A Comprehensive Review of Sustainable Thermal and Acoustic Insulation Materials from Various Waste Sources

Abstract

The growing demand for sustainable and energy-efficient construction has driven significant interest in the development of advanced insulation materials that reduce energy usage while minimizing environmental impact. Although conventional insulation materials such as polyurethane, polystyrene, and mineral wools offer excellent thermal and acoustic performance, they are derived from non-renewable sources, have high embodied carbon (EC) (up to 7.3 kg CO₂-eq/kg), and pose end-of-life disposal challenges. Thus, this review critically examines the emergence of insulation materials derived from natural and recycled sources, which align with circular economy principles by minimizing waste, promoting material reuse, and extending product life cycles. Sustainable alternatives such as sheep wool, hemp, flax, and jute not only exhibit competitive thermal conductivity (as low as 0.031–0.046 W/m·K) and very good sound absorption but also offer low EC, biodegradability, and regional availability. Despite some limitations, including variable fire resistance and thickness requirements, these bio-based insulators present a viable path toward greener building solutions. The review highlights that waste-based insulation materials are essential for sustainable construction due to their low EC, renewability, and contribution to waste reduction, making them a necessary alternative even when conventional materials demonstrate superior short-term performance.

1. Introduction

As global energy demands continue to rise and concerns over climate change intensify, due to rapid urbanization, expanding industrial activities, and continuous population growth, the need for high-performance insulation materials has become more critical than ever. According to the IEA, the building sector is considered one of the major consumers of energy, where major consumption accounts for the heating and cooling of the building, particularly in regions subject to extreme climates such as Canada, Russia, India, and the MENA region [1]. Heating and cooling systems in these areas place substantial stress on energy systems and contribute significantly to greenhouse gas emissions [2]. The Global Status Report for Buildings and Construction highlights that the construction industry contributes nearly 39% of global CO₂ emissions and consumes about 36% of the world’s total energy [3]. Insulation has proven to be one of the most effective strategies for enhancing energy efficiency in buildings. By reducing heat transfer through the building walls, thermal insulation lowers the reliance on mechanical heating and cooling systems. Proper insulation reduces the need for excessive energy use, leading to lower utility costs and a reduced environmental footprint [4,5,6]. At the same time, growing attention is being paid to acoustic comfort, particularly in densely populated urban areas. Exposure to high levels of ambient noise has been associated with a variety of health problems, including stress, headache, sleep disturbances, and other problems [7,8]. Consequently, there is growing interest in insulation materials that provide both thermal resistance and sound attenuation. Beyond construction, insulation materials are also widely used in the aerospace, automotive, oil and gas, and marine industries, where thermal management and noise control are critical to safety, efficiency, and comfort [9].

Conventional insulation materials such as fiberglass, mineral wool (MW), Expanded polystyrene (EPS), and polyurethane foam have long been used due to their affordability and thermal performance. For acoustic insulation, dense materials like rock wool, foam panels, etc., are commonly used because of their high performance. Despite being lightweight, hydrophobic, and effective at blocking heat and sound, these materials’ full lifecycle presents serious environmental challenges. These non-renewable materials are difficult to recycle, can take thousands of years to decompose, and often find their way into terrestrial and marine environments, where they absorb toxic pollutants, affecting ecosystems and food chains [10]. At the same time, the disposal of solid waste from agricultural, animal, and industrial processes presents a growing challenge, particularly in developing countries. Sustainable insulation options, including recycled materials and bio-based composites, offer a safer and more environmentally friendly alternative while maintaining high-performance standards [11,12]. As a result, there is a growing demand for sustainable construction materials, which has increased interest in biodegradable options that provide an environmentally friendly alternative to conventional insulation materials.

The increasing demand for sustainable insulation materials presents significant growth opportunities across multiple industries. Natural fiber-based insulation, such as hemp, date palm, flax, jute, and sheep wool, is gaining attention for its biodegradability, renewability, and low environmental impact [11,13]. These materials offer excellent thermal and acoustic properties and have a lower carbon footprint compared to synthetic alternatives [11,14]. Additionally, recycled materials, such as cotton waste, rubber granulates, and repurposed plastics, are being integrated into insulation products, promoting the principles of circular economy and reducing landfill waste [15,16,17]. Furthermore, advanced insulation technologies, including Phase change materials (PCMs), Aerogels (AGRs), and living walls, are paving the way for highly efficient, sustainable thermal management solutions [18]. These waste-derived materials offer several advantages, including reduced production costs, lower environmental impact, and the potential for innovative insulation solutions with comparable or enhanced thermal and acoustic properties.

