Recent Advances in Integrated Building Cavities: A Comprehensive Review of Fundamentals, Modeling, and Performance Evaluation

Abstract

Currently, building envelopes are evolving from simple passive shields to smart systems. They control the exchange of energy, air, and moisture within the cavities of buildings. This comprehensive review attempts to investigate the potential performance outcomes resulting from the integration of photovoltaic/thermal systems (PV/Ts), phase change materials (PCMs), and heat pumps (HPs). After the screening process, 153 documents were selected to be synthesized, including 64 primary peer-reviewed publications, 20 standards, and 69 reviews or methodological articles. The results from experimental, field, and numerical studies have shown that cavity integration techniques are effective in achieving photovoltaic cooling, recovering useful heat, reducing heat flux, and supporting heat pump operation. Specifically, annual simulations showed a 6.5% reduction in heating electricity and coefficient of performance (COP) improvements of 22–26%, whereas short-term experiments reported COP gains of 3.3% on average and up to 8.6%. Hybrid (solar/thermal and waste heat recovery) system studies reported that combined heating and cooling energy can be saved by more than 50%. Yet, these results substantially varied with climate, cavity configuration, baseline system, control strategy, averaging period, and auxiliary energy usage. In general, PCM-integrated ventilated cavities can reduce summer heat gains while supporting controlled heat storage and release. However, in extreme weather conditions, application of PCMs may not be effective since complete phase transitions cannot be achieved. Therefore, future studies are recommended to focus on climate-responsive designs, validated hygrothermal models and controls, regulatory compliance, and life-cycle assessments.

1. Introduction

1.1. The Evolution of the Building Envelope

The building envelope has historically served as a static barrier to separate indoor spaces from the external environment. This provides structural integrity, thermal insulation, and weather protection. However, conventional static envelopes have limited capacity to adaptively regulate heat, airflow, and moisture transport in response to changing conditions. Airflow and moisture transport are defined by the movement of air and fluid (vapor, liquid, or ice) in the cavity, respectively. Therefore, conventional buildings strongly influence heating and cooling demand [1,2,3,4]. Improving building energy efficiency has become a key component of international climate policy, as indicated in the European Union’s net zero energy building (nZEB) requirements and China’s carbon neutrality target for 2060 [5,6].

As a result, building envelopes are increasingly becoming active and adaptive systems. They can substantially reduce the thermal and electrical energy demands [7,8]. Current research therefore increasingly focuses on dynamic, multifunctional building envelopes that actively manage air, heat, and moisture flow over time. As a result, air cavities become purposefully ventilated spaces within the wall/ façade assemblies [9]. At the same time, they function as thermal buffers, moisture-control airflow channels, and platforms for integrated energy conversion technologies. The idea of the “active building envelope” therefore exhibits a genuine shift in thinking, recasting the envelope from a passive insulation layer into an active component of the building’s energy system [10,11,12].

Rather than adding renewable systems on top of a conventional façade, many contemporary approaches embed these technologies directly into the envelope. The integration provides an additional pathway toward low-carbon and carbon-neutral construction. This shift includes BIPV, solar-air and solar thermal collectors, ventilated and double-skin façades, and adaptive PCM-based layers. Conventional façade surfaces can be redesigned as active interfaces that modulate solar gains, precondition air ventilation, and contribute to on-site electricity [13,14,15]. This transformation aligns with international net-zero-energy and net-zero-carbon agendas.

1.2. The Anatomy of the Air Cavity

Air cavities are functional layers within building envelopes whose geometry and operating mode influence hygrothermal properties and the durability of adjacent materials. They are flexible, acting as mediators for the link between the external environment and the internal room.

• Thermal regulation in airflow: Air cavities support both active and passive thermal control. In active mode, fans/blowers can draw/push air through the cavity with the help of solar-heated air, which is more effective in winter. For example, cavity air can preheat the ventilation air that the heat pump uses as an input. In passive operation, solar heating and buoyancy can drive airflow through appropriately connected cavities. The air becomes buoyant and rises, allowing the façade to “breathe out” warm air in summer without any mechanical input [3].
• Thermal buffering: A ventilated cavity also acts as a thermal buffer zone between the outdoor climatic conditions and the conditioned interior. By maintaining an intermediate-temperature air layer, the cavity may reduce the temperature difference across the principal insulation layer under favorable operating conditions. This may lower heat flux and peak loads on the heating, ventilating, and air conditioning (HVAC) system [16]. If cavity openings are controlled seasonally, the cavity may reduce winter heat loss and summer overheating under favorable conditions. When the cavity is coupled to a heat pump, any resulting improvement in source-air temperature may increase heating COP. This benefit must be evaluated at the system level, including fan electricity, defrost operation, auxiliary heating, and control energy. Mathematically, COP is expressed as the ratio of useful thermal energy output to the electrical work input required to drive the system.
• Moisture management: The air cavity can also protect envelope materials and assemblies. It provides drainage and ventilation paths that allow infiltrated rainwater to drain and dry out. Heat transfer across a cavity results from conduction, convection, and radiation. Cavity depth, surface temperature, airflow pattern, and surface optical properties are some of the critical factors that influence the effective thermal transmittance of the complete wall assembly. Among these, building cavities significantly influence heat exchange with the environment [17]. For integrated cavity systems, intelligent tools should therefore complement rather than replace validated physical models and safety-critical control logic [18,19].

1.3. Objectives, Rationale and Research Questions

Existing studies are limited, since they have particularly focused on ventilated/double-skin façades, BIPV/T systems, solar chimneys, PCM-based envelopes, or adaptive insulation alone. These approaches limit understanding of the physical and functional interface of the integrated cavity. As a result, it becomes difficult to figure out the relationships between heat, air, and moisture flow, energy conversion and storage, and HVAC systems.

Moving beyond reviews of individual technologies, this review establishes the cavity as a common physical interface and clearly shows the opportunities and limitations of integrations. It combines fundamental physical transport processes with the application of active, passive, and hybrid technologies. Finally, it evaluates different approaches that have been used across different climates, geometries, control strategies, and modeling methods. The review answers the following core questions:

• What physical mechanisms control the coupled heat, airflow, and moisture transport through integrated building cavities?
• What are the benefits and drawbacks of system integration in relation to energy, humidity, and comfort under various climates and seasons?
• What methods should be used to address cavity performance issues, and how should uncertainties and validation of these methods be addressed?
• Why do different researchers obtain contradictory outcomes?
• How should promising research setups be implemented into practical building systems?

2. Review Methodology

2.1. Review Design and Scope

This review used a structured comprehensive-review methodology to identify, screen, critically appraise, and narratively synthesize evidence on integrated building-envelope cavity systems. The review covered experimental, field-monitoring, numerical, and coupled system studies concerning ventilated cavities, BIPV/T systems, phase change materials, heat pumps, and associated heat, airflow, and moisture processes. Regulatory standards, guidance documents, and previous review articles were used to provide technical or regulatory context but were not treated as primary performance evidence.

2.2. Information Sources and Search Dates

Web of Science was searched on 21 May 2026, and ScienceDirect was searched on 2 June 2026. The final update search for both sources was conducted on 11 August 2026. The searches covered records published from 1 January 2009 to 11 August 2026 (Table 1). No records published after the final search date were considered for inclusion.

Table 1. Database, terms/keywords, and research articles’ search outcome.

