The literature suggests that, in stable climate conditions, PCMs are particularly advantageous, whereas, over paraffins, the preferred option is salt hydrates, which have low environmental impacts in terms of production and disposal [188].

To analyze the environmental advantages of PCMs, it is important to compare the reduction in GHG emissions produced during the manufacturing and installation stages [189]. A numerical study on a house located in New Castle, Australia, was conducted using EnergyPlus software (DesignBuilder and CondFD approach)for both seasons, where it was observed that integrating a PCM with a layer thickness of 10 mm, melting point of 21 °C, and life span of 50 years, positioned in the wall and roof, can contribute to a reduction of 264 tonnes of CO₂ emissions [187,190]. Another important aspect that contributes to LCA is the payback period of PCMs. A study found an average payback period of 23 years [191], while Mohseni et al. [187] observed a 16.6-year payback period for a PCM-integrated building. Generally, the lifespan of a PCM is in the range of 10–30 years, depending on the type of PCM used, the type of embedding technique, and environmental conditions [71]. Therefore, further research is needed to refine PCM formulations, improve thermal performance, and enhance compatibility with environmental conditions.

3. Incorporation of PCMs into Building Commodities

PCMs’ integration into concrete cementitious materials is an evolving research area aimed at improving the energy efficiency and thermal comfort of buildings. Figure 6 presents a summary of the applicability of PCM integration.

3.1. Concrete, Cementitious, or Other Building Materials

The incorporation of PCMs into construction materials has gained a lot of attention for improving TES and energy efficiency in buildings. PCM integration can reduce the dependency on active heating and cooling systems by regulating indoor temperatures by absorbing and releasing latent heat during the phase transition process [192].

As discussed in Section 2.4, PCM can be integrated into construction materials using different embedding techniques, such as direct mixing, encapsulation, and impregnation techniques [173,193,194,195,196]. Direct mixing is the simplest technique; however, it may affect the mechanical strength and durability of building structures. Encapsulation techniques like microencapsulation and macroencapsulation prevent the leakage of PCMs and thus maintain the structural integrity. Moreover, PCMs can be integrated into low-weight aggregates or porous materials to enhance their compatibility [197].

According to Hunger et al. [109] PCM-enhanced concrete increases the thermal inertia and, therefore, delays heat transfer, which makes these materials advantageous for passive heating and cooling strategies in building applications. Moreover, this strategy can help improve the energy efficiency and sustainability of buildings. However, some challenges remain, such as compatibility with cement hydration, low compressive strength, and leakage during PCM melting [83]. Further research in this field can assist in optimizing performance and durability.

3.1.1. Bricks and Masonry

The addition of PCMs to bricks and masonry structures is a better technique for enhancing the thermal inertia, minimizing temperature variations, and enhancing the overall thermal energy efficiency of building envelopes [198,199,200]. Fired clay bricks are broadly used in the construction sector due to their strength, accessibility, and other mechanical properties [8], which makes them ideal building commodities for PCM integration. This hybrid mechanism can boost the thermal performance of traditional masonry [40]. Many studies have been conducted on this topic, as shown in Figure 7a, which shows the preparation of PCM-integrated bricks, while Figure 7b shows the brick model and its experimental setup.

PCM-enriched bricks can optimize thermal inertia in various ways. Primarily, it improves the thermal mass of the material [199], enhancing its thermal absorption and release capacity, which further stabilizes the internal temperature and minimizes the dependency on heat and cooling systems. Furthermore, the existence of PCMs lessens heat transfer by storing excess thermal energy as latent heat during peak hours and then releasing it when the ambient temperature reduces [39,71,202]. Consequently, this approach maintains the interior comfort at its highest level, which alternately reduces the cooling load in summer and the heating load in winter [62]. However, it delays the time for heat transfer; bricks integrated with PCM delay the transfer of high exterior temperatures to the interior environments. The literature suggests that masonry walls enriched with PCMs can attain a thermal delay of up to 13.3 h [203]. Another study reported a delay of 2 to 3 h during the shoulder seasons [204].

Moreover, maintaining a low temperature is another key benefit of PCM-added bricks. A decrease of 3–7 °C in temperature was observed at the surface of the inner wall, improving the inner thermal comfort. Another study found a decrease of 4.5–7 °C in PCM-incorporated bricks compared to conventional masonry [8], while another study found a reduction of 5–6 °C in walls [199]. These enhancements can result in substantial energy savings, as this approach minimizes the reliance on active cooling and heating systems, which ultimately leads to a decrease in electricity consumption [186]. For instance, a 2008 study reported a 15% decrease in summer electricity consumption in PCM-embedded buildings [202]. Other studies proposed that PCM-added walls can reduce energy consumption by 16.58–68.63% and save 1–7% based on the PCM arrangement [35,40]. In addition, increasing the PCM thickness in summer can enhance energy savings by an additional 2–6% [8].

The effectiveness of PCM-embedded bricks depends on the type of material used. Eicosane and OM35 reduced the heat flow by 8% and 12%, respectively [199]. In contrast, paraffin, RT-25, and capric acid showed reductions of 6.07%, 3.61%, and 8.31%, respectively, in the heat flow [205]. Encapsulated paraffin integrated into bricks provides 147 kJ/kg of LHS and a phase-transition temperature of 38–43 °C [25,27]. In addition, studies suggest that a multilayer configuration of n-eicosane, paraffin, and n-octadecane can further reduce heat flow [127].

In addition, the location of the PCM inside the brick plays a significant role in the thermal performance [17,206,207]. A study suggested that applying a PCM to the insulation layer of walls can enhance energy efficiency [161]. Whereas the placement of PCMs is defined by the exterior weather conditions, such as in cold climates, the ideal location is towards the inside of the wall, and in areas where the temperature is below 40 °C, then the location moves towards the center. However, if the temperature crosses the threshold, the PCM layer should be placed near the interior side for better thermal comfort [17,127,208].

Table 3. PCM integrated bricks for sustainable building envelopes.

Research Study	Brick Embedded with PCM	Position of PCM	Findings
Huang et al. [204]	Hollow bricks with C16H34 PCM	Brick cavities are filled.	Compared to non-PCM bricks, the average heat flow on the inner surface is decreased by 8.57 W/m2. Inner surface and midsection temperatures are increased by 0.99 °C and 3.85 °C, respectively.
Fahrurrozi et al. [205]	Standard bricks have paraffin-based ice bags with a ratio of 50:50	Filled into brick cavities.	Substantially reduced the temperature variance. Showed higher heat transfer rates in the phase transition process.
Ru et al. [206]	Recycled aggregate pavement bricks with PEG-400/SiO2 or Tet/SiO2 composite PCM	Mixed with the brick materials	Compressive strengths after 28 days for PEG-400/SiO2 and Tet/SiO2 were 5.98–18.35 MPa and 10.37–16.36 MPa, respectively. Thermal conductivity is reduced by 14.5–38.3% using PEG-400/SiO2, while 6.4–41.4% using Tet/SiO2 in comparison to ordinary recycled aggregate pavement bricks.
Kumar et al. [207]	Geo-polymer bricks with paraffin wax	Incorporated	Samples dried under solar heat were compared with non-PCM samples. 6.1% higher compressive strength. 19.5% higher tensile split strength. 8.1% higher flexural strength.
Chihab [208]	Hollow-fired clay bricks with 20 wt% of PCM32	Within the brick structure	Heat flux swings decreased by 34%. A delay of 2.5 h in the infiltration time of the external thermal temperature.

summarizes some studies from the literature.

The PCM container shape and design, while integrating it into bricks, also affect thermal performance [8]. In comparison, thin, plate-shaped containers perform better in terms of heat transfer during the PCM phase transition than cylindrical, cuboid, and spherical containers [209,210,211,212]. Another study found that bricks with cylindrical cavities filled with PCM worked better than those with square cavities because of the higher surface area, which resulted in better heat exchange [213]. Additionally, the presence of air-filled cavities in PCM-integrated bricks enhances their thermal properties compared to solid bricks [205,214].

Despite these advantages, some challenges remain, such as compromising the mechanical strength of bricks, particularly when cavities are created [205]. In addition, other potential disadvantages include PCM loss, segregation, and undercooling, particularly when the PCM is directly added to cementitious materials, resulting in uneven dispersion [215]. However, a suitable and sufficient encapsulation technique can prevent these issues [8,216]. Although a few PCMs can result in corrosion, especially in steel reinforcement, this problem can be controlled through compatible material selection and suitable encapsulation techniques [217,218].

In conclusion, the application of PCM-integrated bricks is an innovative approach that contributes to the enhancement of the thermal performance of buildings, augmenting the thermal mass, minimizing heat transfer, providing thermal retardation effects, decreasing interior temperatures, and upgrading energy savings. The optimization of PCM-integrated bricks in a building is contingent upon the type of PCM, the location of the PCM inside the brick and wall, and the structural design of the PCM capsules. Despite these benefits, some challenges, such as reduced mechanical strength, material loss, and corrosion, still exist, which require further research and technological advancement to improve the feasibility and efficiency of PCM-integrated bricks for better sustainable building envelopes.

3.1.2. Plasters and Coatings

Similar to PCM-integrated bricks, plaster, and coatings containing PCMs are applicable in the passive thermal management of buildings [19,59,219]. The incorporation of PCMs into plasters and coatings allows for the storage and release of heat energy, resulting in the regulation of heat transfer in building envelopes [8]. In general, upon heating or an increase in the ambient temperature, PCMs absorb thermal energy, and a phase transition occurs from solid to liquid, which restricts the penetration of heat into the interior [59,166,220]. Conversely, the opposite occurs, which stabilizes the internal microclimate [40,166]. Figure 8 shows the experimental process flow diagram for the fabrication of the composite PCM-loaded plaster.

Generally, integrating these PCM-incorporated plasters and coatings can regulate temperature fluctuations, which further enhances thermal inertia, reduces internal temperature peaks, and results in a reduction in temperature variation and a decrease in the dependency on active heating and cooling systems [17,59,222]. However, as a passive thermal management solution, it does not require external power, which makes it a sustainable solution that promotes energy savings and a decrease in carbon emissions [59,181,223,224]. Furthermore, this type of coating can be applied to ceilings, walls, roofs, and external facades to enhance the thermal performance of building envelopes [62,87,224]. Some studies have mentioned the effectiveness of PCM-enriched plasters. For instance, a study found a reduction of up to 20.8% in the internal temperatures while using SS-PCM acrylic plaster [224] and a decrease of 2.76 °C in temperature fluctuations using PCM-modified plasters [35]. Moreover, PCMs promoted a thermal phase shift by delaying the peak internal temperature by up to 67.26% [224]. However, a 75% reduction in heat transfer was reported in [225].

The latest research has focused on establishing hybrid solutions by applying several PCMs with different melting points, causing an increase in the thermal efficiency over a broad range of temperatures [60,226]. Additionally, the incorporation of advanced materials, such as aerogels, carbon nanotubes (CNTs), or expanded graphite, can enhance the thermal conductivity and avoid losses, such as solid-stabilized PCM [157,158,227,228,229,230]. In general, multiple PCMs are used in thermal coatings, such as eutectic fatty acids, including capric-myristic, lauric-myristic, and palmitic acid blends [224], paraffin-based PCMs [87,181], bio-based alternatives from renewable sources [59], and microencapsulated PCMs enclosed in polymer or silica cases [5]. Moreover, for improved thermal performance, researchers have focused on using advanced techniques like micro and macroencapsulation to enhance stability [231,232], shape stabilization for leakage prevention [159], and the implication of nanomaterials like CNTs to upgrade heat transfer [157]. Despite their great potential, PCM-enriched plasters and coatings face challenges, including high material costs [5,39], durability issues [231,232], and intrinsically low thermal conductivity [5].

In summary, thermal plasters and coatings enriched with PCMs are promising technologies for promoting passive thermal management in building envelopes. This approach offers significant benefits in terms of temperature regulation, thermal comfort, and energy efficiency. Hence, further research is needed to refine the performance, affordability, and durability of PCM-integrated plasters and coatings over longer periods. Thus, this strategy can contribute to the development of more sustainable buildings.