Understanding the importance of thermal and acoustic insulation and selecting proper insulation material is essential to building design and overall energy conservation. This review aims to provide an overview of both conventional and recent advances in insulation materials, particularly those made from natural and recycled (synthetic) resources. It investigates the benefits and challenges associated with waste-derived materials, aiming to produce high-performance insulation panels that offer reduced environmental impact and long-term sustainability. Unlike previous reviews, this paper focuses on the long-term performance of these sustainable alternatives, specifically addressing their durability and fire resistance to assess their practicality for real-world applications. Additionally, it evaluates manufacturing methods and their influence on material properties, providing comparisons to identify the most efficient and eco-friendly approaches. The main focus of this paper can be summarized as follows:

• This review paper explores sustainable thermal and acoustic insulation materials derived from both natural and recycled waste sources, offering solutions to reduce energy consumption and environmental impact. These materials provide significant advantages, including low density, high thermal resistance, strong sound insulation, and cost-effectiveness.
• The paper provides a thorough analysis of the theoretical foundations, production techniques, and key measurement methods used for assessing the thermal conductivity and sound absorption of waste-based insulation materials. It also investigates factors influencing the durability and long-term performance of these sustainable materials in insulation applications.

2. Insulation Materials

Insulation materials play a crucial role in regulating thermal and acoustic performance in building and industrial applications. These materials can be broadly classified into natural and synthetic types, each offering unique properties and benefits. While natural materials prioritize sustainability, biodegradability, and environmental safety, synthetic materials provide versatility, durability, and high performance, making them essential for modern insulation solutions.

2.1. Natural Materials

Natural insulation materials are derived from sustainable sources, primarily categorized into plant fibers, animal fibers, and mineral fibers, as illustrated in Figure 1. Plant fibers such as hemp, jute, flax, cotton, and coir are composed of cellulose and have a porous structure that provides excellent thermal and acoustic insulation properties [19]. These fibers trap air within their cavities, reducing heat transfer and absorbing sound waves, making them effective for regulating indoor temperature and noise. Animal fibers, such as wool and silk, consist of protein-based structures like keratin, which insulate effectively, regulate moisture, and improve indoor air quality. For instance, wool fibers naturally absorb and release moisture without compromising their insulating performance, which is beneficial for creating a comfortable indoor environment [20]. In addition, mineral fibers, such as rock wool and basalt fibers, offer high thermal resistance and durability, making them suitable for both residential and industrial applications. Natural insulation materials are gaining attention due to their environmental benefits, biodegradability, renewability, and potential for cost-effectiveness, contributing to greener and more sustainable construction practices [21,22,23].

2.2. Recycled (Synthetic) Materials

Recycled (synthetic) materials, including synthetic fibers and recycled waste materials, are widely used as insulation due to their versatility, durability, and superior thermal and acoustic performance. Synthetic waste materials such as polymers, glass, ash, slag, marble, ceramics, and sludge, as shown in Figure 1, are being repurposed for insulation, addressing environmental concerns by reducing landfill waste and promoting material reuse. For example, recycled glass is used to produce fiberglass insulation, known for its excellent thermal resistance and lightweight structure. Similarly, Polystyrene (PS) foam derived from synthetic polymers offers high insulation efficiency due to its low thermal conductivity. Synthetic fibers are further divided into organic and inorganic fibers: organic fibers, such as polyester, nylon, and aramid, contain carbon atoms and are lightweight, strong, and flexible, making them ideal for thermal insulation panels and acoustic control systems. In contrast, inorganic fibers, like boron fibers and silica carbide fibers, are non-carbon-based and known for their durability, fire resistance, and ability to perform under extreme temperatures, making them suitable for industrial insulation applications. While synthetic materials provide exceptional performance, their long-term environmental impact remains a concern, encouraging research into enhancing their recyclability and reducing energy-intensive production processes [24,25,26,27,28,29,30,31].

2.3. Role of Sustainable Insulation Materials in Advancing the Sustainable Development Goals (SDGs)

Sustainable insulation materials are not only essential for improving energy efficiency in buildings but also play a broader role in addressing the global sustainability challenges outlined in the United Nations SDGs. With 193 countries committed to the 2030 Agenda for Sustainable Development, the construction sector holds significant potential for contributing to multiple SDGs. As explained in the introduction, traditional insulation materials, such as EPS, fiberglass, and polyurethane foams, are widely used due to their thermal and acoustic properties. However, these materials are often petroleum-based, non-renewable, and challenging to recycle, leading to substantial environmental and health concerns. In contrast, sustainable insulation materials, including those derived from agricultural waste, recycled content, or bio-based sources, offer a lower environmental footprint and align with the broader sustainability agenda.