Databases	Search Field	Search Sets	Publication Coverage	Language Treatment	Records Retrieved
Web of Science	Topic: title, abstract, author keywords, and Keywords Plus	i AND (ii–iv) combined using OR	1 January 2009 to 11 August 2026	English-language filter applied in the database	1009
ScienceDirect	Title, abstract, or author-specified keywords	i AND (ii–iv) combined using OR	1 January 2009 to 11 August 2026	English, assessed during record management	3021
Results identified after full-text review					64

2.3. Database-Specific Search Strategies

The search was implemented as a series of pairwise and multiconcept searches rather than as a requirement that every record contains all five concept groups. Each search combined the building-envelope cavity block with at least one integration or performance block. The results of all search combinations were exported and merged before duplicate removal. The implemented search strings are:

• (“ventilated facade” OR “ventilated façade” OR “air cavity” OR “double-skin facade” OR “double-skin façade” OR “building envelope cavity” OR “ventilated wall”)
• (“BIPV/T” OR “BIPVT” OR “PV/T” OR “building-integrated photovoltaic thermal”)
• (“air-source heat pump” OR “air source heat pump” OR “ASHP” OR “heat pump”)
• (“phase change material” OR “PCM” OR “thermal energy storage”)
• (“building energy” OR “energy performance” OR “airflow” OR “ventilation” OR “moisture” OR “hygrothermal” OR “heat transfer”)

2.4. Eligibility Criteria

Eligibility was determined by using predefined concept groups rather than by counting individual keyword occurrences. To be eligible, a study had to address (i) a building-envelope air cavity or functionally equivalent ventilated-envelope system and (ii) at least one additional targeted concept: photovoltaic/thermal integration, heat pump coupling, phase change thermal storage, coupled heat–air–moisture transport, or cavity-related energy-performance evaluation. Consequently, a publication was not required to contain every term appearing in the search strategy. Search terms were used for retrieval, whereas eligibility was determined through content-based assessment during screening.

2.5. Record Management and Duplicate Removal

All retrieved records were exported from Web of Science and ScienceDirect into Mendeley. Duplicate records were initially identified using DOI. Records without a DOI were compared using normalized title, first-author surname, publication year, and journal/conference title. Automatically identified duplicates were checked manually before removal. Conference and journal versions were not considered duplicates when they contained substantively different methods/results; when the journal article represented an expanded version of the same study, the more complete version was retained for performance synthesis. This procedure removed 1703 duplicate records, leaving 2327 unique records for screening.

2.6. Screening and Study Selection

One reviewer independently screened titles and abstracts against the eligibility criteria. The remaining two reviewers were involved in evaluating the records. The full texts of potentially eligible studies were screened in the same manner. Disagreements were resolved through discussion. Reasons for exclusion were recorded at the full-text stage.

2.7. Data Extraction

The searches identified 4030 records. After removal of 1703 duplicates, 2327 unique records underwent title and abstract screening. At this stage, 643 records were excluded because they did not satisfy the language, publication-type, date, or broad subject-scope criteria, leaving 1684 records for detailed title and abstract eligibility assessment. A further 1531 records were excluded because they did not examine a building-envelope cavity together with at least one additional targeted concept. Full texts were sought for 153 records. Of these, 89 were excluded because of the absence of original technical evidence, resulting in 64 included studies (Figure 1).

Figure 1. Flowchart of the literature search and selection process.

2.8. Evidence-Quality and Reliability Assessment

The included evidence consisted of experimental, field-monitoring, numerical, and coupled modeling studies. Risk-of-bias tools, which were used for evaluating studies, could not be applied to the entire body of evidence. Thus, the review performed a reliability check of each study by means of design-specific criteria. It was aimed at assessing the confidence that could be placed in reported performance outcomes.

Reliability categories were used to qualify the narrative synthesis. They were not used to calculate combined effects because substantial heterogeneity existed in climates, building types, cavity configurations, controls, baselines, averaging periods, and system boundaries. Numerical studies without independent validation and short-duration experiments were retained to identify mechanisms and design trends. Yet, their reported performance values were not interpreted as broadly transferable estimates.

2.9. Synthesis of Performance Metrics

Performance results were collected along with their averaging period, operating condition, climate, and system boundary. Thermal efficiency, heat recovery efficiency, electrical efficiency, COP, and energy savings were considered as different performance measures. Hence, instantaneous/test period average/seasonal/annual COPs and heating/cooling/total HVAC electricity/whole-building energy savings were noted separately. In cases where any of the auxiliary fan or pump electricity, defrost energy, control electricity, and PV electricity credits were not mentioned clearly in the study, it was considered an incomplete or unclear system boundary. Since the included studies were diverse, percentage results were summarized narratively and were not combined as effect estimates.

To characterize the thematic coverage of the review, the eligible primary studies were grouped according to their dominant technical contribution. The classification was used to structure the subsequent narrative synthesis and does not represent reliability/ quality ranking (Figure 2).

Figure 2. Distribution of the included primary studies by dominant evidence category. Of the 64 studies, 40 examined ventilated or double-skin façades, 20 examined BIPV/T-, solar-air-, or heat-pump-integrated envelopes, and 4 focused primarily on PCM- or thermal-storage-integrated envelopes. Each study was assigned to one dominant category to avoid double counting.

3. Fundamentals of Cavity Heat and Moisture Transfer

3.1. Modes of Heat Transfer

Heat transfer in the cavity is a combination of conduction, convection, and radiation phenomena. Their relative influence depends on the geometry of the cavity. For example, forced-convection heat transfer is often estimated using correlations such as Dittus–Boelter or Gnielinski, provided that geometric and flow-regime parameters are important to predict its performance [17,20]. The convective heat-transfer coefficient (hc) also plays a significant role in system performance.

Wind speed and incident radiation strongly affect cavity Nusselt numbers (Nu), which connect convective to conductive heat transfer [21,22]. Heat exchange through radiation in cavities is linearized using the radiative heat-transfer coefficient (hr) [6]. Studies by Vuong et al. [23] and Fu et al. [24] show that increasing the channel’s internal emissivity from 0.1 to 0.9 can increase useful thermal output by 10–15%.

3.2. Moisture Dynamics

Moisture behavior is a coupled phenomenon of air and heat transfer. It is the main cause of condensation, mold growth, and corrosion that affect overall durability. However, cavities are the main path for vapor diffusion and drying, which makes condensation less likely to occur. Condensation occurs when temperature differences and humidity levels in the airstream reach the dew point. In winter, indoor air leaking into the building through the cavities may cool to the dew point. Conversely, during summer, warm, humid air can condense on cold surfaces inside the building [25].

Recent studies have focused on how these spaces are actively managed and how active airflow control can reduce moisture risk by stabilizing conditions. According to C. Zhang et al. [16] and Hailu et al. [26], porous insulation layers reduce condensation risk in exhaust air insulation walls (EAIWs). This exfiltration strategy smooths temperature gradients and provides a continuous drying airflow through the assembly (shown in Figure 3). Likewise, in ventilated façades, the cavity plays a similar protective role as a drained, back-ventilated space. In rainscreen construction, the cavity forms part of the drainage and drying strategy, while open-joint claddings allow infiltrated water and vapor to escape rather than accumulate [27,28]. Experimental and simulation work by Aparicio-Fernandez et al. [3] and Ibane-Puy et al. [29] also show that ventilated cavities can maintain surface temperatures above the dew point in winter. Since heat and moisture transport depend strongly on climate, recent studies have tried to apply reversible/seasonally adaptable airflow strategies. These strategies can switch among inward, outward, and bypass modes to support year-round moisture-safe operation [30].

Figure 3. Building-integrated photovoltaic/thermal (BIPV/T) configuration integrated with ASHP [26].

3.3. Aerodynamics

The cavity airflow is usually divided into three categories: natural, forced, and mixed convection. Natural convection does not need any mechanical fan power. In particular, it is effective in tall cavities where stack pressures generate substantial flow rates [3]. The stack effect is represented by the Rayleigh number (Ra), which represents the ratio of the buoyancy force to viscous dissipation. For high aspect ratio cavities, natural convection becomes turbulent, increasing heat transfer at velocities of 0.1 to 0.3 m/s in solar-air heaters [31].

4. Classification and Configurations of Integrated Building Cavities

4.1. Passive Ventilated Cavity Strategies

4.1.1. Passive Solar Heating

Trombe walls are well-established passive solar systems used for space heating and, in ventilated configurations, natural ventilation [32]. A Trombe wall consists of three layers, namely the glazing layer, air cavity, and the storage wall. Solar energy passes through the glazing layer and warms up the air cavity and the wall surface. The stored heat is passed on to the interior environment through convection and conduction. In recent years, PCMs, PV systems, and controlled air pathways have considerably enhanced the efficiency of Trombe systems [33,34,35].