3.1.3. Glasses and Windows Glazing

PCMs are widely used in glazing units to enhance thermal efficiency by storing thermal energy from solar radiation and allowing visible light to enter indoor spaces [233]. This assists in regulating temperature variance and improving thermal comfort [233,234,235]. At relatively constant temperatures, PCMs absorb and release heat during phase transition [23,56,57]. This phase change process stabilizes the interior temperature of the building envelope [70]. PCM-enriched glazing units offer multiple advantages, such as energy savings. The passive thermal management of this approach helps lower the dependence on active heating and cooling systems [236], improving the thermal comfort of occupants by maintaining a stable interior temperature [233,237] and shifting peak energy demands into off-peak periods [23,56], thus enhancing the overall building efficiency and minimizing temperature fluctuations [37,66].

In the literature, different types of PCMs have been used in glazing units, such as water, paraffin, and eutectic PCMs [233]. Water is circulated in double-glazed chambers [238], as shown in Figure 9. It was further observed that the water-filled chambers reduced energy consumption compared with the air-filled chambers. Further, paraffin-based PCMs have wider applications in glazing [239], while eutectic PCMs usually produce a crystalline mixture in a frozen state and then melt simultaneously [240,241]. Generally, double- or triple-glazed windows integrated with PCMs help reduce heat transfer [59], whereas PCM-enriched double-glazed roofs can lower peak temperatures, resulting in better energy savings [233]. A schematic illustration is shown in Figure 10. Moreover, PCMs are integrated into shutters and facades to enhance the regulation of indoor temperatures.

Additionally, PCMs are incorporated between double or multiple panes inside glazing systems [242,243,244,245,246], which boosts the thermal performance. Usually, the optical and thermal efficiency of PCM-integrated glazing units depends on the state of the materials, such as when the PCM is in liquid form, as it reduces the light transmission. Another factor is the thickness of the PCM layer, where a higher thickness reduces the heat loss [247,248,249]. Other factors that influence the thermal functioning include the melting temperature [250], thermal conductivity [251], and optical properties of the PCM and glass [252].

Despite their benefits, PCMs have some shortcomings, including durability of 10–30 years [71], a reduction in heat gain in colder climates [8], and dispersion effects caused by the porous structure of solid PCMs [39].

In summary, PCM-integrated glazing units are an innovative approach for enhancing the thermal performance, energy savings, and thermal comfort of buildings. However, to address its shortcomings, further research is needed to develop numerical models and conduct various experiments to obtain an optimized design applicable to various climatic conditions.

Figure 9. Alsace Case Pavilion using water in the glazing unit [253].

Figure 9. Alsace Case Pavilion using water in the glazing unit [253].

Figure 10. The left side experimental setup for roof glazing is explained through a schematic diagram on the right side, where T1, T2, T3, and T4 show the number of thermocouples attached, whereas on top, glazing units are shown, filled with PCM (left side liquid state, right side solid-state), reproduced from [254].

Figure 10. The left side experimental setup for roof glazing is explained through a schematic diagram on the right side, where T1, T2, T3, and T4 show the number of thermocouples attached, whereas on top, glazing units are shown, filled with PCM (left side liquid state, right side solid-state), reproduced from [254].

PCM Behavior in Severe Climates

An important concern is the PCM behavior under severe climate conditions. When PCMs are incorporated into building glazing units in severely cold climates, their behavior presents specific technical challenges and opportunities. A prime concern is that, at low ambient temperatures, PCMs, specifically common types like paraffin, often remain in their solid state. This solid phase significantly reduces visual transmittance, decreasing the transparency from approximately 90% in liquid form to around 40% in solid form. Consequently, the amount of solar energy that can enter the building is sharply diminished, counteracting the key benefit of glazing for passive solar heating [233,255].

Furthermore, the inherently low thermal conductivity of many PCMs, combined with the typically low solar radiation in cold regions, can lead to incomplete melting or charging of the material during the day [39]. If the PCM does not fully melt, its latent heat storage potential and ability to absorb and release large amounts of heat at a nearly constant temperature are not fully utilized, thereby hindering its effectiveness as a thermal storage medium for heating purposes. The porous structure of solid-state paraffin can also cause scattering effects, further impacting the optical and thermal performance of the glazing [233]. While PCMs can help mitigate overheating and reduce cooling costs in cold climates, an unsuitable design can inadvertently worsen heating performance. The optimal melting temperature for PCMs in cold climates is critical; for instance, temperatures between 14 and 16 °C have been suggested for Northeast China to ensure melting occurs under lower solar conditions, as opposed to higher ranges that are suitable for moderate or hot climates [256].

To address these limitations, several solutions and future research directions are proposed. Enhancing the thermal conductivity of PCMs, perhaps by incorporating nano-sized reinforcement mediums like nanoparticles or using high thermal conductivity encapsulation materials, can accelerate phase transition and improve energy absorption and release [257]. Strategic placement within the building envelope is also crucial; for cold climates, integrating PCMs into the interior of external walls (rather than roofs) with an optimal thickness and melting temperature has demonstrated better cost-saving potential [258]. Additionally, applying external thermal insulation to glazing systems can reduce the impact of cold weather and promote more effective PCM activation [233]. While passive methods are cost-effective, their heat storage rates can be limited in climates with small temperature differences. Therefore, active strategies, such as controlled night ventilation, can be employed to ensure the complete solidification of the PCM, preparing it for the next day’s cycle [185]. Another strategy is the integration of PCMs with aerogels in building glazing units, which is a notable strategy, particularly as a promising retrofitting solution for existing building envelopes [227]. Aerogels, such as silica aerogel granules, have demonstrated a significant capacity to encapsulate PCMs, with studies showing that they can hold up to 80 wt% of capric acid [259]. This suggests their potential as effective carrier materials for PCMs in transparent or translucent applications. Furthermore, 3D carbon aerogels have been investigated for their ability to enhance the stability of foams and improve the heat transfer performance of PCMs. This is especially relevant for mitigating issues such as nanoparticle agglomeration, which can occur after repeated heating and cooling cycles within the PCM, thus contributing to the long-term reliability of the system. The use of monolithic silica aerogels has also been reported in the in situ preparation of shape-stabilized composite PCMs, which helps prevent leakage and maintain the form of the PCM [37]. While porous nanocarbon materials, such as graphene aerogels, are also considered potential PCM beads, their mechanical properties require further evaluation. It is important to note that while filling the enclosure between glass panes with aerogel can reduce the overall heat transfer coefficient and increase the thermal insulation performance, it does not significantly increase the thermal mass of the glazing system, and the visible transmittance of aerogel typically needs improvement [233]. Despite this, combining aerogels with PCMs in glazing aims to leverage the benefits of both: aerogels for enhanced insulation and structural stability, and PCMs for their latent heat storage capabilities, which can help regulate indoor temperatures and manage solar radiation. This combined approach aims to create a more energy-efficient and thermally responsive glazing system.

Continued experimental investigations are required to understand the long-term durability and maintenance requirements of PCM-enhanced glazing systems in severely cold environments.

3.1.4. Roofing and Wall Materials

PCM-incorporated tiles and panels can be used in walls and roofs for heat management by minimizing heat transfer, thus upgrading the energy efficiency and thermal comfort of building envelopes [17,40,59,71]. This approach is a booming technique for building applications due to its potential to store and release heat during the phase change process [97,170,179,260]. Tiles with PCMs absorb heat throughout the day while changing from solid to liquid, thus storing thermal energy [166,216]. However, the opposite occurs at night [59]. As the concrete roof slab remains at the PCM phase-transition temperature, which ensures a continuous thermal cycle and thus maintains a steady indoor temperature, a schematic diagram is shown in Figure 11. A PCM-integrated tile with a 2 cm thick layer, containing 10% copper nanoparticles, and coated with plaster exhibited better thermal performance [216]. Furthermore, macroencapsulation techniques are utilized for prefabricated roofing systems integrated with PCMs, where it was found that a container with high thermal conductivity is required to optimize heat transfer [87,261,262,263].

PCM-integrated roofs have been found to be beneficial for reducing heat transfer into building envelopes and lowering interior temperatures [264,265]. For instance, in traditional roofing systems, a redesigned PCM shingle decreased the indoor peak temperature by 6.62 °C [216]. Another study found that incorporating macroencapsulated PCM into reinforced concrete roofs improved passive cooling by reducing heat transfer and heat load [53]. Further, it was determined that PCM-enriched roofing sheets have reduced the indoor temperature by 7.2 °C during sunny hours while decreasing the heat flow and heat load into the building by 60.6% and 54%, respectively [258,261]. PCM-integrated roofs thermally perform efficiently compared to walls due to the involvement of thicker PCM layers, better encapsulation, and higher exposure to solar radiation [17,35]. PCMs integrated walls showed a reduction of 7 °C in the interior air temperature, avoiding overheating in summer [17]. Furthermore, the utilization of PCMs in different climate zones performed better, such as PCMs with 25–30 °C melting points, together with light and dark roofing membranes employed in both Rome and Abu Dhabi [8,263]. The studies conducted on PCM-integrated roofs and walls are summarized in

Table 4. Different PCM incorporated into roofs and walls for sustainable building envelopes.

Research Study	PCM-Integrated Walls and/or Roofs	Location of PCM	Findings
Dardouri et al. [261]	Exterior or interior walls and roofs with Infinite R™ PCM have different melting points	External, internal, and double-layer	- Energy demand is reduced by up to 41.6%. - PCMs with a melting temperature of 21 °C and 29 °C favored energy savings during heating and cooling, respectively. - The double layer saved more energy compared to the single layer of PCM.
Hamidi et al. [212]	External walls where PCM is incorporated into terracotta bricks	External walls	- EUR of 74 is achieved for cost savings. - The northeast Mediterranean zone showed 56% of energy savings, while the southeast zone observed no savings.
Hu et al. [262]	Roof with thermochromic coating-PCM	Varied position and thickness	- 19% and 5% drop in total energy consumption and CO2 emissions, respectively. - An increase in the PCM layer thickness further reduced the energy consumption by 29% and CO2 emission by 8%.
Huang et al. [266]	Roof with PCMs RT27, RT31, RT35HC, PT37	The external side of the roof, between the external mortar and the concrete layer	- Upon a rise in the thickness of the PCM layer, a decrease is observed in the peak temperature attenuation of the interior surface of the roof. - The time lag reached 6 h with a phase change temperature of 37 °C. - In Mediterranean conditions, RT31 showed the longest time lag of 5 h, and RT27 showed the lowest inner surface temperature.
Rahimpour et al. [263]	All elements of the building, such as the roof, walls, and floor, with MT21 and MT23 PCMs	0.03 m PCM layer below the plasterboard	- Across five cities in Australia, considering home energy management system (HIMS) control, a decrease of 31.9% in HVAC consumption is found. - Whereas, electricity cost is reduced by 10.6% and 19% in Brisbane and Adelaide, respectively.

.

Numerous factors influence the performance of PCM-integrated roofs and walls, including surface area, material characteristics, layer thickness, location, and climate conditions [263,267,268]. The improvement in the exposed surface area, especially in macroencapsulated PCM panels, motivates the melting process and thus accelerates the thermal energy absorption and release rates [39]. Moreover, the selection of carrier materials depends on their applicability and compatibility. To upgrade the thermal conductivity, the addition of highly thermally conductive materials, like porous graphite nanomaterials or CNTs, is particularly suitable [60,62]. PCM layer thickness is another important factor; upon increasing thickness, the thermal performance is improved to an optimized point, beyond which the effect may decline [269,270]. The placement of the PCM inside the roof structure also affects the potential of thermal regulation, where internal installation is the most suitable location [8]. Furthermore, the performance of PCM-integrated walls changes with ambient conditions [263], with the highest cooling efficiency observed in hot and dry climates [271]. Together, these factors optimize the overall thermal performance of PCM-integrated roofs and walls, contributing to lower energy consumption and improved interior occupant thermal comfort.

Finally, the addition of PCM to roofing and wall materials is an effective solution for developing energy-efficient building envelopes. To further enhance the thermal performance, the factors affecting the thermal performance must be optimized to construct more sustainable roofs and walls.

3.1.5. Floor, Chilled Ceiling, and HVAC Tanks

PCMs have significant applicability across various building elements and systems, particularly in floors, chilled ceilings, and PCM-powered HVAC tanks, to enhance energy efficiency and thermal comfort [272,273].