Hence, the key SDGs supported by sustainable insulation include the following:

• SDG 3: Good Health and Well-Being—by promoting non-toxic, breathable materials that enhance indoor air quality and thermal comfort;
• SDG 7: Affordable and Clean Energy—through improved thermal efficiency that reduces energy demand for heating and cooling;
• SDG 9: Industry, Innovation, and Infrastructure—by advancing eco-innovative material technologies and sustainable manufacturing practices;
• SDG 11: Sustainable Cities and Communities—by enabling the design of low-carbon, energy-resilient buildings;
• SDG 12: Responsible Consumption and Production—through the use of recycled and renewable resources, reducing waste and raw material dependency;
• SDG 13: Climate Action—by lowering greenhouse gas emissions associated with both operational energy use and material production.

Although sustainable insulation materials offer significant environmental and performance advantages, much of the existing literature mainly focuses on economic considerations, particularly initial material costs. This focus often overlooks critical aspects such as energy efficiency, lifecycle emissions, and overall sustainability performance. To realize the full potential of these materials, it is essential to evaluate their long-term environmental impact, resource efficiency, and contribution to climate mitigation. As energy plays a central role across multiple SDGs, sustainable insulation materials provide a scalable and cost-effective means of reducing the built environment’s carbon footprint while supporting broader global sustainability objectives. In the following sections, a comprehensive and critical review of various natural and synthetic insulation materials is presented, highlighting their thermal, acoustic, and environmental performance.

3. Characterization of Insulation Materials: Thermal and Acoustic Insulation Properties

3.1. Theory of Thermal Insulation

Thermal insulation materials are engineered to reduce heat flow by limiting conduction, convection, and radiation, thus improving energy efficiency and maintaining thermal stability across various applications [2,13]. The rate of heat transfer, as indicated in Equation (1), is primarily determined by a material’s thermal conductivity $(\lambda )$, defined as the steady-state heat flow passing through a 1-meter-thick layer of a homogeneous material per unit area due to a temperature difference of 1 Kelvin across its surfaces. This property, expressed in W/mK, is influenced by factors such as temperature, density, moisture content, and porosity. Standards like EN 12664, EN 12667, EN 12939, and ASTM C518 provide methodologies for measuring thermal conductivity [2,13].The equation for heat transfer is presented as follows:

$$Q={\displaystyle \frac{\lambda }{d}}\left(t_1-t_2\right)$$

(1)

where Q represents the heat transfer rate (W/m²) through the material via thermal conduction, d is the thickness in meters, and t₁ and t₂ are the surface temperatures of the insulation material [32].

Another key factor influencing insulation performance is bulk density, defined as the mass per unit volume, including the air spaces within the material, particularly in multilayer configurations, as shown in Equation (2). It is typically expressed in kg/m³.

$$\mathrm{Bulk}\mathrm{Density}={\displaystyle \frac{\mathrm{Mass}\mathrm{of}\mathrm{the}\mathrm{material}}{\mathrm{Volume}\mathrm{of}\mathrm{material}(\mathrm{including}\mathrm{voids})}}$$

(2)

Lower bulk density generally improves insulating properties by trapping more air, which reduces heat transfer due to air’s poor conductivity. Materials with low bulk densities, such as fiberglass (10–96 kg/m³) and EPS (15–35 kg/m³), are ideal for lightweight insulation applications like attics and walls. In contrast, higher bulk density materials, such as mineral wool (30–200 kg/m³) and Extruded polystyrene (XPS) (28–45 kg/m³), are used in applications requiring greater mechanical strength and durability, like under concrete slabs or in industrial settings [33]. Hence, the total heat transfer conductance across multiple layers of different materials can be expressed as per Equation (3):

$$Q={\displaystyle \frac{t_1-t_2}{\frac{d_1}{{\lambda }_1}+\frac{d_2}{{\lambda }_2}+\frac{d_3}{{\lambda }_3}+etc.}}$$

(3)

where $d_1$, $d_2$, and $d_3$ are the thickness of various insulation layers in meters, and ${\lambda }_1$, ${\lambda }_2$, and ${\lambda }_3$ are their conductivities in W/mK, as illustrated in Figure 2. When dealing with layered materials, each layer has its thermal conductivity and thickness, which affect the overall heat transfer. The concept is useful in designing thermal insulation systems or understanding heat loss through walls, roofs, or multi-layered objects [34].

3.2. Thermal Insulation Measurement Techniques

The primary techniques for thermal analysis and measuring the thermal conductivity of insulation materials can be grouped into steady-state, transient, and differential methods, each with specific use cases [35].

3.2.1. Steady-State Techniques

In the steady-state technique, a constant heat flow is established across a sample until the system reaches thermal equilibrium (the temperature difference across the sample remains constant over time). Once equilibrium is achieved, the steady temperature gradient enables the precise calculation of the material’s thermal conductivity using Fourier’s Law of heat conduction. Steady-state measurements often use the guarded hot plate (GHP), heat flow meter (HFM), and Heat Box methods [35]. The steady-state principle was used to measure wood and bark residues’ thermal conductivity [36].