Duzcan and Kara [36] evaluated PCM-integrated Trombe walls against traditional constructions and found that they can reduce annual heating energy use by 11.9–34.3%. This occurs because the PCM layer absorbs excess solar heat during the day and then releases it slowly in the evening and at night (Figure 4). If thermal efficiencies are 58–66%, a Trombe wall provides 25–44% of the total heating needs. Since these systems may overheat during summer, seasonal shading, bypass ventilation, or closure strategies are required.

Figure 4. Trombe wall design: (a) Summer Season, (b) Winter Season.

Solar chimneys use similar physical principles but prioritize ventilation over direct thermal storage [37]. Increasing the height of the sun-heated vertical cavity creates buoyancy-driven pressure differences, thereby increasing the potential for buoyancy-driven ventilation [38]. In cold periods, preheated air can be utilized for space heating, but hot weather conditions require suitably designed inlets and outlets that enable the chimney to exhaust hot air from inside the house.

The greenhouse effect can substantially increase cavity temperature under high solar exposure. When a weakly ventilated cavity combines with high-transmittance glazing and a high-absorptance interior surface, air temperatures within the cavity can rise by 20–40 °C. The key design task is to balance these gains against the need to prevent overheating and maintain comfort across seasonal variations.

4.1.2. Passive Cooling Strategies

Passive cooling strategies use temperature differences in airflow to reduce cooling energy usage. During night purge ventilation, cool nighttime air removes heat stored in the building fabric and cavity system [39,40].

The effectiveness of natural cross-ventilation depends strongly on cavity geometry, inlet and outlet areas, and prevailing wind conditions. Therefore, open-joint rainscreen façades can support exterior ventilation and drying. They support continuous passive airflow that limits solar heat buildup in summer while still allowing the cavity to dry out in winter [17,28,41].

Stack-effect cooling uses buoyancy forces to reverse the usual winter (chimney) effect. The warm air rises and exits through the upper part of the building. Conversely, cool (dense) air enters from the lower end of the building. In raised buildings, the resulting pressure differences can drive significant airflow without a fan. Vertical cavities/solar chimneys can significantly reduce cooling demands. It is suitable for cool nights and moderate daytime temperatures. It may also enhance evaporative/hybrid-ventilation strategies where climatic conditions are suitable.

Table 2 compares selected quantitative studies according to their evidence type, baseline, temporal basis, metric, and reported outcome. The studies used different climates, configurations, operating conditions, assessment periods, and system boundaries. Therefore, their results were treated as separate metrics.

Table 2. Performance evidence for cavity-integrated envelope systems.

Evidence Type	System/Context	Baseline	Temporal Basis	Metric	Reported Outcome	Interpretation	Ref.
Calibrated whole-building simulation	HRV retrofit in an existing Vancouver house	Passive ventilation without HRV	Annual simulation calibrated to monthly utility data	Space heating energy; break-even infiltration	With HRV: 75.49 GJ electricity and 56.70 GJ gas; without HRV: 73.64 GJ and 52.70 GJ. Break-even infiltration: 0.281 h−1	HRV increased heating energy in the studied leaky house; airtightness should precede retrofit.	[7]
Full-scale experiment	Opaque ventilated façade under natural and forced convection	Natural versus forced convection	Four separate 4 h tests, 10:00–14:00, November 2020	Air-temperature rise; collected heat; collector efficiency	Mean natural-convection efficiency: 11%; maximum forced-convection efficiency: 25%	Short daytime results; fan electricity must be considered in net performance.	[17]
Comparative full-scale experiment	South-facing PCM ventilated façade; two Mediterranean test cubicles	Identical cubicle without PCM façade	Winter 2012; free-floating, controlled-temperature, and demand-profile tests	Indoor temperature; HVAC electricity	PCM façade supplied heat and reduced HVAC electricity	Performance varies with operating mode; not an annual saving.	[20]
Full-scale outdoor experiment	Semi-transparent PV double-skin façade with forced convection	Tested airflow and wind conditions	Selected summer tests	PV temperature; electricity; recovered heat; heat-recovery effectiveness	Heat recovery effectiveness reached approximately 81%	Effectiveness is not equivalent to annual HVAC energy savings.	[21]
EnergyPlus simulation	BIPV/T–BISAH-assisted ASHP in a net-zero-energy house	Same house with reference ASHP and no solar-air assistance	Annual simulation	Space-heating electricity	Annual space-heating electricity decreased by 6.5%	Result reflects a low-load passive house and should not be generalized to other buildings.	[23]
Cold-climate TRNSYS simulation	BIPV/T-assisted ASHP	Conventional outdoor-air ASHP	Cold-climate heating simulation	Heating COP	COP increased from 4.2 to 5.3, approximately 26.2%	Simulated, climate-specific result; auxiliary-energy inclusion must be stated.	[26]
Whole-building system simulation	Residential BIPV/T-assisted ASHP in Canada	Conventional outdoor-air ASHP	Annual simulation	Annual heating COP	COP increased from 2.74 to 3.35, approximately 22%	Annual simulated result; dependent on model validation and auxiliary-energy accounting.	[31]
Experimentally validated model with TRNSYS	Naturally ventilated BIPV/T double-skin envelope	Building without active BIPV/T envelope	Climate-based simulations for several French locations	PV output; heating and cooling demand	Cooling demand increased slightly; heating-demand effect was weak	Integration did not consistently reduce energy demand.	[35]
Full-scale outdoor experiment	Climate-responsive ventilated façade with PCM and controlled airflow	Alternative PCM and ventilation-control cases	Five-week summer campaign	Air preconditioning; heat flux; thermal resistance	Air-preconditioning effectiveness reached 81–87%, depending on PCM	Selected summer performance; strongly affected by solar radiation and fan control.	[42]
Controlled experiment	Air-type PV/T-assisted heat pump water heater	Study-specific reference operating condition	Short-term tests	Water-heating COP	Average COP improvement: 3.3%; maximum: 8.6%	Maximum value is not seasonal; water-heating COP should not be generalized to space heating.	[43]
Comparative full-scale experiment	PCM ventilated façade for Mediterranean cooling	Identical cubicle without PCM façade	Selected summer cooling tests	Cooling demand; indoor temperature; PCM regeneration	Cooling demand decreased when night ventilation enabled PCM solidification	Performance depends on nightly PCM regeneration.	[44]
Laboratory experiment and CFD validation	Open-joint ventilated façade	Laboratory temperature and stereoscopic-PIV measurements	Pseudo-steady-state laboratory cases	Cavity temperature and airflow	CFD reproduced measured temperature and airflow distributions	Validation evidence, not annual energy-performance evidence.	[45]
CFD and standards comparison	Ventilated façade evaluated using CFD and simplified methods	Standard-based calculation assumptions	Steady/scenario-based CFD cases	Airflow and façade heat transfer	CFD captured local thermal and airflow behavior not represented by simplified methods	CFD results alone do not establish annual HVAC-energy savings.	[46]

Notes: ASHP = air-source heat pump; BIPV/T = building-integrated photovoltaic/thermal; BISAH = building-integrated solar-air heater; CFD = computational fluid dynamics; COP = coefficient of performance; HRV = heat-recovery ventilator; PCM = phase change material; PIV = particle image velocimetry.

4.1.3. Structural Configurations

Airflow patterns and thermal performance depend heavily on the ventilated façade’s structural geometry [47]. Open-joint rainscreen openings can facilitate pressure control if the cavity and openings are appropriately designed. This can moderate pressure differences and support drainage and ventilation to keep the cavity dry. Whereas closed-joint systems create a dead air space that provides thermal insulation with limited drying and heat dissipation.

Comparisons of open- and closed-joint configurations have yielded important design insights; for example, Sanjuan C, S et al. [48] studied open-joint façades in sunny outdoor conditions. The results showed that natural airflow velocities ranged from 0.1 m/s to 0.25 m/s, and temperatures increased by 30–35 °C above ambient. As a result, the heating loads are reduced by 10–15% in winter. However, open joints increase thermal losses during cloudy periods and require careful balancing of ventilation rates.