PCMs are integrated into floors in various ways to provide thermal energy storage. This can involve a single PCM layer, multilayer solutions with different PCMs, or systems with capillary channels for PCM circulation, sometimes integrated with heat pumps [62]. For instance, a system with a permeable PCM layer (3 cm thick, 20 °C transition temperature) under the floor, resting on a concrete slab with an air box, maintained an indoor temperature 1.5–2.1 times longer than a reference solution and achieved a daily energy storage of 1.79 MJ/m² [272]. Experiments on concrete for floor applications with a paraffin mixture (23 °C melting point) resulted in a 16% decrease in the maximum temperature and a 7% increase in the minimum temperature [269]. Double-layer PCM floor solutions with different transition temperatures have been shown to reduce surface temperature fluctuations and heat flows, increasing energy release in peak periods by 41.1% for heating and 37.9% for cooling, compared to standard solutions [274]. Active underfloor heating systems, such as electrical or hydronic hot water systems, are enhanced by PCM integration, with studies showing that PCM (latent) storage has double the discharge time of sand (sensible) storage, significantly improving the thermal mass for floor heating [275,276,277]. Barzin et al. [273] conducted a study involving PCM-impregnated gypsum wallboards above an electric floor heater, which achieved total cost savings of 18.8% and energy savings of 28.7%. Optimal PCM melting temperature close to the indoor set point is crucial for such applications.

PCMs have been widely studied for their ability to regulate indoor temperatures in the context of chilled ceilings. Solutions include ceiling panels with capillary networks for PCM circulation. One such panel, consisting of a steel board with capillary tubes filled with gypsum plaster doped with microencapsulated PCM (22 °C transition temperature, 13 kg PCM/m²), could melt for about 7.5 h under a 40 W/m² heat load, storing 290 Wh/m² and contributing to indoor temperature regulation [278]. Another application involved micro-encapsulated PCM in aqueous suspension (18 °C transition temperature) used as a heat transfer fluid in ceiling cooling circuits, with 40% PCM concentration proving effective [62]. Experimental studies have shown that exposed PCM integrated into suspended ceilings can reduce operative temperature by up to 3.3 °C, keeping it below 26 °C during operative hours. However, challenges like acoustic and aesthetic concerns regarding exposed PCMs exist. Researchers have also explored the combination of PCMs with night ventilation in packed bed-shaped ceilings for active cold storage [59].

PCMs play a critical role in energy efficiency and load shifting for PCM-powered HVAC tanks and active cooling systems. Studies have investigated PCMs as thermal storage media for refrigeration and air-conditioning systems [37]. An integrated latent heat thermal energy storage (ILHTES) system, combining a PAHX with an air-conditioning unit, was designed and evaluated for residential buildings. This system uses cool outdoor air at night to charge the PCM, and during the day, ventilation fans discharge the cooling energy into the interior. Optimal PCM selection for such systems is crucial, with RT25 consistently outperforming others, leading to an Energy Saving Ratio (ESR) of 16% to 44.7% across various European cities for the entire cooling season [49]. The optimal PCM melting point should be close to the indoor set point of the HVAC system, typically between 23 and 25 °C. Such systems can significantly reduce peak load demands and offer energy savings [47,49], with reported reductions in electrical energy consumption ranging from 7% to 41% in fresh air cooling systems [48]. An active PCM storage system demonstrated the potential for daily energy consumption to decrease by up to 23% and cost reductions of up to 42% during warmer seasons [279]. PCM integration with vapor-compression cooling systems can reduce daily energy consumption by 0.9%, daily accessible cooling energy by 2.9%, and electrical peak load by 47% for 154 L of PCM [280]. These applications highlight PCMs’ ability to shift peak energy demand to off-peak hours and provide precise control of required energy.

Inter-Floor Void Formers

The integration of PCMs into inter-floor void formers or similar hollow floor structures represents a substantial approach to boost building energy efficiency and thermal comfort by leveraging the latent heat storage capabilities of PCMs. This method allows for the efficient storage and release of thermal energy within the horizontal elements of a building, contributing to load shifting and indoor temperature steadiness. Recent research has emphasized several ways in which PCMs are incorporated into such floor systems. One notable development involves hollow concrete floor panels that utilize shape-stabilized polymer composite PCM enclosed in their cavities [53]. This design inherently addresses potential leakage problems by integrating the PCM directly into the construction material of the floor itself, efficiently increasing the floor’s thermal inertia, storing thermal energy, and shifting peak loads to sustain stable indoor temperatures [262]. Similarly, some systems use concrete block systems with hollow cores for thermal energy storage, where cool ambient air can be circulated through these cores at night to discharge stored heat during the day. The active management of the stored energy in the floor panels is a key benefit [47].

Beyond hollow cores, other integrations feature permeable PCM layers installed under the floor and supported by a concrete slab with an air box, demonstrating the ability to maintain internal temperatures for extended periods and facilitate daily energy storage [272]. The viability of impregnating lightweight aggregates with PCMs for floor construction solutions has also been explored, offering an effective method for thermal energy storage, particularly in cases where direct PCM incorporation may be challenging. For active heating, PCM can be combined with electrical heating floor systems to enhance thermal comfort and decrease energy utilization, particularly in regions with mixed power pricing. Studies have also shown that integrating multiple PCM layers with diverse transition temperatures in floor solutions can decrease surface temperature variations and heat flows, significantly enhancing energy release during peak periods for both heating and cooling compared with conventional systems [281]. These applications demonstrate the flexibility and effectiveness of integrating PCMs directly within floor assemblies to optimize the thermal performance and energy usage of buildings.

4. Factors Affecting Thermal Performance

Numerous factors (as shown in Figure 12) have been thoroughly addressed in each section; these factors influence the thermal performance of PCM-enriched building materials like bricks, windows, roof glazing units, walls, and roofs. The important factors are comprehensively addressed in

Table 5. Summary of factors that influence the performance of PCM incorporated in building envelopes.

Factors	Description	Impact on Performance Evident from Literature
Melting Temperature [8,37,67,187,233,250,282,283]	The temperature at which the PCM changes from a solid state to a liquid state	PCMs’ melting temperature must be in the range of the average temperature of the application; As for building envelope uses, the PCM melting point is 22–28 °C. In literature, it is found that PCMs having melting points of 19–29 °C performed efficiently. Another study found that 21 °C-PCM utilized in roofs and walls showed efficient results. Furthermore, it was investigated that PCM layers having melting points of 22, 24, and 26 °C do not have an efficient impact, and it was determined that the performance depends on environmental conditions and volumetric quantity. It is found in the literature that in China, for hot summers and cold winters, the suitable melting temperature is 25–31 °C, whereas for northeast China, it is 24–26 °C. It was found that a PCM having 32 °C utilized in a glazed roof decreased the peak temperature by 16.3 °C. Moreover, a 47.5% reduction was observed in the energy consumption and a 40 min increase in the time delay. Another study suggests that using PCM with a melting point of 19–34 °C can reduce energy consumption.
PCM thickness [28,117,187,216,233,269,270,284]	The thickness of the PCM layer inside building commodities	Application of expanding the PCM thickness can enhance the thermal properties to some extent, further increasing it can also reverse the trend; Literature suggested different thicknesses, such as a PCM having a thickness of 10–30 mm, improved the thermal performance, whereas a 16 mm thick PCM employed in a glazed roof decreased the energy consumption and improved the indoor thermal environment comfort. Another study conducted in China used a PCM layer having a thickness of 12–30 mm. Moreover, it was found that a 5–10 mm thick layer of PCM increased the interior thermal comfort while reducing the heating and cooling loads, and temperature variations.
PCM position [40,60,62,128,262,285,286,287,288,289,290]	The location of the PCM inside the construction materials, such as roof, floor, wall, and inside the building envelope, like exterior, middle, and interior	The location of the PCM inside the building envelope substantially impacts its performance in regulating the interior temperature, further, the proximity of the PCM to the inside of the spaces is more beneficial, therefore, the best position will allow the PCM layer to completely melt during the day and solidify at nights. It is found in the literature that putting PCM in the ceiling is more efficient than putting it in the wall. The middle location of the element can enhance the building’s annual performance. For cooling purposes PCM layer must be placed near the outside of the building element, whereas, for heating purposes, the position must be changed to the interior side. Literature proposed that the use of the PCM-incorporated bricks showed better results when PCM was placed close to the interior side. Another study observed that placing a PCM in the wall’s middle layer decreased temperature fluctuations.
Volumetric Change [83,233]	An alteration in the volume of the PCM during a phase change process	A slight change in the volume during the phase transition process is necessary to avoid any confinement issues. A study suggested that the cavity of the glazing unit is not filled because of the volumetric expansion when PCM melts.
Vapor pressure [83]	The pressure applied by a vapor in thermodynamic equilibrium with its condensed phases	The ideal PCM for incorporation into building materials must have as low a vapor pressure as possible to avoid any confinement issues.
Density [38,71,216]	Mass per unit volume of PCM	Higher densities are preferable so that the material can occupy minimal volume. It is observed in previous research that a PCM having a density of up to 1640 kg/m3 is used. Whereas increased microencapsulation of PCMs can decrease the density of the materials.
Optical properties [61,233,291]	The transference, absorptance, and reflectance of the glazing and PCM	Solar transmittance can be affected by the thickness and extinction coefficient of the PCM, where the solar transmittance of glazing can change between 0 and 0.8. A study observed a 39.5% reduction in the heat gain while using PCM. Another research study utilized Prisma solar glass in TIM-PCM walls and showed improvement in thermal performance.
Latent heat [52,165,187,292,293]	The amount of heat stored or released during the phase transition of PCM	A higher latent heat of fusion per unit mass is necessary, which permits fewer materials to keep the required amount of energy. Literature suggests that the required amount of latent heat fusion is higher than 120 kJ/kg. A study utilized a PCM having 161.8 kJ/kg latent heat of fusion. Paraffin wax is utilized with a latent heat of fusion of 190 kJ/kg. It is found in the literature that PCMs up to 248 kJ/kg are used in different research.
Thermal conductivity [71,80,294]	The ability of PCM to conduct heat	Higher thermal conductivity of PCM is beneficial because it increases the rate of thermal charging and discharging, whereas PCMs with low thermal conductivity can benefit in insulation. The studies suggest that PCMs having thermal conductivities greater than 0.5 W/mK are preferable.
Chemical stability [20,52,60,71]	The capability of PCM to resist chemical degradation over time	PCMs should have suitable chemical stability when exposed to operating temperatures. Furthermore, PCM must have steady phase change properties over many cycles. In literature, it is found that organic PCMs exhibit better thermal and chemical stability. A study on paraffin-based PCMs showed that it has good chemical stability and resists degradation in high PH environments. Also, it exhibits a non-corrosive nature.
Specific heat [295]	The property of PCM that indicates the amount of heat required to change its temperature by a specified amount. It is measured for both liquid and solid phases of PCM	A higher specific heat capacity directly increases the thermal inertia of the building envelope. This helps in stabilizing indoor temperatures by regulating heat transfer and reducing temperature fluctuations caused by external conditions.
Climate [279]	The usual weather conditions, like temperature, solar radiation, and moisture content	The weather conditions affect the optimized position of the PCM. Guo et al. [279] tested the performance of a PCM wallboard under real solar radiation and seasonal weather. It was found that efficiency is very dependent on environmental conditions and volumetric quantity.

, drawing outcomes based on evidence from the literature. These materials contain some internal factors, material properties, and external factors.

5. Assessment of PCM-Added Construction Commodities

To assess the influence of PCM addition to building envelope materials characterization and its validity through numerical modeling and simulation before integration. The subsections provide a comprehensive discussion of PCM evaluation and its evidence from the literature.

5.1. Experimental Assessment Techniques

The analysis of PCM-enriched building materials is generally based on experimental methods to investigate their thermal performance, energy savings, cost-effectiveness, and sustainability in building applications [60]. The most persistent experimental techniques include differential scanning (DSC), thermal conductivity analysis, and field tests [296,297]. Figure 13 highlights various evaluation techniques. Table 6 summarizes the characterization techniques.

DSC is a widely used method for analyzing the thermophysical properties of PCMs by measuring the heat transfer during phase transition as a function of temperature [293,298,299]. This technique provides information about enthalpy, melting and solidification temperatures, and heat storage capacity [71]. For instance, accurately determining the melting point and latent heat of microencapsulated PCMs (MPCMs) and MPCM-UHPCs using DSC requires a heating and cooling rate of 5 °C/min, a temperature accuracy of ±0.1 °C, and an enthalpy error margin within ±1% [233]. Furthermore, for effective characterization, DSC develops connectivity between the specific heat capacity and temperature [49]. However, this technique has a shortcoming, which tests a small sample size, resulting in affecting the accuracy of overall thermal property measurement [300,301].