3.2.2. Transient Techniques

Transient techniques involve applying a heat pulse or step change to a material and observing its temperature response over time. They contrast with steady-state methods, where a constant temperature gradient is established across the material, and the heat flux is measured. The key transient techniques are the Transient Plane Source (TPS) (hot disk), Laser Flash Analysis (LFA), and Transient Line Source (TLS) (needle probe) methods [35]. The thermal analysis of agriculture product strawboard Solomit panels and hemp insulation materials was determined using a needle probe [37]. The needle probe analyzes thermal responses to heat flux pulses, allowing for effective measurement outside laboratory conditions.

3.2.3. Differential Scanning Calorimetry (DSC)

DSC is a thermal analysis technique used to study the thermal properties of materials, including their heat capacities, phase transitions, and thermal stability [38]. While DSC is not a direct method for measuring thermal conductivity, it provides valuable information that can be used in indirect thermal conductivity calculation. Two pans are used in a DSC setup; one contains the material sample, and the other is an empty reference pan. Both pans are subjected to the same controlled heating or cooling rate, and the instrument records the heat flow into or out of the sample relative to the reference. When the sample undergoes a thermal event, such as melting, crystallization, or a glass transition, it absorbs or releases heat, creating a difference in the heat flow between the sample and reference pans. This difference is measured as a function of temperature or time [38].

3.3. Theory of Acoustic Insulation

Acoustic insulation, or soundproofing, refers to the process of controlling sound transmission between different environments by reducing the amplitude of sound waves as they pass through materials, reflect off surfaces, or absorb energy by converting it into another form, such as heat [39,40]. The primary objective of acoustic insulation is to enhance sound absorption while minimizing sound propagation. The effectiveness of acoustic insulation is influenced by several factors, including the properties of the materials used, their structural composition, and the type of noise—whether airborne or impact noise [39]. A key parameter in assessing a material’s ability to absorb sound is the SAC, which is defined as the ratio of sound energy absorbed by a material to the total incident sound energy, and it is evaluated in Equations (4) and (5) [12]. The SAC is measured through standardized testing methods such as ISO 354 or ASTM C423 [41,42], with values ranging from 0 (complete reflection) to 1 (total absorption). Materials with higher SAC values, such as porous and fibrous structures, are commonly used in acoustic insulation to reduce reverberation and improve soundproofing performance [12].

$$W_a=W_i-W_r-W_t$$

(4)

$$SAC={\displaystyle \frac{W_a}{W_i}}$$

(5)

where ${\mathrm{W}}_i$, ${\mathrm{W}}_r$, and ${\mathrm{W}}_t$ are the incident, reflected, and transmitted sound power, respectively, and ${\mathrm{W}}_a$ is the absorbed sound power.

In addition, material composition, material thickness, and surface texture also affect the SAC [39]. Different materials have different sound absorption properties. For example, porous materials like foam and fiberglass are good sound absorbers, while dense materials like concrete and metal are poor sound absorbers. On the other hand, thicker materials generally absorb more sound than thinner materials, and rough surfaces tend to absorb more sound than smooth surfaces. Additionally, installation techniques like creating air gaps or using damping materials enhance insulation by preventing sound from traveling through solid paths or vibrating structures [39].

The Noise Reduction Coefficient (NRC) is another metric used to evaluate a material’s sound absorption capabilities. It represents the average sound absorption across a range of frequencies, typically from 250 Hz to 2000 Hz, which covers most human speech and common noise. The NRC is expressed as a value between 0 (perfect reflector) and 1 (perfect absorber) [43].

3.4. Acoustic Insulation Measurement Techniques

The measurement of the sound absorption coefficients can be conducted using various established methods. These methods include the impedance tube, the reverberation room, the reflection (or in situ), and the sound intensity method [44].

3.4.1. Impedance Tube Method

The impedance tube method defined by standard ISO 10534-2 [45] is a commonly used technique for measuring the SAC, especially for smaller samples and in lab settings. In this method, a sound source is placed at one end of a tube, with the material sample at the other end. Microphones are positioned along the tube to measure incident and reflected sound waves, as shown in Figure 3. The frequency range is typically limited, as it depends on the diameter of the tube (usually effective for low-to mid-frequency ranges) [46,47].

3.4.2. Reverberation Room Method

It is also known as the diffuse-field method and is standardized under ISO 354. The reverberation room measures the absorption coefficient in a large room with highly effective surfaces, allowing sound to reflect and diffuse in multiple directions. This setup is ideal for evaluating the sound absorption of large samples and is effective across a broad frequency range, particularly for assessing materials with complex sound-absorbing properties over various frequencies [48].