In contrast, closed-joint systems offer higher thermal resistance (U-values of 0.1 W/m²·K) under optimal conditions [16]. However, they generally offer less ventilation-assisted drying and heat rejection than open/mechanically ventilated cavities. To overcome these drawbacks, hybrid configurations are suggested, as they offer a flexible and promising approach to developing a dynamic envelope [42].

4.2. Active Heating and Cooling Systems

4.2.1. Forced Ventilation Cavities

Forced ventilation cavities can provide more controllable airflow and closer HVAC integration than naturally ventilated cavities [49]. This results from mechanical fans that maintain controlled airflow rates. The cavity acts as an active energy-management component [50,51]. Choi et al. [43] studied fan-assisted BIPV/T systems under winter conditions, achieving thermal efficiencies of 30–33% and outlet air temperatures of 25–30 °C. Under the tested operating conditions, variable-flow operation produced an average COP improvement of 3.3% and a maximum improvement of 8.6% relative to the study metrics. These short-term experimental results should not be interpreted as annual system-level improvements [16,43].

The performance of these systems depends heavily on airflow-rate design, duct geometry, and pressure-drop management. Although higher airflow can increase total heat removal, it may lower outlet-air temperature and thereby reduce the usefulness of the recovered heat. For this reason, Delisle and Kummert [52] reported favorable channel velocities of approximately 0.1–0.5 m/s for the investigated collector configurations. Other studies by Rounis et al. [53] and L. Xu et al. [54] found that wind-induced pressure fluctuations can cause inlet flow variations of 15–25%. However, a well-designed inlet can reduce this to less than 5%.

For cavities filled with porous materials, pressure losses can be examined using Darcy’s law. Using this relation, Z. Zhang et al. [55] and Hailu et al. [26] demonstrated a 40–50 mm thick porous layer at an exfiltration velocity of approximately 0.00286 m/s. The results showed reduced total energy and improved heat recovery while affecting the required fan power (Figure 5). Variable-speed fans with PID controllers have a significant role in real-time adjustment of airflow. They can regulate solar radiation, cavity temperature, and heating demand. This indicates that control strategies should incorporate weather forecasts, heating/cooling demand, humidity, and equipment constraints. It helps turn the building envelope into a responsive, energy-efficient thermal boundary [42,56].

Figure 5. Schematic diagram of air exfiltration through a porous layer.

4.2.2. Exhaust Air Recovery

Exhaust air recovery systems may be particularly valuable in buildings with substantial ventilation demand. These systems recover thermal energy from exhaust air before it is discharged outdoors [57]. The exhaust air insulation wall (EAIW) is a new heat-recovery approach that uses porous material to recover heat from exhaust air before releasing it to the outside environment [58]. Figure 6 shows indoor air flowing through a porous layer, while the inner wall surface temperature stays near the indoor environment. The thickness and exfiltration rate of the porous materials play a dominant role in preventing heat conduction. C. Zhang et al. [16] showed that the daily total heat gain of the EAIW can be approximately eliminated for an exfiltration rate of 0.005 m/s. Another numerical study by Zhang C. et al. [30] shows the influence of thickness on energy output. As per this specific study, the optimal thicknesses of the porous material component are 40 mm and 50 mm, achieving minimum annual overall energy consumption of 0.51 kWh/m² and 0.48 kWh/m², respectively [30].

Figure 6. Schematic heat and airflow paths through an air-permeable porous layer in an exhaust air insulation wall.

These integrated methods have several advantages over traditional air-to-air heat exchangers. They require no extra ductwork and allow localized heat recovery in each conditioned zone [6,59].

Soudian and Berardi [42] used MICRO-V façades and achieved an 81% heat-recovery efficiency. The study used a bidirectional design to enable efficient counterflow heat exchange between fresh and exhaust streams. Similarly, J. Xu et al. [60] and Yan et al. [54] developed a hybrid PV/T wall with dual air channels and PCM. The evaluated configuration achieved reported thermal efficiencies of 55–57% and maintained indoor temperature within approximately 0.5–1.2 °C of the setpoint under the tested conditions. Despite these strong results, designing an efficient EAIW requires careful trade-offs. For example, pressure losses within the porous layer increase the fan’s power demand, and in extremely cold climates, condensation risks need close attention.

4.2.3. Heat Pump Integration

Integrating air-source heat pumps with building-envelope cavities is an emerging area of research in energy efficiency. By using cavity air as the heat source/sink, ASHP performance can be improved. Cavity design, climate conditions, and control strategies are key factors influencing a heat pump’s COP [61]. Depending on the weather conditions, the cavity acts as a moisture-control and thermal buffer (potentially warmer source air in sunny winter conditions) [62].

Reported COP improvements were strongly dependent on the system and averaging period [63]. For example, Hailu et al. [26] reported a simulated heating COP increase from 4.2 to 5.3, equivalent to 26.2%, under the investigated cold-climate operating conditions relative to the specified reference configuration. Choi et al. [43] reported a 3.3% average and 8.6% maximum increase during selected experimental operating periods. Kamel et al. [11] reported an annual simulated heating COP increase from 2.74 to 3.35, approximately 22%, relative to a conventional ASHP configuration. The study also shows that preheated air from BIPV/T collectors reduced electricity demand for space heating by 20% (Figure 7).

Figure 7. Scheme of the integration of BIPV/T–ASHP and thermal energy storage systems [11].

Recent research has explored different configurations in which the cavity serves as a thermal-storage medium [64]. For example, Dannemand et al. [65] reported COP values of 3.8–4.2 for the investigated PVT-assisted heat pump configuration, compared with the traditional one. The corresponding percentage difference was 15–20%.

4.3. Materials and Integration

4.3.1. Integration of Photovoltaic Modules

Building-integrated photovoltaic/thermal systems are an important development in multifunctional building-envelope design [66,67]. The system can generate electricity and heat simultaneously by mounting PV modules directly in the building envelope (Figure 8). Airflow removes heat from the rear surface of the PV modules, thereby reducing cell temperature. The warm air can then be used for space heating, acting as a preheated source air stream for heat pumps [68,69,70].

Figure 8. Integration of BIPV/T with a multispeed heat pump (MSHP) [23].

BIPV/T systems can be opaque/semitransparent, solid-backed with PV cells embedded in glass [71]. Because opaque systems absorb all solar radiation, they achieve higher thermal efficiency, unlike semitransparent systems, which sacrifice some thermal performance for daylighting gains and a better architectural look [23]. The reviewed configurations reported thermal efficiencies of approximately 30–40% and electrical efficiencies of 10–15%.

The preferred cavity depth (50 mm to 150 mm) depends on airflow mode, geometry, climate, pressure drop, and design objective. This balance involves effective heat transfer and structural requirements. Meanwhile, air velocities between 0.5 m/s and 1.5 m/s have been found to provide adequate cooling of PV modules [53,72,73]. Since they disturb the boundary layer and promote turbulence in the flow, internal features (baffles/turbulators) can further improve heat transfer by 20–30%. According to Rounis et al. [53], multiple-inlet BIPV/T designs can achieve higher electrical (1%) and thermal (14–25%) efficiencies than single-inlet systems, as shown in [53].

4.3.2. Solar/Thermal Systems

Building façades with solar-air collectors are important for space heating. Their simplicity is especially suitable for cold climates [74]. Specifically, unglazed transpired collectors (UTCs) are a simple, cost-effective method in which cladding absorbs solar radiation and preheats air drawn through the perforations [75,76]. Choi et al. [43] examined air-type PV/T collectors with outlet temperatures (5–10 °C). They achieved thermal efficiencies of 30%, higher than conventional designs. This disrupts boundary layers and increases turbulence, enhancing convective heat transfer [43,77].

Despite their greater complexity, hybrid (solar/thermal and waste heat recovery) systems can achieve thermal efficiencies of 50–60% [78]. However, freeze protection, condensation control, and leakage remain concerns, particularly in cold climates.