Thermal conductivity evaluation is another significant tool for studying the thermal behavior of PCMs and their integrating materials [296]. This method measures the thermal conductivity, phase change characteristics, and heat capacity, which are essential for building envelope performance. A laser thermal conductivity analyzer is one of the instruments used to characterize PCM paste samples [40].

Table 6. Summary of experimental characterization techniques.

Research Study	Experimental Characterization Technique	Type of PCM Used	Purpose of the Technique
Berthou et al. [289]	DSC	Eutectic PCM	To investigate the thermophysical properties of eutectic PCMs.
Cellat et al. [118]	DSC	Strontium-based powder (Sr(OH)2.8H2O)	To test the desired properties of the PCM, like melting temperature and enthalpies.
Chelliah et al. [197]	Viscometer with cooling and heating elements	OM18, HS22, OM29, OM32, and OM37	To measure thermal conductivity in solid and liquid states.
	KD2 Pro-thermal property analyzer		To measure the thermal conductivity of PCMs.
	DSC		To measure the phase transition temperature of PCMs.
Ren et al. [38]	DSC (NETZSCH DSC 200 F3)	Microencapsulated PCM (MPCM)	To quantify the melting point and latent heat of MPCM.
	Laser thermal conductance instrument (LFA467)		For characterization of thermal properties of MPCM in paste samples.
	FT-IR (FT-IR 650)		To analyze chemical compatibility between MPCM and the cement matrix.
	SEM (S-570 SEM, Hitachi in Japan)		To examine the microstructure of MPCM in the cement.
	TGA (TGA, SDT650 TA)		To observe the impact of MPCM on hydration products and thermal stability.
	Mercury intrusion porosimetry (MIP, AutoPore IV 9500, Micromeritics Instrument Corp, Norcross, GA, USA)		To evaluate the pore parameters of ultra-high-performance concrete.
Farulla et al. [218]	DSC	Pluslce X25 (pristine PCM)	To check the stability of the material and assess the main parameters, like phase change enthalpy and specific heat.
Pisello et al. [302]	CC-TPS method (through HOT Disk TPS 2500S unit and ATT DM 340 SR climatic chamber, Gothenburg, Sweden)	Paraffin-based MPCM	To check the dynamic thermal behavior of the PCM-integrated concrete walls by determining volumetric specific heat and thermal diffusivity.
Boussaba et al. [297]	TGA	Bio-based PCM	To analyze the thermal stability of bio-based PCM in natural clay, cellulose fibers, and graphite.
	SEM		To examine the uniform distribution in the matrix.
	FT-IR		To analyze the Chemical stability of bio-based PCM in natural clay, cellulose fibers, and graphite.
	DSC		For the characterization of thermal properties.
Yinping et al. [284]	T-history	Various PCMs, including salt hydrates and paraffin-based	To determine the heat of fusion, thermal conductivity, and specific heat of various PCMs.

Table 6. Summary of experimental characterization techniques.

Research Study	Experimental Characterization Technique	Type of PCM Used	Purpose of the Technique
Berthou et al. [289]	DSC	Eutectic PCM	To investigate the thermophysical properties of eutectic PCMs.
Cellat et al. [118]	DSC	Strontium-based powder (Sr(OH)2.8H2O)	To test the desired properties of the PCM, like melting temperature and enthalpies.
Chelliah et al. [197]	Viscometer with cooling and heating elements	OM18, HS22, OM29, OM32, and OM37	To measure thermal conductivity in solid and liquid states.
	KD2 Pro-thermal property analyzer		To measure the thermal conductivity of PCMs.
	DSC		To measure the phase transition temperature of PCMs.
Ren et al. [38]	DSC (NETZSCH DSC 200 F3)	Microencapsulated PCM (MPCM)	To quantify the melting point and latent heat of MPCM.
	Laser thermal conductance instrument (LFA467)		For characterization of thermal properties of MPCM in paste samples.
	FT-IR (FT-IR 650)		To analyze chemical compatibility between MPCM and the cement matrix.
	SEM (S-570 SEM, Hitachi in Japan)		To examine the microstructure of MPCM in the cement.
	TGA (TGA, SDT650 TA)		To observe the impact of MPCM on hydration products and thermal stability.
	Mercury intrusion porosimetry (MIP, AutoPore IV 9500, Micromeritics Instrument Corp, Norcross, GA, USA)		To evaluate the pore parameters of ultra-high-performance concrete.
Farulla et al. [218]	DSC	Pluslce X25 (pristine PCM)	To check the stability of the material and assess the main parameters, like phase change enthalpy and specific heat.
Pisello et al. [302]	CC-TPS method (through HOT Disk TPS 2500S unit and ATT DM 340 SR climatic chamber, Gothenburg, Sweden)	Paraffin-based MPCM	To check the dynamic thermal behavior of the PCM-integrated concrete walls by determining volumetric specific heat and thermal diffusivity.
Boussaba et al. [297]	TGA	Bio-based PCM	To analyze the thermal stability of bio-based PCM in natural clay, cellulose fibers, and graphite.
	SEM		To examine the uniform distribution in the matrix.
	FT-IR		To analyze the Chemical stability of bio-based PCM in natural clay, cellulose fibers, and graphite.
	DSC		For the characterization of thermal properties.
Yinping et al. [284]	T-history	Various PCMs, including salt hydrates and paraffin-based	To determine the heat of fusion, thermal conductivity, and specific heat of various PCMs.

Field tests involve full-scale buildings or experimental cells integrated with PCM to monitor their thermal response under ambient conditions [19,35,187]. This test involves monitoring variations in energy consumption, internal temperature, and thermal comfort parameters for longer durations [297,303]. To determine the effectiveness, the PCM-enriched structure was compared with a PCM-free reference structure [71]. For example, in a study on phase-change frame walls (PCFW), a 38% reduction was observed in the maximum heat flux of the wall [304]. In addition, microencapsulated PCMs in tubes inside small cubicles were investigated to monitor the thermal behavior. This approach helps assess the impact of PCM incorporation in real-world applications [8,49,187,305].

In addition to the aforementioned techniques, a few more techniques can help in measuring the thermophysical properties. It includes differential thermal analysis (DTA) and T-history analysis. Further, chemical characterization, thermal cycling analysis, and Fourier transform infrared spectroscopy (FT-IR) are suitable for analyzing the chemical stability of materials [38,302]. Scanning electron microscopy (SEM) allows for determining the microstructure of the PCM [139,140,141,302], while thermal gravimetric analysis (TGA) is used for thermal stability evaluation [306]. Devices like hot disks can be used for thermal conductivity measurements [307].

Finally, this technology requires further optimization studies to identify the best properties and advanced PCMs based on specific applications to enhance their thermal efficiency. In addition, economic analysis must be conducted in parallel with thermal behavior evaluation to examine the cost-effectiveness and economic sustainability of using PCMs in building envelopes.

5.2. Numerical Simulation and Modeling Techniques

PCM-added buildings require numerical modeling and simulation to examine and optimize the best possible parameters for improving thermal management. The software used for these applications includes EnergyPlus, TRNSYS, RadCool, CoDyBa, BSim, PCM Express, and WUFI, which help in assessing heat transfer, energy efficiency, and indoor thermal comfort [57,244,277,278]. Figure 14 shows the various tools used in this context. This section is further elaborated through two approaches, i.e., material scale and the building scale.

5.2.1. Material Level Modeling

Material-level modeling of PCMs is crucial for designing effective thermal storage systems within buildings due to the complex coupled fluid-thermal transfer processes involved in their phase change [308]. By mathematically simulating heat transfer mechanisms and flow evolution, this model can accurately predict the thermal performance, such as the temperature distribution and heat flux within building elements [28,221,309]. This deep understanding is essential for optimizing the integration of PCMs by identifying the most suitable PCM properties (e.g., melting temperature and latent heat) [26,28,37,264], determining the optimal quantity and thickness of PCM layers [8,258,310], and selecting their ideal location within building components like walls, roofs, or concrete structures, to maximize energy savings and enhance thermal comfort [8,129,186,264,311].

Given the inherent nonlinearities of melting and solidification and the challenges posed by fluctuating ambient conditions, the sophisticated evolution of mathematical and numerical approaches across multiple length scales [28,49,55,267,308] is required. This evolution allows for the accurate capture of complex coupled fluid-thermal phenomena, such as gravity-induced natural convection and temperature-dependent material properties, which were previously simplified or overlooked in earlier models [312]. Furthermore, these advanced models, ranging from nano-structured PCM composites to macro-scale building components, enable the comprehensive optimization of PCM integration strategies. This includes precisely identifying optimal PCM properties (like melting temperature and latent heat), determining the most effective quantity and thickness of the PCM layers, and pinpointing their ideal placement within various building elements like walls, roofs, or concrete structures. Such detailed modeling ultimately aims to maximize energy savings, enhance thermal comfort, and effectively manage peak loads under diverse and dynamic real-world climatic conditions [8,17,26,49].

Stefan Problem and Modeling Methods

The Stefan problem serves as the fundamental theoretical foundation for understanding solid-liquid phase change phenomena, classically formulating it as a moving boundary problem, where a sharp interface separates distinct solid and liquid phases [55,313], as shown in Figure 15. In its original theoretical development, key simplifying assumptions include one-dimensional heat transfer solely by conduction [312], constant material properties for each phase [314], and an isothermal phase change occurring at a single, fixed melting temperature [313]. While essential for initial theoretical understanding and serving as a baseline for validating subsequent numerical models, its direct applicability in real building scenarios is limited due to these inherent simplifications.

In practical building applications, PCMs rarely undergo isothermal phase transitions; instead they exhibit non-linear behavior over a temperature range, commonly referred to as the “mushy region” [28]. Furthermore, real PCM properties, such as density, viscosity, and thermal conductivity, are often temperature-dependent and can drastically change during phase transition. Crucially, the classical Stefan problem neglects gravity-induced natural convection in the liquid phase, which is a dominant heat transfer mechanism that significantly influences the melt rates and overall thermal performance of building elements like walls, roofs, and concrete [312]. Additionally, buildings are subjected to complex multi-dimensional geometries and dynamic, fluctuating ambient conditions, including time-varying solar radiation and air temperatures, which deviate considerably from the constant boundary conditions assumed in analytical Stefan solutions [28,314]. To overcome these limitations, particularly the non-linear behavior of melting and solidification over a temperature range (the “mushy region”) and temperature-dependent properties under fluctuating ambient conditions, two primary numerical approaches have evolved: the enthalpy method (EM) and the effective heat capacity method (EHCM).

The enthalpy method formulates the solid-liquid phase change as a total heat content approach, combining sensible and latent heat into a single enthalpy term within the energy conservation equation [28]. This transforms the complex moving boundary problem into a more manageable fixed-grid problem, eliminating the need to explicitly track the phase interface [315]. It is widely used and well-suited for a broad range of cases, from isothermal phase changes to those occurring over a temperature range, and is generally implemented in commercial computational fluid dynamics (CFD) packages, such as ANSYS Fluent [312,315]. However, it can face challenges in handling supercooling and may exhibit temperature oscillations in a mesh [315].

In contrast, the effective heat capacity method implicitly incorporates the latent heat effect by modifying the specific heat capacity of the material across the phase transition temperature range [300,316,317,318,319,320]. This approach is computationally simpler because temperature is typically the sole primary variable to be solved and discretized [321]. While conceptually simpler, its accuracy heavily depends on the precision of the apparent heat capacity curve used, which is often derived from experimental techniques like Differential Scanning Calorimetry (DSC) [322]. When accurately determined, the DSC-based effective heat capacity method can provide a more precise prediction of PCM temperature than the enthalpy method by better capturing the non-linear, non-uniform nature of latent heat absorption and release during phase transition [49]. However, it is sensitive to the selected phase-change range and is not applicable to purely isothermal phase changes [315].

In terms of applicability across scales, the effective heat capacity method is frequently employed for macro-scale building energy models, where PCMs are often treated as homogeneous layers within larger building components like walls, roofs, or concrete [323,324]. This simplifies the complexity of the “mushy region” for whole-building simulations. Conversely, the enthalpy method, especially when integrated into advanced CFD simulations, is crucial for more detailed and granular analyses of PCM behavior that necessitate accounting for complex phenomena like gravity-induced natural convection in the liquid phase and detailed temperature-dependent material properties [308]. These advanced models offer deeper insights into the melt progression and heat transfer dynamics within specific geometries, enabling the optimization of PCM integration under diverse and dynamic real-world climatic conditions.