3.4.3. Reflection (or In Situ) Method

The reflection method is used to measure sound absorption directly on-site, particularly on floors, walls, or ceilings, making it suitable for in situ measurements. Typically, a microphone is positioned in front of the sample surface, and a speaker directs sound waves at it. In some cases, a rotating microphone or loudspeaker array is used to capture reflected sound from multiple angles. It is capable of measuring a broad frequency range depending on the setup and follows the procedure outlined in ISO 13472-1 [49,50].

3.4.4. Sound Intensity Method

It measures the energy of sound absorbed by material by capturing the difference in sound intensity in front of and behind the materials. A sound intensity probe (with dual microphones) is positioned near the sample to measure the sound intensity at different points in front and behind it. This method is suitable for both lab and in situ measurements and is ideal for testing large surfaces or materials where sound pressure-based techniques are difficult to apply [51].

4. Fabrication Techniques of Natural Fiber Composites

Natural fiber composites combine natural fibers, such as flax, hemp, and bamboo, with a polymer matrix like epoxy, polyester, or vinyl ester. The choice of manufacturing technique is critical to ensuring the desired performance, size, and quality of the composite material. Factors such as shape, size, properties (strength, stiffness, and weight), manufacturing costs, production rates, and raw material characteristics all influence the selection process. Common manufacturing techniques for natural fiber composites include hand lay-up, compression molding, extrusion molding, injection molding, spray lay-up, and resin transfer molding (RTM) [19]. Hand lay-up is one of the simplest and most cost-effective methods, where fibers are manually arranged in a mold, and resin is applied to bond the layers. This process is ideal for small-scale production but is time-consuming and labor-intensive, with the potential for inconsistent quality [52,53]. Compression molding uses heat and pressure to form pre-impregnated material in a closed mold. It provides high production rates, excellent dimensional accuracy, and the ability to create complex shapes, making it popular in industries like automotive and aerospace. However, it requires high capital investment, reliance on prepreg materials, and a lengthy curing process [54,55]. Extrusion molding involves forcing heated material through a die to create continuous profiles. It offers high production rates and material uniformity but is typically limited to simpler shapes and higher tooling costs [56].

Injection molding is highly suitable for creating intricate, high-precision parts, offering consistent quality and fast production cycles. It is cost-effective for mass production but requires significant initial investment and is best suited for thermoplastics [57,58]. Spray lay-up is used for producing large, complex shapes and is efficient for large-scale production. It involves spraying a mixture of resin and fibers onto a mold, ideal for applications in the aerospace, automotive, and marine industries. However, it raises environmental concerns due to the release of volatile organic compounds (VOCs) [53,59]. RTM uses a closed mold to inject resin into pre-placed fibers, offering excellent dimensional accuracy and surface quality. RTM is ideal for complex parts but has high setup costs and slower processing times. It is widely used in industries like aerospace, automotive, and wind energy for high-performance applications [60].

5. State of the Art of Sustainable Building Insulation Materials

Researchers have extensively studied the thermal and acoustic insulation properties of sustainable materials, along with agricultural and industrial by-products, to explore their potential for reuse in the construction sector [12]. The objective is to develop sustainable alternatives to conventional insulation materials that offer similar or improved performance while minimizing environmental impact. This section reviews these materials, outlining their key thermal and acoustic characteristics based on the findings of various studies.

5.1. Thermal Insulation Materials

A thermal conductivity value of less than 0.07 W/mK is usually associated with a thermal insulation material. Conventional thermal insulators are usually produced from petrochemicals (mainly PS) or highly processed natural materials such as glass and mineral wool. Recently, there has been more focus on using natural and recycled materials in thermal insulation. These materials are summarized in Figure 1. They span over a wide range of materials, from natural plant or animal sources to other recycled materials such as polymers, glass, and marble.

5.1.1. Natural Fibers

Plant Fibers

Plant fibers, primarily composed of cellulose, are organic materials sourced from various parts of plants, including stems, leaves, seeds, and bark. They can be classified into categories such as fruit, grass, bast, stalk, wood, and leaf fibers. Each type of plant fiber possesses unique properties—such as strength, durability, absorbency, and texture—that make it suitable for diverse applications. The key thermal properties of these fibers are discussed in the following section.

• Fruits

Thermal insulation materials based on fruits (peels, stones, and seeds) or tree parts (stem/trunk, leaves, and surface fibers) were recently explored by multiple researchers. Fruits such as fig [61], banana [62], luffa [63], pineapple [64], areca nut [65], palm dates [66], peach, apricot, and plume [67] are used as thermal insulator material. The thermal conductivity of pure, binder-free fruit-based insulators ranges from 0.047 to 0.132 W/mK, with densities between 76.5 and 276 kg/m³ [33,68,69]. Researchers have also incorporated fruit-based materials into composites with substances such as cardboard, PS, clay, resin, cornstarch, cement, sand, and gypsum, resulting in improved thermal conductivity [70]. For instance, composite panels made from pineapple crown leaves and polyester achieved a thermal conductivity of 0.0197 W/mK [64].