4.3.3. Phase Change Materials (PCMs)

PCMs can absorb and release latent heat during their phase change. This process can moderate cavity temperatures, delay heat transfer, and shift thermal loads. As a result, it increases the latent thermal-storage capacity of building-envelope systems. In an integrated ventilated cavity, PCM charging and discharging occur during the day and night, respectively. PCMs used for building applications are often selected with transition ranges near expected operating temperatures, frequently between 20 and 30 °C, with a storage thickness of 10–20 mm. Researchers use various techniques to prevent leaks and ensure durability. Typically, paraffin waxes, salt hydrates, and bio-based materials are macro-encapsulated in panels/tubes for this purpose [79].

Research by de Gracia et al. [44] showed that PCM-integrated ventilated façades can lower the heating demand by 30–40%. Other recent studies have evaluated multitemperature PCMs and strategic placements. For example, Ke et al. [80] found that internal placement is more suitable for cold climates, whereas exterior placement is better in warm seasons. The liquid fraction, which represents the proportion of melted PCM, is another critical issue. As stated in Li et al. [81], 70–90% of PCM Trombe walls melted on sunny winter days. The PCM required approximately 4–6 h after sunset to solidify.

PCMs store excess solar heat during the day and release it at night. In warm-season operation, night ventilation can regenerate the PCM by promoting solidification [44]. Figure 9 shows the operational modes of ventilated façades with PCM.

Figure 9. Operational modes of ventilated façade: (a) PCM Solidification Period, (b) PCM Melting Period, (c) Overheating Prevention Period, (d) Nighttime Ventilation.

5. Modeling and Performance Evaluation Techniques

There is no simulation platform that resolves airflow, heat transfer, moisture, thermal storage, PV generation, HVAC operation, and controls with equal detail and computational efficiency. Model selection should therefore be based on research questions rather than an assumed hierarchy of accuracy [82,83].

5.1. Computational Fluid Dynamics

Computational fluid dynamics (CFD) has become an essential tool for analyzing complex airflow and heat-transfer behavior within building cavities. It can model variable airflow, including laminar and turbulent flows. In particular, the k–ω-based and transition shear stress transport (SST) models may improve the ability to predict flow regimes from laminar to fully turbulent flow [84]. Studies by Ioannidis et al. [21] used CFD to develop Nusselt number correlations for BIPV/T cavities under both forced and natural convection. The results showed that existing correlations for smooth ducts underpredicted heat transfer by 20–50%. This is because of the PV module geometry and flow disturbances. The CFD-derived correlations reportedly produced errors below 10% over Reynolds numbers of 10,000–26,000 for the evaluated geometry and boundary conditions.

Additionally, CFD can optimize cavity geometry for uniform flow distribution. Specifically, Rounis et al. [53] studied multiple inlet BIPV/T systems and showed that inlet spacing and size influence flow distribution. In a well-optimized design, the flow uniformity is within 5% across the cavity, compared to 30% in a conventional design. Other studies have integrated CFD with building energy simulation to understand the interaction between cavity airflow and full-building thermal performance. This coupled approach enables accurate prediction of energy savings and thermal comfort, accounting for the dynamic behavior of cavity systems [85].

5.2. Whole-Building Energy Simulation

Whole-building energy simulation (BES) tools can represent cavity systems in annual building energy assessments. Among those, TRNSYS and EnergyPlus are the most widely used platforms. They provide component models for ventilated façades, BIPV/T systems, and PCM integrations. For instance, TRNSYS Type 567 and Type 568 model glazed and unglazed BIPV/T systems, respectively. These help to estimate cavity performance under realistic climate conditions and operational schedules.

EnergyPlus includes a wide range of cavity models from simple resistance–capacitance networks to detailed airflow networks. The Airflow Network model simulates natural and forced ventilation in cavities. However, these models require careful selection of input parameters, specifically convective heat-transfer coefficients. Vuong et al. [23] modified EnergyPlus to include improved BIPV/T models with variable emissivity and multi-inlet configurations, enabling parametric analysis of these design variables.

Validation studies have shown that whole-building simulations can achieve reasonable accuracy when calibrated with experimental data. For example, R. S. Kamel and Fung [86] verified that the TRNSYS model of a BIPV/T–ASHP system reported normalized error measures of approximately 5–10% for selected outputs. These are promising performance indicators when compared with field measurements. Calibration is particularly important for uncertain parameters (surface emissivities and thermal resistances). The key differences among simulation software include the ability to simulate transient heat transfer, moisture transport, and multiphysics coupling. Additionally, they differ in their support for PV/T, PCM, HP modeling, and Python/API integration. Rather than using a single software package, coupling multiple packages can yield good accuracy. However, these approaches are computationally expensive and complex. The actual runtimes depend on model detail and hardware (Table 3).

Table 3. Modeling capabilities and validation requirements.

Approach	Appropriate Application	Main Represented Processes	Computational-Cost Drivers	Principal Limitation	Minimum Validation Evidence	Ref.
EnergyPlus	Annual energy, HVAC, schedules, controls, daylighting	Whole-building heat balance, HVAC, airflow network, selected moisture models	Zones, surfaces, HVAC complexity, time step, airflow network, outputs	Local cavity flow and turbulence are simplified	Temperature, airflow, heat flux, and annual energy	[87,88,89]
EnergyPlus plus WUFI	EnergyPlus detailed component moisture	Building energy and one-dimensional heat-moisture transfer	Number of component cases, time step, coupling frequency	Often sequential; limited multidimensional airflow	Energy plus temperature, RH, moisture content, and drying	[87,89,90]
TRNSYS with TRNFLOW	Solar-assisted systems, storage, ASHP, multizone airflow	Dynamic components, HVAC, storage, COMIS airflow	Component count, time step, iterative coupling, controls	Network airflow rather than local flow field	Airflow, temperatures, useful heat, electricity, COP	[91]
CFD	Local airflow, joints, geometry, PV cooling, condensation	Velocity, pressure, temperature, turbulence, radiation	Mesh, dimensionality, time step, turbulence, radiation	High setup and computational cost	Mesh independence plus local velocity, pressure, temperature, and heat flux	[45,89]
Hygrothermal modeling	Condensation, drying, moisture storage, freeze–thaw	Coupled heat and moisture transport	Layers, time step, climate, material data	May not represent pressure-driven multidimensional flow	Temperature, RH, moisture content, condensation	[90]
Modelica/FMI co-simulation	ASHP, controls, storage, digital twins	Dynamic systems and control interaction	Solver stiffness, communication step, exchanged variables	Synchronization and conservation errors	Component validation, interface checks, whole-system operation	[92,93]
Reduced order/surrogate model	Optimization and real-time control	Dominant states and simplified dynamics	Training data, states, scenarios	Limited extrapolation	Independent seasonal and fault-condition testing	[94]

5.3. Field Studies and Experimental Validation

Field studies are essential for validating simulation models and demonstrating the real-world performance of cavity systems. Bock M. [4] reported on the Building Active Steel Skin Envelope (BASSE) project, which installed BIPV/T systems in a temperate climate residential building. This installation consists of 35 panels and generates 3321.14 kWh/year, which is 30.4% efficiency, and the heat pump COP is 4.1–4.6. The system reduced building heating loads by 34.3% in cold climates. de Gracia et al. [20] investigated a PCM-integrated ventilated façade, which reduced electrical energy consumption for space heating by 19–26% under severe winter conditions. The PCM provided thermal storage, which optimized the performance by eliminating peak heating demand (2–4 h).

Long-term monitoring studies provide insights into system durability and the effectiveness of ventilated-façade control. The tested MICRO-V façade achieved a reported heat-recovery effectiveness of 81% under the evaluated operating conditions [42]. However, this effectiveness should not be interpreted as annual HVAC energy savings or evidence of long-term PCM performance. Additionally, solar radiation also influenced cavity temperatures more than ambient conditions.

5.4. Climate-Specific Performance Strategies

Cold-climate evidence should not be generalized to hot–humid/hot–arid operation. For example, solar-warmed cavity air can support heating-mode ASHP operation in cold climates but may reduce cooling-system performance if supplied to a condenser in hot conditions. PCM performance also depends on whether local day–night conditions permit repeated melting and solidification. Natural ventilation must also be evaluated psychrometrically in humid climates because thermally enhanced air may still introduce excessive moisture.