In contrast, the Heat Source Method (HSM), also known as the enthalpy-porosity method, is a numerical technique for modeling phase-change phenomena [315]. Its fundamental concept involves separating enthalpy into sensible and latent heat, and treating the latent heat as a distinct source term within the classical heat equation. This approach allows HSM to directly solve for temperature as the primary variable, which contrasts with the enthalpy method that primarily solves for total enthalpy. HSM is highly versatile for Finite Element (FE) simulations because it can be effectively employed for gradual, sharp, and isothermal phase processes, which is a significant advantage over methods like ACCM that struggle with sharp or isothermal changes. It is often described as highly intuitive, quickly converging, and providing good accuracy, particularly when natural convection plays a significant role in heat transfer. However, HSM’s accuracy can depend on the mesh size, potentially leading to numerical diffusion at the solid-liquid interface [55,315]. Additionally, in some commercial software implementations like Ansys Fluent’s Solidification & Melting (SM) model, a linear relationship is assumed for the liquid fraction over the melting range, which may not accurately capture the non-linear behavior of real PCMs. The “mushy zone constant” used in the SM model is often an undefined parameter that may require tuning to fit experimental data, potentially making it less universally applicable or prone to solver divergence if not correctly adjusted [312]. A summary of the various modeling techniques is presented in

Table 7. Summary of Enthalpy method, effective heat capacity method, and heat source method for PCM in Buildings with CFD Packages.

Method Used	CFD Package Used	Context/Findings
Enthalpy Method & Effective Heat Capacity Method [319]	MATLAB (using FDM)	Context: Experimental validation of transient nonlinear heat transfer processes within PCMs, analyzing how factors like melting/freezing temperature ranges, the width of the phase change temperature ranges, and liquid fraction consideration impact the accuracy of both the enthalpy and effective heat capacity methods. Findings: The Effective Heat Capacity Method consistently demonstrated higher accuracy than the Enthalpy Method, primarily because its treatment of latent heat is closer to the actual, non-linear physical process. The enthalpy method’s averaging of latent heat over a temperature range is a simplification that does not fully capture the non-uniform, non-linear nature of phase change (which follows a parabolic DSC curve). For the enthalpy method, narrower temperature ranges around peak temperatures resulted in more accurate results. It was concluded that if melting and freezing temperature ranges differ, each range should be used for accurate calculations. Considering the liquid fraction is important for PCMs, where the phase transition is incomplete.
Solidification and Melting (SM) Model (Enthalpy-Porosity Formulation) & Apparent Heat Capacity (AHC) Method [312]	ANSYS Fluent	Context: Compares the Solidification and Melting (SM) model and the Apparent Heat Capacity (AHC) method in ANSYS Fluent, including the simulation of natural convection using temperature-dependent density, validated against experimental and literature data. Findings: The AHC method is favored by the authors, as it accurately captures the non-linear phase transition behavior without needing an arbitrary “mushy zone constant” (which is often used to fit experimental data in the SM model and can lead to non-physical melting or solver divergence). The SM model applies a linear relationship for the liquid fraction, which is less accurate for real PCMs with non-linear phase transitions.19 While SM can yield similar temperature curves, it often shows larger deviations from measured data. Both methods are highly dependent on the selection of thermophysical properties.
CFD-based Solidification and Melting Model (implicitly uses linear method for properties, likely effective heat capacity variation) [309]	ANSYS Fluent	Context: Analyzes heat transfer in building walls using Sodium Hydrogen Phosphate PCM, aiming to manage thermal transfer and store energy for cooling. Findings: The numerical model showed good similarity with experimental results. The PCM walls demonstrated a damping effect on indoor temperature fluctuations, reducing maximum and increasing minimum temperatures by approximately 5 °C compared to non-PCM walls. The PCM’s behavior, including solidification and absorption of accumulated energy, contributes to reducing inner temperature variance. The study found that results depend significantly on PCM thickness, inlet cooling temperature, and mass flow rates.
Enthalpy Method & Effective Capacity Method [325]	Not specified (various simulation tools like Energy-Plus are discussed)	Context: Provides a general overview of progress of PCM technology usage in building applications. Findings: EM was employed to account for the different thermophysical properties in a composite wall system. EHCM was utilized for PCM simulation in radiant floor systems. Overall, PCM models are found to significantly decrease heating and cooling loads by augmenting the thermal storage capacity of building envelopes.
Modified Specific Heat (effectively Apparent Heat Capacity) [326]	COMSOL Multiphysics (3.5)	Context: Focuses on the solid-liquid phase change heat transfer process within a simple rectangular enclosure heated from one side, assessing the influence of the PCM melting temperature range. Findings: The model successfully simulated melting behavior by modifying the specific heat to account for the latent heat of fusion across the PCM’s melting temperature range. The validation clearly demonstrated the impact of incorporating a “mushy region” (represented by the modified specific heat) on the temperature profile in the liquid PCM and the behavior of the melting front. It was noted that numerical results tended to underpredict the total heat flux and yielded lower temperatures in the liquid PCM phase over longer periods.
Enthalpy Method & Apparent Calorific Capacity Method (ACCM) & Heat Source Method (HSM) [55]	Not specific (Mentions commercial codes like FLUENT and COMSOL in a general context)	Context: Reviews numerical tools for modeling phase change problems, specifically focusing on the Stefan problem and fixed grid methods for PCMs in cement-based composites. Findings: The ACCM and HSM are preferred for finite element procedures because temperature is the sole dependent variable. The HSM is considered the best choice for finite element simulations and PCM analysis as it can be applied to gradual, sharp, and isothermal phase processes. The Enthalpy Method (EM) uses enthalpy to distinguish sensible and latent heat. The ACCM is simpler as it lumps sensible and latent heat, while HSM treats latent heat as a source term in a temperature-based equation.
Various Models (Single-phase additional heat source, Equivalent specific heat capacity (single/two-phase), Enthalpy (single/two-phase), Multiscale models) [322]	Not specified (General Numerical Methods)	Context: Reviews and analyzes various mathematical models for the thermal-hydraulic processes of microencapsulated phase change slurry (MEPCS). Findings: Enthalpy models are considered more flexible than additional heat source and effective specific heat capacity models because they do not require difficult-to-obtain input correlations. Two-phase models are generally more accurate than single-phase models for heterogeneous flow, but demand higher computational cost and may oversimplify the phase change inside the capsule. Multiscale models offer the highest accuracy and can describe non-linear behavior and subcooling effects, but are complex and currently less widely adopted. All macroscopic models reviewed assume a “homogenization temperature” inside the capsule, which limits their ability to capture non-linear phase change behavior.
Effective Heat Capacity Method (DSC Cp-T method) [49]	Modelica (Dymola)	Context: Investigates the design and evaluation of an integrated latent heat thermal energy storage (ILHTES) system for residential buildings, combining a PAHX with an air conditioning unit. Findings: This method provided heightened accuracy in effectively predicting both PCM temperature and air outlet temperature, outperforming both the standard effective heat capacity and enthalpy methods.5 The study’s long-term simulations (covering an entire cooling season) allowed for a comprehensive evaluation of the system’s efficiency, overcoming the limitations of short-duration CFD simulations. Using RT25 PCM in Budapest resulted in a 32.4% energy saving ratio (ESR), with RT25 generally outperforming other PCMs (RT18, RT20, RT27) due to its ability to leverage larger temperature differences with the nighttime outdoor environment for more efficient heat transfer and solidification.
Apparent Heat Capacity Method [327]	ISOLAB code (building simulation code)	Context: Empirical validation of a building thermal model that incorporates PCM into a complex roof structure. The model was integrated into a multi-zone building simulation code. Findings: The numerical thermal model successfully predicted the dynamic thermal behavior of PCM, showing encouraging results for various wall types (with or without PCM). The method employs a smoothed Heavisine function to manage the discontinuity in specific heat during the phase change. For proper simulation, it required the use of small, fixed time steps and careful adjustment of mesh size and solver settings.

.

Conduction-Only vs. CFD Modeling

In the modeling of PCMs for building applications, the choice between conduction-only and a computational fluid dynamics (CFD) approach strongly depends on the complexity of the heat transfer phenomena, the level of detail required, and the available computational resources [312,328,329,330,331]. Models based on conduction alone are often considered adequate when heat transfer is dominated by thermal diffusion and natural convective effects are negligible or intentionally suppressed [332,333,334]. Such conditions occur, for example, when PCMs are integrated into building elements such as walls, floors, or roofs in the form of thin layers (e.g., thicknesses of about 2 cm), where heat transfer can be approximated as one-dimensional [332]. Furthermore, in composite materials containing small fractions of PCMs, such as modified cements, the conductive approach significantly simplifies the analysis by avoiding the modeling of natural convection in the liquid phase [335,336,337]. Thermal conduction is also dominant in the solidification phase of the PCM, i.e., in the thermal system discharge [338]. In these cases, the classical Stefan problem, which describes the solid-liquid transition assuming conduction only, provides a solid theoretical basis and can be solved both analytically and numerically [326]. However, when heat transfer occurs in three dimensions or when natural convection in the liquid phase of the PCM becomes significant, CFD becomes indispensable [339]. During melting, for example, gravity-induced convective motions (Rayleigh-Bénard convection) can accelerate energy transfer and reduce charging times, effects that conduction-only models cannot capture [308]. Similarly, in ventilated configurations, such as roofs or walls equipped with PCMs [55,340,341,342,343,344], or in active HVAC systems, where the PCM interacts with air flows, CFD allows for a realistic representation of the fluid-solid thermal coupling [49,308]. It is also particularly useful when analyzing microencapsulated PCMs or two-phase materials, where phenomena such as sedimentation, density variation, and complex geometries occur [322]. Common tools for these simulations include commercial software such as ANSYS Fluent, which adopts the enthalpy-porosity method, COMSOL Multiphysics, OpenFOAM for open-source Navier-Stokes models, and environments such as MATLAB or Modelica for custom numerical modeling [49,319,328,330,331,345,346,347,348,349]. Among the methods used, some are discussed in Section Stefan Problem and Modeling Methods and Table 7. Other approaches include the use of the Boussinesq approximation to model natural convection without resorting to complex multiphase models [350], the Volume of Fluid (VOF) method to represent interfaces and volumetric expansion in encapsulated PCMs [312], and numerical schemes such as the Finite Volume Method (FVM) and the Finite Element Method (FEM) [55,308,326]. The latter is particularly effective in enthalpy or heat capacity methods, as temperature is often the only variable that needs to be determined. Finally, RANS models, such as RNG k-ε, are often used to model turbulent flows in ventilated or HVAC systems [332]. Regardless of the type of modeling adopted, validation with experimental data is essential to ensure the reliability of the model. Comparisons with experimental results obtained from test cells, thermal storage units, or heat exchangers demonstrate that CFD models, especially those based on the EHCM supported by Cp–T measurements, provide an accurate representation of the thermal behavior of PCMs in buildings.

5.2.2. Building Performance Simulation

Building Performance Simulation (BPS) operates primarily at the building or system scale, aiming to predict and optimize overall thermal comfort and energy consumption [315]. Tools like TRNSYS and EnergyPlus are examples of BPS software that simulate dynamic changes in indoor conditions and energy requirements on a large scale. However, the accuracy and comprehensiveness of these building-level simulations are fundamentally reliant on detailed insights derived from material-level modeling. For instance, although BPS tools are effective for large-scale temperature and energy analyses, their simplified moisture exchange models often do not fully account for the complex coupling of heat and moisture transfer phenomena within the building envelope materials [351]. Therefore, a deeper understanding of material behavior, such as phase change processes in PCMs, at a smaller scale (e.g., through methods like apparent heat capacity or enthalpy-porosity), is crucial for enhancing the fidelity of BPS. In practice, this hierarchy is often addressed through approaches like co-simulation, where a dynamic building simulation tool is coupled with a more precise model for hygrothermal behavior at the component or material level [327]. Furthermore, CFD analyses, which operate at a more granular level, can validate material behavior models for components such as PCM panels in roofs or walls, providing critical data that can be used in larger system evaluations [352]. This integration ensures that material-specific thermal phenomena, such as natural convection in liquid PCM or heat transfer in different PCM configurations, are accurately represented within the broader building performance assessment.