Date palm fibers have been investigated for their potential to enhance the thermal insulation properties of building materials [71]. In a study by Raza et al. [72], composites were developed by blending expandable PS with date palm surface fibers (DSFs) at varying weight percentages (0% to 40%). The mixtures were processed at 200 $°\mathrm{C}$ with a torque of 150 N/m, then molded using stainless steel forms and compressed with a hot press at 180 $°\mathrm{C}$ for 20 min. The thermal conductivity test was performed using a laser Comp FOX-200 heat flow meter. The thermal conductivity decreases as the DSF percentage increases from 0% to 20%, from 0.082 W/mK to 0.053 W/mK. Whereas, for 30% and 40% DSF percentages, the thermal conductivity slightly increases from 0.054 W/mK to 0.06 W/mK [72].

In [73], thermal insulation boards based on wheat bran were produced using water only or a banana peel slurry as a binder, as shown in Figure 4. Two different samples of the approximate size of 60 × 60 × 12.5 mm3 were prepared using only wheat bran (WB) and wheat bran with blended banana peels (WBBPs) at different densities. The thermal conductivity measurement shows that both the WB and WBBPs samples provide insulation performance, with values between roughly 0.050 and 0.065 W/mK [73].

• Grass

The grass plant family includes cereal crops like maize, wheat, rice, barley, and millet, as well as grasses used for animal feed. Various species, including reed, corn, elephant grass, sugarcane, agave, miscanthus, furcraea foetida, water hyacinth, and alfa, have been explored as sustainable thermal insulation materials [61,74,75,76,77,78].

Reed is a plant that grows along riverbanks, characterized by straight stems (up to 6 m) and thin leaves (30–60 cm). Its rapid growth, which interferes with agriculture, leads farmers to pull it up, making it a widely available raw material [79]. Researchers tested reed without a binder and mixed it with cardboard or rice husk. The thermal conductivity of the tested materials ranged between 0.0602 W/mK [80] and 0.089 W/mK [81]. Meanwhile, polyvinyl alcohol (PVA) has also been used as a binder with natural materials, such as sugarcane bagasse, achieving a thermal conductivity of 0.034 W/mK [75]. In another study, corn cobs formed an insulation panel with minimal processing, as shown in Figure 5. The thermal conductance test was performed for eight different configurations (labeled cfg # 0-cfg # 7). The lowest thermal conductivity achieved in that study was 0.14 W/mK [82].

• Wood

Wood waste is commonly generated during post-consumption or at the wood-processing stage. Additionally, forest residue resulting from major weather events could produce large quantities of wood waste. Wood waste from sawmills and other millwork companies was examined as a thermal insulation material [83]. Despite having slightly higher thermal conductivity than common inorganic insulation materials, wood waste is economically attractive due to being a low-cost by-product. Wood sawdust was used in one study to produce polyurethane wood composite (PU-WC) by using a large amount of wood waste without the addition of a catalyst composite, as shown in Figure 6 [84]. The thermal conductivity of the composites depends on the amount of wooden filler. The results appeared to show that the thermal conductivity slightly decreases with increasing filler amount. The lowest thermal conductivity achieved in the study was 0.110 W/mK for an 80% weight percentage of wood filler.

• Bast fibers

The materials investigated in this section were jute, hemp shive, and cotton [65,85,86]. They were used with binders such as polypropylene, polyester, clay, cardboard, potato starch, and resin. Hemp shive fibers achieved the lowest thermal conductivity (0.02–0.03 W/mK) among other bast fibers when tested with potato starch as a binder [87]. Jute fibers have been extensively studied in the literature and rank second among all produced fibers, valued for their recyclability, biodegradability, and low energy requirements for production [88]. The effect of areal density, thickness, and the number of layers of jute felt on the thermal insulation property was studied by Samanta et al. [89]. Jute felts were prepared using needle pinching of fiber fleece. The jute felt exhibiting an areal density of 500 g/m² demonstrated superior specific thermal insulation efficiency. By using four layers with a total thickness of 1.81 mm, the thermal insulation attained a value of 341 m²K/kW, which is substantially comparable to that of glass wool (342 m²K/kW), nitrile rubber (320 m²K/kW), and PS (381 m²K/kW) insulation materials. Furthermore, the cost of jute felts for four layers was notable compared to synthetic alternatives.

• Stalk fibers

Stalk fibers are obtained from the straws of commonly cultivated plants such as rice, wheat, oats, maize, and barley, as well as from other crops like bamboo and tree wood [90]. Rice, being one of the most produced plants, generates a significant amount of agricultural waste. This waste has been successfully used as an insulation material in multiple studies [80,91].