A Canadian modeling study reported a 6.5% reduction in space-heating electricity for one BIPV/T-ASHP configuration, but this is a cold-climate modeled outcome rather than a transferable universal result [23]. Table 4 shows strategies, risks, and control techniques for cavities under different climate conditions.

Table 4. Strategies, risks, and control techniques for cavities under different climate conditions.

Climate	Main Objectives	Suitable Strategies	Main Risks	Required Controls	Ref.
Cold/severe cold	Heat retention, solar preheating, drying, ASHP support	Sealed buffer, controlled solar collection, recirculation, BIPV/T-ASHP	Night heat loss, condensation, frost, defrost, weak solar coincidence	Temperature difference, solar irradiance, humidity, heating demand, frost protection	[95]
Temperature/mixed	Seasonal switching, useful daylight, night ventilation	Multimode cavities, adaptive shading, PCM, bypass	Rapid switching, transitional overheating, unstable natural flow	Deadbands, humidity, demand, forecasts, PCM state	[23,95]
Hot–humid	Solar rejection, PV cooling, drainage, latent-load control	Exterior-to-exterior ventilation, shading, controlled exhaust	Condensation, mold, humid-air entry, incomplete PCM regeneration	Dew point, humidity ratio, rainfall, surface temperature	[95]
Hot–arid	Solar rejection, night cooling, PV cooling, PCM regeneration	Daytime exhaust, night flushing, shading, storage	Dust, ultraviolet degradation, extreme heat, reduced night cooling during heat waves	Irradiance, night temperature, storage state, dust/fouling condition	[42]

5.5. Regulatory Compliance, Codes, and Approval Requirements

Integrated cavity systems may simultaneously function as exterior walls. They may therefore be subject to building, fire, structural, energy, moisture, acoustic, product-certification, inspection, and maintenance requirements. Therefore, they must fulfill the regulatory domains (Table 5).

Table 5. Regulatory domains and their approval evidence.

Regulatory Domain	Main Cavity-Related Requirement	Typical Approval Evidence	Ref.
Building/planning	Occupancy, height, appearance, heritage, projections	Permit drawings, planning approval, code matrix	[96,97]
Fire safety	Compartmentation, cavity barriers, combustibility, emergency shutdown	Assembly tests, fire strategy, fire-stop details	[96,98]
Structural/façade	Wind, seismic, fatigue, falling components	Structural calculations, tests, field review	[96,99]
Energy/envelope	OTTV, RTTV, effective U-value, airtightness, whole-building performance	Compliance calculations and approved energy model	[97,100]
Moisture/water	Drainage, drying, air barrier, vapor control, condensation	Hygrothermal analysis, water and air testing	[90,96]
Acoustics	Exterior noise, flanking, fan and vibration noise	Laboratory or field acoustic testing	[101]
Electrical/PV	Wiring, grounding, isolation, connectors, interconnection	Product certification, electrical permit, inspection	[96,102]
Mechanical/HVAC	Ventilation, filtration, fan power, dampers, condensate	Mechanical calculations, balancing, commissioning	[96,97]
Certification	Component and complete-system compliance	Listings, evaluation reports, technical documentation	[96,97]
Inspection/testing	Verification of concealed and functional work	Mock-up, field tests, professional assurance	[103]
Operations/maintenance	Access, cleaning, replacement, emergency procedures	Operations and maintenance plan, inspection schedule, training	[103,104]

Building-integrated cavities may not only meet mandatory envelope energy criteria. They should also ensure predicted energy savings or COP improvements. Overall thermal transfer value (OTTV), residential thermal transfer value (RTTV), effective U-factor, and building performance are some of these criteria. The OTTV summary sheet incorporated the U-value, glazing area and type, shading coefficient, and absorptivity. But OTTV cannot directly quantify HVAC electrical consumption, PV electrical production, fan electrical consumption, control effectiveness, and heat pump COP. Conversely, lower simulated annual energy does not automatically establish OTTV compliance. Different countries have different jurisdictions and approval routes. For example, Canada and the United States of America use the National Energy Code of Canada for Buildings (NECB) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) or the International Energy Conservation Code (IECC), as shown in Table 6.

Table 6. Country-based jurisdictions and their approval routes.

Jurisdiction	Framework	Envelope Metric/Pathway	Cavity Parameters	Approval Route	Ref.
Hong Kong	OTTV Code and Buildings Department guidance	OTTV for applicable envelopes	U-values, glazing, shading, absorptivity, density, PV coverage, cavity treatment	OTTV calculation and summary sheet submission	[105,106]
Hong Kong, residential	Residential energy guidance	RTTV, glazing visible-light transmittance (VLT) and reflectance, natural ventilation	Glazing ratio, shading, façade reflectance, openings	Applicable residential submission	[107]
Hong Kong, building services	Building Energy Code (BEC) 2024	Building-services efficiency, control, metering, HVAC requirements	Fans, pumps, controls, heat pumps, cavity-connected systems	Required declarations, forms, and compliance process	[108]
Canada	NECB as locally adopted	Effective transmittance, airtightness, tiers, performance paths	Insulation, thermal bridges, rails, fire stops, airflow, controls	Provincial, territorial, or local compliance	[100,109]
United States	Locally adopted IECC or ASHRAE 90.1	Prescriptive envelope, trade-off, energy cost budget, baseline model	U-factor, solar gain, air leakage, shading, mechanical energy	COMcheck or approved performance documentation	[110,111]
European jurisdictions	National implementation of energy-performance legislation	National U-values, primary energy, solar gain, whole-building methods	Thermal bridges, ventilation, glazing, PV, controls	National approval and conformity process	[112,113]

5.6. Code Compliance and Regulation

Because integrated cavities may be regulated simultaneously as façade, fire-safety, structural, energy, moisture-control, acoustic, electrical, and mechanical systems, they must pass through research evidence to approved pathways. The relevant authority having jurisdiction must also accept this evidence through the procedure it establishes (Table 7).

Table 7. From research evidence to approved and operational cavity systems.

Stage	Principal Question	Typical Evidence	Common Failure in Existing Research	Required Outcome	Ref.
Research performance	Does the concept work under defined conditions?	CFD, building energy simulation (BES), hygrothermal modeling, laboratory or test-cell data	Limited climate, geometry, duration, or measured variables	Validated performance within a stated domain	[45,89,114]
Design optimization	Which configuration performs best?	Parametric and multiobjective analysis	Optimization omits fire, moisture, access, cost, or auxiliary energy	Preferred solution subject to practical constraints	[7,115]
Code compliance	Does the design satisfy legally adopted requirements?	Code analysis, mandatory calculations, alternative-solution report	Research metric treated as equivalent to compliance metric	Documented jurisdiction-specific compliance	[8,99,116]
Certification and testing	Are products and the complete assembly acceptable?	Listings, certifications, accredited components and assembly tests	Reliance on component certificates without assembly evidence	Traceable evidence for the proposed configuration	[116,117,118]
Authority approval	Will the authority accept the design and evidence?	Permit submission, professional assurance, consultation and peer review	Approval assumed because a numerical model was validated	Formal acceptance or permit authorization	[8,116]
Construction	Was the approved configuration installed correctly?	Shop drawings, field reviews, photographs and test reports	Substitutions and concealed defects	Verified as-built conformity	[117,119]
Commissioning	Does the system operate safely in all intended modes?	Functional tests, balancing, sensor checks, alarm and override testing	Energy mode tested without fault or fire modes	Accepted commissioning report	[103,119,120]
Operation	Does performance remain safe and durable?	Monitoring, inspections, maintenance and recalibration	No access, maintenance, fault-detection, or replacement plan	Ongoing performance and compliance plan	[120,121,122]

5.7. Integrated Evaluation Matrix

The full life of an integrated cavity cover is evaluated, from installation through operation, maintenance, and replacement. Aside from energy efficiency and comfort considerations, moisture safety, auxiliary power, cost, and commissioning should also be considered. Accessibility of clean and replacement components, and cost and carbon lifecycle evaluation, should also be taken into consideration [123]. As shown in Table 8, the system is only viable if its benefits remain greater than its costs and environmental impacts.