Key Elements in BPS

To accurately predict a building’s dynamic response and energy consumption, BPS models incorporate a range of multidomain inputs:

• HVAC systems: These systems are deeply integrated with PCM layers in various configurations, such as walls and ceilings, to improve energy efficiency [332]. PCMs can be part of active TES systems, like PAHX, coupled with air conditioning (AC) units. This integration aims to mitigate the peak load demands of heating and cooling systems, thereby boosting operational efficiency. For instance, cool outdoor air at night can charge the PCM, and during the day, ventilation fans help discharge cooling energy into the interior space [49]. Studies have explored the functionality of PCMs beyond passive regulation, extending to direct contact with heating and cooling cycles within HVAC components like ducts, evaporator coils, and heat exchangers [322,353,354,355].
• Occupancy schedules: The building type must be identified to establish the predominant occupancy throughout the daytime, which directly influences internal gains and energy loads. The varying occupancy periods for different building types (e.g., offices, schools, and residences) determine their specific cooling demands. Occupant behavior is a crucial factor influencing building performance [49,352].
• Transparent components (e.g., windows): These components significantly influence solar gains and play a role in the PCM charging process. Highly insulated buildings, particularly passive houses with substantial window areas (e.g., approximately 20% of the floor area for daylight), can experience elevated indoor temperatures during summer due to solar gain. Incorporating PCMs into large window areas can help reduce indoor temperatures. However, current commercially available PCMs may have low solar transmittance during their melting phase, potentially reducing the daylight factor in double-glazed windows [356].
• Lighting systems: These systems contribute to the internal heat loads within a building. While the direct heat contribution from lighting systems is part of the overall internal gains, which also include equipment and people, models focusing on cooling demand often simplify or exclude detailed accounting of these specific heat sources when prioritizing occupant-related loads [325].
• Typical Meteorological Year (TMY) data: This provides essential local climate inputs for an accurate BPS. Daily temperature profiles are critical for designing free cooling applications. BPS studies often utilize real climate data, such as variable daily temperature profiles, for the numerical investigation of PCM units. Comprehensive climatic databases, like those supporting energy certification, provide meteorological reference data for specific cooling seasons, enabling long-term simulations that cover entire cooling periods and diverse climatic conditions. The effectiveness of PCM integration is highly dependent on climate conditions [180,352,357,358,359,360,361,362].
• Internal loads: These encompass heat contributions from equipment, people, and appliances within the building. For cooling demand analysis, the internal load associated with occupants is a primary focus, while contributions from equipment and appliances may sometimes be simplified or excluded in specific analyses. Overall, internal loads are a key factor influencing the performance of buildings [363].

Building Performance Simulation Tools

BPS tools are essential for analyzing the integration of PCMs within building systems, and their accuracy is often enhanced by incorporating detailed material-level modeling or by validating material properties. These integrations allow for a more nuanced understanding of how PCMs influence overall thermal comfort and energy consumption. Table 8 summarizes the use of EnergyPlus, TRNSYS, and WUFI in conjunction with material-level modeling, along with their contexts and findings. Among various tools, EnergyPlus is a versatile tool used in studies to assess the energy performance of building envelopes integrated with PCMs, including walls and roofs, across various climatic conditions [332]. It can also model a simple test room for thermal comfort analysis when PCMs are integrated into the internal partitions [325]. For example, it has been used to evaluate the energy performance of building envelopes with PCM in hot, subtropical regions [351,364]. However, a notable limitation is that detailed CFD modeling, when coupled with building simulations, can lead to high computational requirements. Furthermore, EnergyPlus, like some other building energy simulation software, primarily focuses on simulating temperature variations and energy requirements on a large scale, and its moisture exchange models at the wall scale often rely on a simplified approach that does not account for the coupling of heat and moisture transfer phenomena through the building envelope [351].

On the other hand, TRNSYS is recognized as a Building Energy Simulation (BES) software that is frequently employed as a modeling tool to simulate latent heat TES units and entire buildings for comprehensive techno-economic assessments and multi-objective optimization [49]. It excels at describing dynamic changes in indoor relative humidity and temperature due to external climate, hygrothermal loads, and moisture buffering effects [164]. A new module, Humi-mur, has also been developed and implemented in TRNSYS, specifically for the precise representation of mass transfer in materials in contact with indoor air [365]. Despite these strengths, similar to EnergyPlus, TRNSYS’s moisture exchange models at the wall scale often rely on a simplified approach that may not fully account for the coupled heat and moisture transfer through the building envelope.

Moreover, Wärme Und Feuchte Instationär (WUFI) is a prominent hygrothermal simulation software lauded for its specialization in coupled heat and moisture transfer and stands out as one of the few BPS tools that integrates a full moisture model [366,367,368,369,370]. These models incorporate detailed material properties and use input parameters derived from the experimental characterization of materials, including both thermal and hydric properties, to simulate the coupled heat and moisture transfer in components containing PCMs. WUFI can also employ methods like the enthalpy method to describe the phase change process for such materials. However, the primary focus of WUFI is on component-level heat and moisture behavior, and the provided sources do not elaborate on its applicability or computational intensity for comprehensive whole-building energy consumption analysis or complex HVAC system modeling [332].

Table 8. Various studies on building performance simulation.

BPS Tool	Context/Study Description	Conjunction with Materials-Level Modeling	Key Findings
EnergyPlus [49,371,372]	Wooden Building with Latent Heat TES Unit (Bordeaux, France): Evaluated the thermal performance of a lightweight wooden building with integrated latent heat TES units.	A dynamic simulation model of the TES unit was first developed in MATLAB and validated with experimental data at the component/material level. This validated MATLAB model was subsequently integrated into EnergyPlus via a co-simulation framework for comprehensive building-scale evaluation.	Enabled a comprehensive evaluation of the building’s performance with integrated TES units.
	Swedish Passive House with PCM Air-Heat Exchanger (Stockholm, Sweden): Assessed a comfort cooling strategy to achieve a good indoor climate during summer.	MATLAB code was used to analyze the thermodynamic properties of the PCM storage prior to simulation in EnergyPlus. This involved detailed material property characterization informing the BPS model.	The PCM could remove a substantial amount of excessive temperatures (degree hours), contributing to a good indoor climate during summer.
TRNSYS [49,315,373,374,375,376]	Passive Buildings with Latent Heat TES Units (Stockholm): Conducted a comprehensive techno-economic assessment of passive buildings incorporating latent heat TES units under specific climatic conditions.	TRNSYS was used to model both the TES unit and the building, implying the development or use of detailed, user-defined components or modules to precisely represent PCM behavior and its interaction with building systems within the TRNSYS environment.	A multi-objective optimization algorithm identified cost-effective solutions that simultaneously reduced cooling demand and life cycle costs.
	Building with Phase Change Humidity Controlling Material (PCHCM) Wallboards (Wuhan, China): Studied the effects of PCHCM wallboards on building energy consumption and indoor hygrothermal environment.	The PCM effect was integrated using the effective heat capacity method, a common material-level modeling approach for phase change, which accounts for the relationship between specific heat capacity and temperature. Kunzel’s model was additionally used to describe hygrothermal behavior at the material level.	The PCHCM wallboard significantly reduced energy consumption and improved the indoor hygrothermal environment.
	Development of New TRNSYS Modules for PCM Walls: Focused on enhancing TRNSYS’s capability to simulate PCM-integrated building components.	Involved the establishment and experimental verification of new, user-defined TRNSYS modules designed to accurately represent the heat transfer model of PCM rooms or latent heat storage walls, based on material-level characteristics like latent heat utilization ratio.	Improved the accuracy and capability of TRNSYS for modeling PCM-enhanced walls and rooms.
WUFI [377]	Wood-Frame Walls with Macro-Packed PCM (MPPCM): Assessed the hygrothermal performance of building components containing PCM (n-octadecane).	WUFI PRO 5.3 was utilized, a BPS tool particularly known for its focus on coupled heat and moisture transfer in materials and its integration of a full moisture model. This indicates a direct application of material-level hygrothermal modeling within the software.	Demonstrated improved hygrothermal performance and the potential to mitigate mold growth risk when MPPCM was used as a vapor retarder replacement.

Table 8. Various studies on building performance simulation.

BPS Tool	Context/Study Description	Conjunction with Materials-Level Modeling	Key Findings
EnergyPlus [49,371,372]	Wooden Building with Latent Heat TES Unit (Bordeaux, France): Evaluated the thermal performance of a lightweight wooden building with integrated latent heat TES units.	A dynamic simulation model of the TES unit was first developed in MATLAB and validated with experimental data at the component/material level. This validated MATLAB model was subsequently integrated into EnergyPlus via a co-simulation framework for comprehensive building-scale evaluation.	Enabled a comprehensive evaluation of the building’s performance with integrated TES units.
	Swedish Passive House with PCM Air-Heat Exchanger (Stockholm, Sweden): Assessed a comfort cooling strategy to achieve a good indoor climate during summer.	MATLAB code was used to analyze the thermodynamic properties of the PCM storage prior to simulation in EnergyPlus. This involved detailed material property characterization informing the BPS model.	The PCM could remove a substantial amount of excessive temperatures (degree hours), contributing to a good indoor climate during summer.
TRNSYS [49,315,373,374,375,376]	Passive Buildings with Latent Heat TES Units (Stockholm): Conducted a comprehensive techno-economic assessment of passive buildings incorporating latent heat TES units under specific climatic conditions.	TRNSYS was used to model both the TES unit and the building, implying the development or use of detailed, user-defined components or modules to precisely represent PCM behavior and its interaction with building systems within the TRNSYS environment.	A multi-objective optimization algorithm identified cost-effective solutions that simultaneously reduced cooling demand and life cycle costs.
	Building with Phase Change Humidity Controlling Material (PCHCM) Wallboards (Wuhan, China): Studied the effects of PCHCM wallboards on building energy consumption and indoor hygrothermal environment.	The PCM effect was integrated using the effective heat capacity method, a common material-level modeling approach for phase change, which accounts for the relationship between specific heat capacity and temperature. Kunzel’s model was additionally used to describe hygrothermal behavior at the material level.	The PCHCM wallboard significantly reduced energy consumption and improved the indoor hygrothermal environment.
	Development of New TRNSYS Modules for PCM Walls: Focused on enhancing TRNSYS’s capability to simulate PCM-integrated building components.	Involved the establishment and experimental verification of new, user-defined TRNSYS modules designed to accurately represent the heat transfer model of PCM rooms or latent heat storage walls, based on material-level characteristics like latent heat utilization ratio.	Improved the accuracy and capability of TRNSYS for modeling PCM-enhanced walls and rooms.
WUFI [377]	Wood-Frame Walls with Macro-Packed PCM (MPPCM): Assessed the hygrothermal performance of building components containing PCM (n-octadecane).	WUFI PRO 5.3 was utilized, a BPS tool particularly known for its focus on coupled heat and moisture transfer in materials and its integration of a full moisture model. This indicates a direct application of material-level hygrothermal modeling within the software.	Demonstrated improved hygrothermal performance and the potential to mitigate mold growth risk when MPPCM was used as a vapor retarder replacement.

5.3. Practical Applications via Case Studies

The literature provides insights into various case studies on the applicability of PCMs in building materials, emphasizing their potential to improve thermal mass and energy efficiency. As discussed, PCMs can store and absorb energy during a phase-change process, resulting in enhanced thermal comfort and energy conservation [8,17,71]. Multiple studies have been conducted to explore several types of PCMs, their incorporation into building commodities, their effects on thermophysical properties, and interior temperature regulation.

Hu et al. [262] studied the performance of PCM-enriched walls under various climate conditions in China. The study revealed a 6% reduction in energy consumption and a 1% decrease in CO₂ emissions under warm ambient conditions. It was further determined that PCM placement can play a crucial role in energy savings, where the inner wall placement showed 1–7% more effectiveness than the outer insulation. An increase in the PCM layer thickness improved the energy savings by 2–6% in extreme climates. Similarly, Ahangari et al. [351] found an increase in thermal comfort from 73% to 93% in dry ambient conditions in Iran using a double PCM layer. A decrease in heating energy consumption of 17.5% and 10.4% in dry and semi-arid areas, respectively.