Rice straw (RS) was used to enhance the thermal properties of Portland cement mortar (CM) [92]. Test samples of 40 × 40 × 160 mm mortar, consisting of cement and sand, were produced with varying rice straw content and mineral wool from 0% to 5%. Four different types of samples were produced: control mortar (CT) without any RSF addition, CT-RS with RSF addition, CT-MF with MWF addition, and CT-MF-RS with both MWF and RSF addition [92]. After curing the samples for 28 days, thermal conductivity was measured on cylindrical cement mortar specimens with a diameter of 30 mm and a thickness of 18 mm. The study demonstrated that both rice straw and mineral wool improved the thermal conductivity of mortars, with greater addition of RSF and MWF resulting in lower thermal conductivity. The 50% rice straw mortar achieved the lowest thermal conductivity of all mortars, as shown in Figure 7. From the figure, it is clear that the best insulation performance was observed with rice straw compared to mineral wool fiber.

Animal Fibers

Materials of animal origin, such as feathers, wool, leather, etc., were tested as sustainable and renewable insulation instead of conventional insulating materials. Leather [93], feather [94], and sheep wool [95,96] tested separately or combined with other fibers showed varying performance. Studies showed that insulation materials made from animal wastes, such as sheep wool and goose down, exhibit low thermal conductivity coefficients, ranging from 0.055 W/mK to 0.068 W/mK [97,98]. In order to analyze the thermal insulation property, waste wool fibers were mixed with RPET fibers in 50/50 proportions in the form of a two-layer mat [95]. Another set of three samples from 100% waste wool and 100% RPET fibers was also prepared. All samples were tested for thermal insulation, moisture absorption, and fire properties. Moreover, the behavior of the samples under high humidity conditions was evaluated. Two layers of 50% waste wool and 50% RPET mat provided the best insulation, moisture absorption, and fire properties. The RPET/waste wool mats have adequate moisture resistance at high humidity conditions without affecting the insulation properties [95].

In [96], the authors evaluated the thermal and hygrothermal performance of sheep wool insulation using steady-state measurements across varying temperatures and material densities. Thermal conductivity was tested at mean temperatures of 10 °C, 20 °C, 30 °C, and 40 °C, with sample thicknesses ranging from 80 mm to 40 mm, as shown in Figure 8, corresponding to bulk densities of 20–40 kg/m³. The results showed that the thermal conductivity increases with temperature and decreases as the bulk density increases due to the reduced air pore volume. In terms of hygrothermal behavior, sheep wool demonstrated high moisture absorption (up to 30%) without significant loss of insulating properties. Up to a moisture content of 20%, the thermal conductivity remained largely unaffected, confirming the material’s stability under varying humidity levels.

In another study, Erkmen et al. [97] investigated the development of an insulation material using agricultural and animal wastes such as wood shavings, wheat straw, and goose down, which were used for developing thermal and water insulation material, as shown in Figure 9. Polyvinyl acetate acrylic copolymer resin styrene, a water-based environmentally friendly resin, was used as a binder. In this study, all components were coated with a hydrophobic agent and polymer binder to prevent biological degradation [97]. The optimal mixture of 125 g of binder, 80 g of wood shavings, and 30 g of wheat straw achieved a thermal conductivity of 0.06 W/mK. The addition of goose down further reduced the thermal conductivity. To address water absorbency, 5% Polydimethylsiloxane (PDMS) was used, lowering water absorption from 175% to 20%. The final samples showed thermal conductivity between 0.055 and 0.0681 W/mK and improved waterproofing properties. These results indicate that materials derived from animal waste have the potential to serve as lightweight and sustainable insulation for buildings [97]. However, feathers, despite their excellent thermal conductivity, face certain challenges. For instance, the biodegradation and decomposition of feathers can be an issue if the time between collection and washing/disinfection is not short [94].

Barbanera et al. [99] investigated the thermal properties of building insulation panels made from recycled leather scraps with various compositions and adhesives, comparing their performance to other insulation materials. The leather scraps were chopped and mixed with a polyvinyl acetate binder. Thermal conductivity measurements conducted using a hot box system demonstrated good thermal performance. The panels showed thermal conductivities between 0.104 and 0.108 W/mK, indicating effective thermal properties suitable for insulation applications [99].