Table 8. Whole-life performance framework for integrated cavity systems.

Evaluation Domain	Recommended Indicators	Required Evidence	Principal Risk If Omitted	Ref.
Operational energy	Annual heating and cooling energy, fan and pump electricity, PV output, recovered useful heat, auxiliary consumption	Calibrated simulation or monitored annual data	Gross savings may conceal auxiliary-energy penalties	[120,121]
Initial cost	Design, materials, equipment, testing, installation, commissioning, approval	Contractor estimates, supplier quotations, project cost records	High capital cost may make the system economically impractical	[124,125]
Installation complexity	Labor hours, trades involved, installation sequence, tolerances, façade access	Construction planning, mock-up, field records	Delays, errors, damaged membranes, or incomplete fire stopping	[125]
Maintenance access	Access time, removable area, safe isolation, cleaning frequency	Maintenance plan and physical access review	Components cannot be inspected, cleaned, or replaced	[125,126]
Component replacement	Service life and replacement interval for fans, dampers, sensors, filters, PCM, PV, and controls	Manufacturer data and field evidence	Repeated cost, carbon, and downtime remain unaccounted	[126]
Reliability	Failure frequency, availability, degraded performance, fail-safe state	Failure-mode analysis and fault testing	Energy loss, condensation, overheating, or unsafe operation	[103]
Control robustness	Sensor accuracy, forecast error, switching frequency, communication loss, manual override	Functional testing and monitored control data	Simulated savings are not maintained in operation	[103,121]
Moisture durability	Condensation hours, moisture content, drainage, drying, mold and corrosion risk	Hygrothermal analysis and field measurements	Hidden deterioration and indoor-air-quality problems	[127,128]
Fire-safety maintenance	Barrier continuity, damper operation, penetration control, inspection interval	Fire strategy and inspection records	Concealed fire and smoke pathways	[129]
Acoustic durability	Sound transmission, fan noise, vibration, silencer condition	Laboratory or field acoustic tests	Non-compliance or occupant disturbance	[130]
Service life	Performance retention and degradation rate	Aging tests and long-term monitoring	Initial performance is assumed to remain constant	[126]
Embodied carbon	Product, transport, installation, replacement, maintenance, and end-of-life impacts	Life-cycle inventory and environmental declarations	Operational savings may not offset added material impacts	[131,132]
Life-cycle cost	Net present cost, maintenance, replacement, failure, energy, and residual value	Cost model with sensitivity analysis	Simple payback misrepresents long-term value	[124,133]
End-of-life	Reuse, recycling, disassembly, hazardous-material handling, disposal	Disassembly and waste-management plan	Difficult separation, waste, and lost material value	[132]

6. Contradictory Findings and Their Sources

Most apparent contradictions are conditional or methodological rather than disagreements in fundamental physics. A finding should be classified as a genuine trade-off only when the same design variable improves one objective while reducing another under comparable conditions.

6.1. Energy Consumption

One of the most debated themes is the effect of ventilated cavities and double-skin façades on the overall building energy use. A double-skin façade cavity can serve as a thermal buffer, control moisture, minimize heat transfer, and provide passive ventilation. Multiclimate assessments by Aksamija [134] show that double-skin façades perform better as thermal buffers compared to traditional curtain wall systems. However, some other reviews are more cautious regarding the double-skin façade, as they claim that it is very sensitive to the design process, procurement, and control systems. For instance, poor coordination of glass, shading, cavity depth, ventilation slots, and controls might cause overheating, an uncertain payback period, and a gap between simulations and real performance. Additionally, a cavity that performs well as a heat-retaining buffer in cold and temperate climates may behave as a solar heat trap in hot climates unless shading and exhaust ventilation are carefully managed [49,134,135].

6.2. Natural Ventilation

Another controversial issue arises regarding the effectiveness of natural ventilation in building cavities. Some studies emphasize buoyancy-driven ventilation, where solar radiation heats the cavity air, increasing stack pressure and improving heat extraction. Other studies state that wind pressure can affect the flow pattern, particularly at extremely high exterior wind speeds. As a result, if the model assumes buoyancy is the dominant force, it may overestimate solar-cavity performance. This provides an explanation for why certain experiments and CFD studies demonstrate that there are successful results for ventilation flow, whereas other studies demonstrate poor or reversed airflows [136,137]. Such results may not contradict the fundamental laws of physics but rather the boundary conditions used.

6.3. Cavity Geometry

Suggestions also vary regarding cavity width and geometric configuration. Some studies report that narrower cavities can enhance convective heat-transfer coefficients and heat recovery. This enhancement makes them useful in heating-dominated applications. Alternatively, several studies show that wider cavities may increase air movement, reduce overheating, and improve façade ventilation. Related disagreements appear in BIPV/T systems also. If the air gap is within the optimum range, increasing the air gap behind photovoltaic panels can lower module temperature and improve electrical efficiency. Research on solar chimneys also showed a similar pattern. Larger dimensions may strengthen airflow, but excessive air gaps can reduce air velocity and weaken the stack effect [138,139,140].

6.4. Phase Change Materials

PCM performance studies in integrated cavities present mixed conclusions. The PCM-assisted Trombe wall and ventilated cavity wall systems tend to avoid overheating, delay heat transfer, transfer cooling loads, and keep room temperature stable. As stated by Liu et al. [141], the integration of PCM in a ventilated Trombe wall helps to reduce the annual cooling energy use by 20.8% in Changsha, China. But some research works have also reported that the efficacy of PCM may be constrained when it does not completely solidify at night. As a result, PCM integration may be highly effective in climates with effective day–night temperature swings, but less successful in climates where the PCM remains partially melted for prolonged periods [142,143]. The disagreement therefore lies not in whether PCM works but in whether the local climate and control strategy allow it to complete its thermal cycle.

6.5. Solar Chimneys

The solar chimney is often referred to as a promising passive ventilation technique. However, there is no unified opinion on how effective the technology is. Some of the studies reveal that the solar chimney significantly boosts natural ventilation owing to transformation of the solar energy into buoyant flows. However, review studies also show contradictions in experimental findings. Solar chimney performance depends strongly on geometry, absorber material, inclination angle, cavity depth, thermal mass, PCM integration, and outdoor conditions. Liu et al. [140] found that a combined solar chimney system combining an inclined-roof chimney with a Trombe-wall configuration could perform better. At the same time, their results showed that different modes suit different ambient-temperature ranges. This indicates that an optimized design for summer ventilation may not be ideal for winter heating or air preheating [144]. Thus, contradictions in solar chimney studies often arise because researchers compare systems designed for different seasonal objectives and performance targets.

6.6. Building-Integrated Photovoltaic/Thermal Cavities

BIPV/T cavity systems introduce another crucial trade-off. Increasing ventilation to the back side of the PV panels reduces panel temperature and improves electricity production efficiency. Rising cell temperatures reduce PV performance, causing a direct drop in electrical output. Therefore, studies on BIPV/T systems often highlight the value of air ventilation, water circulation, or PCM integration for controlling PV temperature. However, the extracted heat can be either advantageous or problematic depending on how the system is integrated into the building. Heat recovery can be utilized for space heating/air preheating in winter but not in summer. The same heated cavity in summer will cause increased energy usage, and the temperature can go beyond the threshold if it is not properly exhausted or insulated from the indoor environment. Thus, research that examines the electrical efficiency of PV may provide positive results, while whole-building studies may show fewer or even conditional benefits due to the thermal effect and auxiliary energy consumption. Ventilating the cavity can lower module temperature and improve electrical efficiency because PV performance generally decreases as cell temperature rises [67,138,139,145].