Zhu et al. [372] used paraffin-based PCMs integrated into the Trombe wall in Wuhan, China, by applying PCM in double layers with different melting points, such as 30 °C on the external side and 18 °C on the internal side. The cooling and heating loads decreased by 9% and 15%, respectively. In another study in Amman, BioPCM^® with varying melting points of 21–29 °C was analyzed by demonstrating the importance of thickness, configuration, and climate data. A comparative assessment is conducted on BioPCM^®, InfiniteR PCM, and WinCo EnerCiel PCM while considering important factors like application, operational temperature range, cost, and thermal stability [37].

Additionally, studies have investigated the role of PCMs in building envelopes by exploring their potential to regulate interior temperatures and minimize reliance on active heating and cooling systems. Usually, PCMs promote excessive heat storage during the daytime and then release it at night to stabilize indoor temperatures [73,378,379,380,381,382]. Moreover, PCM integration can lower the peak hours by up to 4 °C, improve thermal inertia, and decrease energy consumption [379]. To maintain interior comfort in residential and commercial settings, the ideal melting points of PCMs are between 20 and 26 °C [86].

Al-Rashed et al. [380] analyzed the thermal performance of various PCMs such as RT-31, RT-35, and RT-42 in Kuwait. It was found that higher melting points were correlated with superior thermal regulation. Similarly, Cascone et al. [375] augmented PCM-enriched opaque envelope components for office buildings under Mediterranean conditions. Moreover, research studies indicate that PCM incorporation into buildings can reduce overheating, as found by Figueiredo et al. [376], who reported a reduction of 7.23% by providing 35.49% efficiency to the PCM addition. Mohsin et al. [187] determined that adding a 10 mm thick layer of PCM with a melting point of 21 °C incorporated into roofs and walls minimized the temperature variance, shifted the peak energy demand, and improved interior thermal comfort. Figure 16 shows a case study of the PCM melting temperature over energy savings. An experimental study in Iraq suggests that PCM integration into buildings can significantly improve thermal behavior under high outdoor temperatures. The roof and east wall of the PCM-added rooms showed better thermal performance, with maximum recorded temperature differences of 3.75 °C and 2.75 °C, respectively. Moreover, compared to non-PCM rooms, the thermal comfort was increased by 11.2% and 34.8% based on DHR and MHGR metrics [35]. The experimental setup is shown in Figure 17.

Furthermore, investigations have been conducted to assess the impact of PCMs under various climatic conditions. In the hot weather of Mexico, grey-coated roofs attained a reduction of 6.4 °C and 22.2% in interior surface temperature and cooling load, respectively, whereas white-coated roofs showed a reduction of 14.7 °C to 15.4 °C and 58.1% to 62.7% [383]. In Shanghai, PCMs with melting points of 24 °C and 22 °C were ideal for summer and winter conditions, contributing to 96.2% energy savings [384]. In contrast, studies on desert areas determined that PCMs with high melting points and greater thickness increased energy savings by 17.97% to 34.26% while reducing the peak temperature by 2.04 °C [180]. Moreover, a research study on PCM-integrated buildings in Delhi revealed a reduction in heat gain of 12.6–36.2% for roofs and 10.4–26.6% for walls [385]. In Hangzhou, China, increasing the composite PCM layer thickness from 1 to 5 mm enhanced the energy charging and discharging rates during the day and night. This resulted in a 6.7 °C decrease in the peak temperature and improved thermal regulation. Further, a 5 mm thick PCM layer integration enhanced the duration of thermal comfort by 12% compared to a reference wall, thus demonstrating its potential for improving interior thermal comfort and energy efficiency in buildings [386]. In tropical savanna regions, PCM integration reduced the indoor temperature by 3.28 °C, while minimizing the temperature variance by up to 2.76 °C, and reduced the energy utilization by 16.58–68.63% [387]. In different climate regions, PCM integration reduced the building temperature by 2 °C while reducing the heat by up to 75%, and resulted in a time shift of 3 h [225]. Under tropical weather conditions in India, the application of PCM led to a decrease in peak temperature ranging from 7.19% to 9.18%, while the thermal amplitude was reduced by 0.67% to 59.79% when considering both the roof and walls. Additionally, the PCM cubicle exhibited a time lag of 120 min for the east and north walls, whereas the roof, west, and south walls experienced a 60-min delay compared with the reference cubicle. Moreover, the integration of PCM significantly lowered the cooling load by 38.76%, resulting in an estimated daily electricity cost savings of 0.40 US$ [75].

Different studies have revealed that the addition of PCM to buildings contributes to annual energy savings of 4000–10,000 kWh and a maximum reduction in temperature of 2 °C during the summer season. Moreover, discomfort hours are reduced by 500 h yearly [388]. In Amarah, Iraq, a study found that PCM integration in buildings led to a 4.6 °C maximum operative temperature [35]. In a few European cities like Rome, Vienna, and London, a 0.5 °C reduction in operative temperature is observed in numerical studies in a conditioned environment [389]. Similarly, research outcomes from Qatar and Melbourne showed a 2.5 °C decrease in temperature and a 21.6% average enhancement in thermal comfort [297,390].

Research on PCM-enhanced construction materials, such as mortar panels, PCM-encapsulated bricks, and roof structures, has revealed another aspect. Kong et al. [144] found that mortar-based tubes and spherical PCM capsules exhibited the same thermal performance, while flat-plat capsules showed unsteady temperature distribution. Akeiber et al. [385] used paraffin-based PCMs having melting points of 19 °C and 44 °C. PCM-1 (19–37 °C) performed better by reducing discomfort hours and peak heat gain. However, simulation studies utilizing EnergyPlus software have shown the effectiveness of PCMs during peak summer conditions. A study showed an average temperature fluctuation reduction (ATFR) of 5–6 °C, with a peak heat gain reduction of 38–59% and a 6 °C reduction in operating temperature. Furthermore, PCM integration resulted in a 2 kg/day reduction in CO₂ emissions and 250 Iraqi dinars/day of economic savings [186]. Another numerical study using ANSYS ensured that a PCM-enriched room can maintain human comfort levels for 94.1% of the day in comparison to a reference room, having values of 26.7%, while reducing temperature fluctuations by 64.4% [129].

In conclusion, these findings highlight the significant advantages of PCMs in improving the thermal effectiveness of buildings. Their intrinsic capability to efficiently absorb and release thermal energy controls the indoor temperature, minimizes energy demand, and enhances overall thermal comfort.

6. Advantages and Shortcomings of PCM Integration

PCM incorporation into building commodities can benefit society in terms of sustainability in the construction industry. Moreover, to enhance the usability of PCMs in building materials, the challenges must be addressed. The following subsections address the advantages and disadvantages of PCM-integrated construction materials.

6.1. Advantages

PCMs’ incorporation is an effective approach to enhance the energy efficiency of buildings, substantially decrease energy consumption, mitigate peak loads, enhance interior thermal comfort, and minimize dependency on HVAC systems [40,186]. PCM addition into building envelopes and structural elements makes them more sustainable and environmentally friendly [391,392,393].

6.1.1. Energy Savings

PCMs play a significant role in energy savings in buildings because they can store energy. Due to the phase transition process, PCM absorbs and releases thermal energy, further reducing the reliance on traditional heating and cooling systems [39,71].

Integrating PCMs into building envelopes is important for enhancing energy efficiency [37]. Simulation studies have found that PCM-integrated buildings require 1125 kJ of energy compared to 1348 kJ for a reference room, resulting in a 17% decrease in energy consumption [26]. Furthermore, exhibiting long-term efficiency, PCM integration can save energy by 11% to 13.4% [3]. Additionally, particular applications like PCM-integrated radiant floor systems have proven effective in minimizing energy demand [5,39]. Likewise, the incorporation of PCM in glazing units decreased energy consumption by up to 47.5% [254].

PCMs are highly efficient in passive systems, which regulate the temperature without the input of any electrical or mechanical energy [394]. In the cold climate of Morocco, PCM-enriched buildings showed a 17% decrease in energy consumption for cooling and heating purposes, contributing to 13.31 kWh/m²/year compared to the national average of 25 kWh/m²/year [26].

6.1.2. Reduction in Peak Load

PCMs’ incorporation into building commodities can assist in shifting energy usage to off-peak hours due to the absorption and release of thermal energy during higher demand periods, thereby reducing the load on heating and cooling devices [22,23,24,28]. This load regulation capability minimizes the peak demand and further contributes to substantial savings in energy costs. For instance, buildings using PCMs in winter for heating purposes showed a saving of 65%, with a daily reduction of 47% (0.83 kWh) in energy consumption, while in the summer season, the savings reached 42%, with a daily energy consumption reduction of over 23% (0.27 kWh) [395]. In Tehran, Iran, a study found that utilizing PCMs for space cooling reduced energy consumption by 10% in January and up to 30% in March and April [396].

On the other hand, active PCM storage systems can enhance efficiency by storing solar energy in winter and ensuring free night-time cooling in summer. The stored energy can be utilized later, ensuring an effective reduction in dependence on active heating and cooling loads [17].

6.1.3. Enhanced Indoor Comfort

PCMs can maintain a stable interior temperature and enhance occupant thermal comfort [40,397]. By regulating temperature fluctuations, PCMs can promote a steady and comfortable indoor environment [8,42]. According to the literature, incorporating PCMs into walls can reduce indoor air temperatures by preventing excessive heat buildup in summer, thereby lowering the indoor temperature by up to 7 °C [17]. Moreover, adding PCMs to buildings can minimize average room temperature changes by 5–6 °C, further enhancing occupant comfort [186]. Compatibility of the interior temperature with the transition temperatures of PCMs can reduce the dependency on HVAC systems while providing a thermally comfortable environment [37]. In addition to building applications, PCMs can enhance sleep quality and decrease heating and cooling needs when incorporated into clothing and bedding [17,382,398].

In addition, buildings with radiant floor cooling systems embedded with PCMs can offer a higher level of thermal comfort [399]. In Morocco’s hot weather conditions, PCM-integrated bricks decreased overheating in summer by lowering the interior temperature by 3.4 °C [26].

6.1.4. Reduced HVAC Dependence

PCM-added buildings, which passively regulate indoor temperatures and thus reduce dependence on energy-intensive HVAC systems [187,200], further lower energy costs and carbon emissions [59,400]. The inclusion of RT-25 PCM enhanced the cooling system efficiency by attaining an energy-saving ratio (ESR) of 32.4% [49]. Moreover, PCM-enriched buildings have a lower energy use intensity (EUI) than those with conventional insulation, contributing to a significant decrease in the overall energy efficiency [37].

6.2. Shortcomings

PCMs have the potential to optimize the thermal performance of the building; however, their implementation on a larger scale requires further research to address critical issues such as phase change losses, thermal degradation, high costs, compatibility with building materials, and fire safety issues [39,401,402].

6.2.1. Leakage Issue

One of the main challenges associated with PCM integration into a building is the loss of PCM, especially during the phase transition from solid to liquid [116,120]. Direct incorporation of PCMs can lead to leakage during melting, deteriorating their compatibility with construction materials and further enhancing the risk of fire [117]. Even the utilization of macroencapsulation techniques cannot eliminate the leakage issue; if any damage occurs during the construction of retrofitting, it can cause leakage [53,402]. For example, flexible bag-shaped shells tend to deform, thereby deteriorating the system integrity [273,275,401,402,403,404].

Shape stabilization and advanced encapsulation can substantially mitigate this shortcoming. In addition, solid-solid PCMs (SSPCMs) are emerging as alternatives to be utilized after pretreatment with chemical modifications [39].

6.2.2. Thermal Degradation

PCMs can undergo thermal degradation over the years, compromising their long-term efficiency and deteriorating their thermal properties [401]. Particularly, eutectic PCMs exhibit thermal instability due to the presence of impurities, which further reduces their latent heat storage capacity after multiple cycles of use [402]. Therefore, further research is necessary to assess the actual efficiency of PCM-integrated buildings while considering durability and maintenance aspects [3,52]. However, the choice of materials plays a vital role in ensuring the optimal performance of PCM-added concrete over time, as the interconnection between PCMs and building materials can influence the rheological, mechanical, and durability properties of the concrete structure [52].

6.2.3. High Cost

The high initial cost of PCMs is a substantial barrier to their broad applicability in the building sector [77]. Generally, PCMs are more expensive than conventional materials, and their incorporation usually requires additional design and construction efforts [39,77]. Therefore, a comprehensive cost-benefit assessment is required to analyze the economic suitability of PCM applications in buildings [3,71]. Another important factor is the payback calculation, as in some regions, it may exceed the expected lifespan of the buildings [39,60]. One solution is to explore cost-effective alternative materials such as wood chips [17].

6.2.4. Compatibility with Building Materials

To maintain the structural integrity and durability of building envelopes, the compatibility of PCMs with other construction materials is significant [17]. Few PCMs have interconnectivity issues with certain sealants, adhesives, or insulation materials, which generally affect the lifespan and efficiency of the building envelope [103]. PCM addition through direct incorporation can alter the mechanical properties of the final material, especially at higher temperatures, where liquid PCMs reduce the water content [71,116]. Additionally, leakage during PCM’s liquid phase and interference with cement hydration may compromise the strength of the concrete [104,105,106,117]. Paraffin-based PCMs are stable in higher PH environments; however, they may not form strong bonds with concrete hydration products, affecting the selection of materials for building applications [403]. Moreover, it was suggested that supplemental cementitious materials and lightweight epoxy-coated aggregates might be a safe way of incorporating PCMs into concrete structures [52].

6.2.5. Fire Safety Concern

Another important shortcoming is fire safety, as organic PCMs are highly flammable [8,39,404]. Integration of highly concentrated PCMs into wall coverings and insulation materials can increase fire risk [405]. To reduce the risk of high flammability of organic PCMs, multiple strategies have been developed, such as the use of fire retardants, chemical modifications, and protective coatings [406]. Additionally, by law, compliance with fire safety regulations is important to ensure the safe use of PCMs in buildings [407,408]. In addition, to reduce fire hazards during the incorporation of PCM into building structures, a thorough risk assessment should be performed.

6.3. Building Code Provision for PCM Integration

The advantages and disadvantages of integrating PCMs into building commodities were discussed in the preceding section. However, this requires a link to a practical regulatory and policy framework. Building codes and incentives significantly impact the adoption of new materials. In this context,

Table 9. PCMs integration into building materials and their relevance to building codes and incentives.

PCM Benefit/Barrier	Relevant Building Code/Standard	Incentives/Programs	Remarks
Reduced peak cooling/heating loads [28,37,52]	PCM use helps achieve goals related to energy efficiency, such as those implied by the Building Energy Efficiency Standard (BEES). Guidelines like Indicator 7.1.2 of the UN SDGs emphasize efficient fuels and technology combinations for household energy use, including space heating, to ensure health benefits.	The application of PCMs for thermal comfort indirectly supports UN SDG 7.b.1, which relates to increasing the installed capacity of power plants generating electricity from renewable sources. Investment promotions in energy infrastructure and technology upgrades for sustainable energy services are relevant.	PCMs are used for thermal load shaving and shifting, reducing cooling/heating loads, and managing building material temperature. PCM integration can reduce peak cooling/heating loads and shift peak demand to off-peak hours, thereby alleviating pressure on the electrical grid. This can significantly reduce the reliance on mechanical heating and cooling systems.
Improved indoor thermal comfort [52,258,397,409]	ASHRAE 44 guidelines are used in analyses of discomfort hours. ASHRAE Standard 55-2004 is mentioned in the context of predicting comfort. The EN-ISO 52120 standard guides smart heating control with resetting temperature setpoints.	UN SDG 7 (affordable and clean energy), to which PCMs contribute, aims to ensure health benefits through efficient fuels and technology.	PCMs enhance thermal comfort by moderating indoor temperature fluctuations and maintaining stable conditions. They are essential for offering better temperature control, especially in buildings prone to overheating during summer.
Lower HVAC system size/operating costs [3,8,37,186,409]	PCM incorporation has shown better performance for cooling load reduction than for heating load reduction, impacting system sizing. Optimal PCM selection is highly dependent on site meteorological data and desired indoor temperature settings.	Potential annual energy savings of 5476.14 kWh, translating to a cost reduction of 1150 USD/year, and an estimated payback period of four years for PCM incorporation in external walls have been reported in a study. Cost savings in peak cooling loads and electricity cost savings (ECS) have been observed.	PCMs reduce energy consumption, leading to lower energy expenses and operational costs for heating and cooling systems. Energy savings up to 30% for wallboard-based PCM have been achieved.
Enhanced renewable integration (thermal storage) [17,52,62]	UN SDG 7 (affordable and clean energy) directly links to PCM’s contribution to promoting clean energy sources. PCM use in buildings allows them to meet energy efficiency goals and minimize negative environmental effects.	UN SDG 7 indicators specifically mention solar energy, solar PV, solar thermal energy, and solar heating applications as areas where PCM contributes. Investment promotions in energy infrastructure are key to supporting the adoption of renewable energy technologies.	PCMs are effective thermal energy storage media that can store and release heat during phase transitions. They are applied to enhance the thermal mass of building structures, enabling the harnessing of solar energy and reducing reliance on non-renewable sources.
Cost premium of PCM materials [17,37,71,410]	Cost-effectiveness is a crucial criterion for PCM selection and integration into buildings.	While some studies report a payback period of four years, others indicate payback periods ranging from 10 to 23.31 years. Thorough assessment and cost analysis are essential for the practicality and financial feasibility of overall building applications.	PCMs can be more expensive than conventional insulating materials, and their integration may require more engineering and construction work. Micro/nanoencapsulations are typically the most expensive PCM incorporation techniques.
Installation complexity (retrofits) [8,37,52,411]	Thermally Activated Building Systems (TABS), while enhancing the building envelope, cannot be installed in existing buildings as they require implementation during initial construction.	The application of PCM in retrofitting projects for energy-efficient transformation of existing structures is identified as a future scope to attain the UN SDGs.	PCM is typically integrated during the construction process or added as a separate layer within the building structure. While some PCM panels can be installed during the construction phase, they can also be directly attached during the retrofitting phase using adhesives or fasteners.

highlights the connection between PCM integration and major building code provisions and incentive programs in key markets.

7. Recent Progression and Future Potential

Considering the potential of using PCM in building materials, it is also essential to assess the sustainability factor of how PCM integration can move towards a greener, cost-effective, and energy-efficient solution in the construction sector.

7.1. Innovation in Approaches

Recent advances in PCM integration into building applications have aimed to develop innovative encapsulation methods using nanotechnology and bio-based PCMs to increase thermal efficiency, sustainability, and safety [39,412].

Micro-and nanoencapsulation approaches involve the development of core-shell containers to avoid any loss of liquid PCM [62]. These techniques enhance the heat transfer rate by improving the surface area and minimizing reactions with surrounding materials [287]. Microencapsulation usually requires the insertion of PCMs into containers with a size range of 1–300 μm, where the outer shell is made of silica or organic polymers [76]. On the other hand, in macroencapsulation, the PCMs are poured into large-scale containers like panels, bricks, and tubes, which assist in limiting heat ingress, especially during periods of higher solar radiation [39,53]. For instance, Abass et al. [258] integrated concrete roofs with macroencapsulated PCMs, enhancing passive cooling while decreasing the interior temperature by up to 7.2 °C during the hottest hours and minimizing heat transfer by up to 60.6%. Furthermore, another study developed a wall with tubes coated with macroencapsulated PCMs, which maintained the surface temperature around 20.5 °C in summer and 26.8 °C in winter [413]. Additionally, to achieve optimal performance, it is important to choose suitable shapes and geometric parameters while using the macroencapsulation technique [53], which further affects the mechanical strength, corrosion resistance, thermal stability, and flexibility of the material [52].

Another growing research area is the development of bio-based PCMs (BPCMs) [37], which are naturally occurring BPCMs derived from renewable resources such as beeswax, palm oil, coconut oil, and soya oil, which offer a more sustainable option than other PCMs [37,414,415]. However, the thermochemical stability of BPCMs remains a challenge and requires further research [416]. In addition, to promote energy savings and enhance living comfort, PCMs can be integrated with other technologies, such as hybrid systems, in which PCMs are integrated with radiant thermal control techniques, thermochromic windows, and passive radiative cooling coatings to increase energy efficiency [417].

In summary, the latest developments in the field of PCM-embedded building envelopes focus on innovative methods for encapsulation while combining nanotechnology with BPCMs to enhance thermo-chemical stability. These advancements have the potential to optimize energy savings in multifunctional building envelopes. Further, addressing the current challenges through the implementation of advanced research can ensure the broad utilization of PCM technology in the building sector.

7.2. Advancement in Policy Support

To support PCMs’ integration into building codes for the improvement of energy efficiency and thermal comfort, policy development is required. These developments are necessary to promote sustainable building practices and contribute to global energy conservation efforts. Considering that the building sector accounts for a solid share of worldwide energy consumption, policies aimed at this issue can be vital in achieving international targets for minimizing energy consumption and CO₂ emissions [392,418]. Furthermore, policies focusing on the provision of economic incentives and financing programs can further boost the adoption of PCMs, particularly in regions with extreme weather conditions and high energy demand [71]. On the other hand, PCM integration usually supports United Nations SDGs, such as SDG 7 (affordable and clean energy), SDG 11 (sustainable cities and communities), and SDG 13 (climate action), which further strengthens their contribution to worldwide sustainable strategies [3].

Moreover, the incorporation of PCMs in building codes requires a shift towards performance-based standards by considering the thermal response and energy conservation potential of these materials [52,71]. Many studies have simulated PCM performance under national building codes, which have benefited this field with more realistic assessments [19]. Some building codes define EUI targets that can be achieved by PCM utilization. For example, a study conducted in Amman, Jordan, found that PCM usage with optimal specifications achieved an EUI of 117.42 kWh/m²/year, which is approximately the ideal value of 115.14 kWh/m²/year defined under BEES. This shows the compliance level of PCM integration into buildings to support energy efficiency regulations [37,419]. However, in areas where energy regulations are poor, there is a risk that building practices will not be aligned with the SDGs [52].

Future research should aim to analyze the environmental impacts of PCMs from production to disposal through LCAs [420] and conduct longer field studies to assess their long-term performance [71]. A holistic approach is essential to ensure sustainable development by integrating environmental conservation, energy efficiency, and sustainable building practices [421,422]. Moreover, advanced numerical simulation and modeling approaches are required to accurately analyze energy consumption and environmental impacts [52].

In conclusion, the development and execution of supporting policies for the incorporation of PCMs into building codes is a key step to enhancing the sustainability and performance of buildings. This development can contribute to energy consumption reduction, thermal comfort improvement, and reduction in environmental concerns, thus aligning with the SDGs.

8. Conclusions

The utilization of PCM technology in buildings is a groundbreaking strategy that promises energy-efficient and environmentally sustainable futures. PCMs are renowned for their ability to absorb and release substantial amounts of latent heat during phase transitions, effectively moderating indoor temperatures and significantly reducing energy consumption for heating and cooling. This capability contributes to a more stable and comfortable interior environment, with remarkable improvements. It is evident from the literature that PCMs can be integrated into building commodities through various techniques, such as direct incorporation, immersion, encapsulation, form-stable, and shape-stable PMs. PCMs can be integrated into any building commodity, like walls, roofs, floors, and glazing units. Due to the passive heat transfer nature of PCM, it can enhance building energy efficiency by reducing HVAC dependence and shifting peak loads. Alongside experimental work, PCM integration into construction commodities has also been modelled through simulation and modeling techniques on two levels, i.e., the material level and building energy simulation. However, the integration of PCMs requires careful attention because multiple factors can affect the thermal behavior of PCMs. The factors include the PCMs’ latent heat capacity, position, thickness, density, change in volume, and melting point. In addition, PCM incorporation into building commodities still faces some challenges, such as

• The selection of an adequate type of PCM and its long-term durability, cost-effectiveness, and effectiveness for specific applications.
• The compatibility of PCMs with construction materials and their optimization for integration.
• Optimization of PCM encapsulation to prevent leakage and further deterioration of building materials.
• Lack of incentivizing policies for the implementation of PCM integration into building materials.
• Utilization of PCM in severe weather conditions, specifically in the case of glazed units.

Hence, it is of utmost importance to conduct future research using both experimental and numerical approaches to develop innovative techniques that can contribute to the cost-effective production of PCMs, better encapsulation methods, long-term durability, and better compatibility with construction materials. Thus, PCMs can be utilized on a large scale contributing to an energy-efficient and more sustainable future.