6.7. Modeling Uncertainty

A final major source of disagreement comes from modeling methods. CFD and other numerical simulations are valuable because they can capture detailed airflow, temperature, radiation, and convective heat transfer processes inside cavities. However, simulation results can vary significantly depending on the model’s input. They depend on model dimensions (2- or 3-dimensional), the turbulence model, radiation treatment, mesh generation, and boundary conditions. According to Melgaard et al. [137], CFD studies of double-skin façades still lack consistent validation and benchmarking practices. In several cases, models are proven only against temperature measurements. As a result, two studies may claim validation while producing different airflow estimates [135] because they use different validation variables or assumptions. Therefore, the contradiction is methodological rather than purely physical and is not directly comparable across studies. Table 9 summarizes the causes of disagreements in integrated cavity studies.

Table 9. Causes of disagreement in integrated cavity studies.

Issue	Main Reported Disagreement	Principal Explanatory Variables	Classification	Ref.
Overall energy performance	Strong savings versus limited or negative savings	Climate, baseline wall, seasonal operation, fan energy, control	Conditional and system-boundary difference	[46,95]
Natural ventilation	Stable upward flow versus weak or reversed flow	Wind pressure, buoyancy, joint geometry, external exposure	Boundary-condition and modelling difference	[46,95]
Cavity depth	Narrow cavity preferred versus wide cavity preferred	Pressure drops, local velocity, airflow volume, objective	Genuine multiobjective trade-off	[23,95]
PCM performance	Strong peak reduction versus limited benefit	Transition temperature, thickness, placement, regeneration	Climate and control dependency	[46,146]
BIPV/T ventilation	Improved PV output versus reduced thermal usefulness	Airflow, PV temperature, outlet temperature, fan power	Genuine electrical–thermal trade-off	[31,91]
ASHP integration	Large versus modest COP improvement	Source temperature, averaging period, defrost, auxiliary energy	Operating condition and boundary difference	[146,147]
Moisture performance	Ventilation improves drying versus increasing condensation	Vapor pressure, surface temperature, rainfall, airflow direction	Genuine climate-dependent reversal	[85,90]
CFD results	Different airflow and heat-transfer predictions	Turbulence model, radiation, mesh, 2D/3D, inlet conditions	Methodological difference	[45,148]
Whole-building simulation	Different annual savings	Weather file, occupancy, baseline, simplified cavity model, ideal controls	System-boundary and model-resolution difference	[87,88,89]
Control strategy	Strong modeled benefits versus uncertain field operation	Forecast error, sensor drift, actuator failure, switching logic	Operational and reliability difference	[93,149]

7. Challenges, Research Gaps, and Future Directions

7.1. Simulation and Validation Constraints

In integrated cavities, a single modeling software cannot comprehensively describe all the hygrothermal properties and control processes. For instance, CFD can model the detailed airflow and heat-transfer dynamics. However, it cannot capture whole-building hygrothermal behavior. Coupling such computational tools improves process coverage but creates challenges in boundary exchange, time-step synchronization, convergence, and computational cost. Moreover, many models are validated only against temperature [114]. Future studies should use common benchmark cases and validate temperature, airflow, pressure, heat flux, moisture, PV output, auxiliary energy, and control operation against independent measurements.

7.2. Climate-Specific Evidence

Existing evidence is concentrated on cold and temperate climates, with fewer multiseason studies in hot–humid and hot–arid regions. This limits the transferability of reported performance. A cavity that supports solar preheating and ASHP operation in a cold climate may increase cooling demand in a hot climate, while ventilation that promotes drying in one location may introduce moisture in another. PCM performance also depends on whether local conditions permit complete melting and solidification [95,150]. Future research should compare systems across representative climates using consistent buildings, baselines, controls, metrics, and system boundaries.

7.3. Design and Control Complexity

Most studies focused on a single outcome, for example, PV electric generation, thermal efficiency, COP, or peak-load reduction. However, they lack a common framework for cavity design. For example, airflow may improve PV cooling but reduce useful outlet-air temperature. Also, larger PV coverage may reduce daylight, and fire/acoustic treatments may restrict airflow [115,151]. Therefore, future assessments recommend integrating energy, moisture, daylighting, comfort, safety, fan power, cost, and carbon through multiobjective optimization.

7.4. Maintenance and Lifecycle Performance

In most studies, maintenance, reliability, and replacements are rarely included in performance assessments. However, components like fans, dampers, filters, sensors, controllers, PV connections, and PCM containers should be incorporated because they have shorter service lives than the façade itself [151,152]. Therefore, future studies should assess not only the maintenance access but also cavity cleaning, fire-stop inspection, component replacement, system downtime, performance degradation, embodied carbon, lifecycle cost, and end-of-life recovery.

7.5. Research Performance and Practical Implementation

Simulations or laboratory performances do not prove that a cavity system is fit for building use. It needs full-scale research to explain how the expected performance will change when the cavity system has been incorporated with structural support, drainage, fire protection, acoustics, electricity, and maintenance. The major practical gap is the absence of a validated pathway connecting experimental performance to reliable occupied-building operation. Therefore, researchers should follow pathways from research performance to practical implementation pathways, such as performance research, design optimization, certification, approval, construction, and commissioning.

8. Limitations of the Review

8.1. Database Coverage

The search was limited to Web of Science and ScienceDirect. Although these sources cover a substantial portion of the relevant building energy and envelope literature, studies indexed exclusively in other databases or reported in non-indexed conference proceedings, theses, and technical reports may have been missed.

8.2. Language and Publication-Type Restrictions

The restriction to English-language and selected peer-reviewed publication types may have excluded relevant regional studies and reports. Publication bias is also possible because systems producing favorable results may be more likely to be published than systems showing neutral or adverse performance.

8.3. Heterogeneity of the Included Evidence

The included studies have substantial heterogeneity in climate, building type, cavity geometry, orientation, flow cavity, control strategy, and reference system. Experimental studies ranged from short controlled tests to multiweek campaigns. Numerical studies had steady-state, seasonal, or annual simulations. Such heterogeneity did not allow for statistical combinations and direct comparisons of the results.

9. Conclusion and Recommendations

9.1. Conclusions

This review has explored the recent advancements in integrated building-envelope cavities. It emphasizes their ability to create thermal buffers, solar energy harvesting, PV cooling, moisture control, thermal energy storage, ventilation, and HVAC operations. Yet, no literature provides universally applicable values for energy savings, COP enhancement, solar energy harvesting, and peak-load reductions. Based on these findings, the review concludes that:

• Cavity ventilation is useful only where there is heat recovery and rejection, photovoltaic cooling/drying without excessive heat loss, moisture transfer, or use of auxiliary power.
• Cavity geometry cannot be assessed using a single rule. The best cavity width depends on the design goal: heat recovery, passive cooling, PV temperature reduction, indoor comfort, or ventilation enhancement.
• PCM can shift and attenuate loads when its transition range matches cavity conditions and complete regeneration occurs.
• BIPV/T systems involve a trade-off among PV cooling, outlet-air temperature, thermal recovery, fan energy, shading, and daylighting.
• Cavity-assisted ASHPs benefit when cavity air provides a useful source-temperature advantage after fan power, defrost, auxiliary heating, and control are included.
• Simulation accuracy is highly dependent on the model, variables, climate, and operating mode.
• Finally, the applications of predictive control and digital twins have significant importance for physical-model validation, fault conditions, and fail-safe operation.

9.2. Recommendations

• Future researchers are recommended to prioritize real-world safety by carefully evaluating heat, air, and moisture dynamics. Additionally, it needs careful integration of fire and electrical protection and ensuring a default to safe states during unexpected failures.
• To make sure that the design process succeeds from the laboratory setting to the construction process, researchers must comply with relevant codes and employ certified materials.
• To get accurate results, future studies should conduct multiseason field monitoring, clearly separate actual experimental results from computer simulations, track background auxiliary energy use, and finally validate metrics against standardized baselines.
• Investigations must look beyond initial operations. It is recommended to evaluate total lifecycle costs and carbon footprints, considering realistic maintenance and failure scenarios and designing components for end-of-life disassembly and recycling.
• Lastly, researchers suggested the creation and use of predictive control systems and digital twins that will stay safe and grounded in physical reality and human involvement.