Other Natural Fibers

Natural materials that were not of either vegetal or animal origin were grouped in this section. Fungus, mud, and coal ash have been tested in several studies as thermal insulation materials [100,101,102]. A spent mushroom substrate (SMS) and empty fruit bunch (EFB) fibers were used to develop a composite panel using a sandwich technique, as illustrated in Figure 10 [103]. Five samples with varying fiber ratios were made (100 SMS: 0 EFB, 80 SMS: 20 EFB, 60 SMS:40 EFB, 40 SMS: 60 EFB, and 0 SMS: 100 EFB). The thermal conductivity of these samples was measured using a hot disk thermal constant analyzer. The thermal conductivity value ranged from 0.231 to 0.310 W/mK. The lowest value was obtained by the sample made from 100 SMS: 0 EFB, whereas the highest value was achieved using the 0 SMS: 100 EFB sample.

A fungal-based biocomposite with wheat straws, mycelium, and polypropylene for construction materials was investigated by Raut et al. [100]. The cultivation of Ganoderma lucidum mucoromycetes was mixed with wheat straws and polypropylene embedded with spores from Bacillus amyloliquefaciens. Compared with PS, the fungal biocomposite achieved a better thermal insulation capacity with a similar strength to PS for construction applications. The thermal conductivity of fungal biocomposite is lower than PS by 4% at 40 $°\mathrm{C}$, 7% at 25 $°\mathrm{C}$, and 18% at 10 $°\mathrm{C}$.

The thermal conductivity of different types of natural materials is summarized in

Table 1. Thermal conductivity of different types of natural materials (NM).

NM Type	Fiber	Matrix	Fiber Weight Ratio (%)	Bulk Density (kg/m3)	Thermal Conductivity (W/mK)	Test Method	Ref.
Fruit	Fig Stem	Cardboard	40	343.8	0.100	Hot plate	[79]
	Fig Stem	Binderless	100	157	0.072	Guarded hot plate	[81]
	Banana Stem	Polyvinyl alcohol, Carboxymethyl cellulose, glutaraldehyde	60	0.043	0.036	Tci Thermal Conductivity Analyzer	[104]
	Luffa	Clay	59	397.6	0.101–0.116	Heat flow meter (ISO 8301, DIN EN 1946-3)	[88,105,106]
	Date Palm Wood	Polylactic acid	30	1200 (approx.)	0.069	Thermal conductivity meter (ASTM C1045-07)	[71,107]
	Date Palm Leaflet	Cement, sand, gypsum	30	42.6	0.048	Cold and hot plate (DIN 52612)	[108,109]
	Date Palm Surface Fibers	PVA		203	0.038	ASTM C 1045-07	[107,110]
	Date Palm Surface Fibers	Polystyrene	60	937	0.053	ASTM C 1045-19	[72]
	Date Palm Components	Cardboard	40	226.6–312.8	0.074–0.081	Hot plane	[111]
Grass	Corn Cobs	Binderless	100	195	0.14	Movable hot box	[82]
	Corn Husk	PVA		200	0.0386	Guarded hot plate ASTM C177-85	[112,113]
	Miscanthus	Cement	30 (vol.)	554	0.09		[76]
	Furcraea foetida	Lapox L12 Epoxy, hardener K6	20	720	0.132	Guarded hot plate ASTM C177-19	[77]
		Cement			0.374–0.513
		Gypsum			0.269–0.389
Wood	Wood waste			117	0.048–0.055	Hot plate	[83]
Bast Fibers	Hemp Shives	Potato starch		208	0.062		[86]
	Flax Shives	Potato starch		215	0.053
Stalk Fibers	Triticale Straw	pMDI glue	90	200	0.033		[114]
Other plant fibers	Olive Leaves	Cardboard	40	315.1	0.086	Hot plate ASTM E1461-13	[79,115]
	Sunflower Straw			155	0.0469	Thermal constant analyzer	[116]
	Rice Husk Ash				0.477–0.571	Transient heat conduction	[24]
Animal	Chicken Feather	Polypropylene	70		0.059	JIS 1412-2	[117,118]
	Turkey Feather	Polyurethane foam	3	39 (approx.)	0.0291	ASTM C518	[119,120]
	Sheep Wool		100	235	0.0322	Guarded hot plate ASTM C177	[121,122]
Others	Spent Mushroom Substrate		100	800	0.231		[103]
	Coal Fly Ash	PVA	1.25–5	100–190	0.042–0.050	Thermal conductivity analyzer	[102]
	Coal Fly Ash	Gypsum	60	850	0.325 (approx.)	Hot wire method (EN 99315, 1998)	[123,124]
	Coal Fly Ash	Borax, calcium carbonate	40	460	0.36	Thermal analyzer	[125]

.

5.1.2. Synthetic Waste Materials

Synthetic polymer-based thermal insulation materials (SP-TIMs), including polyethylene, polyimide, polyvinyl chloride, and PS, are extensively utilized in construction due to their low thermal conductivity, lightweight nature, high flexibility, and hydrophobic properties. Yasira et al. investigated the feasibility of incorporating plastic waste into brick production as an eco-friendly alternative to traditional bricks [126]. They produced four types of bricks containing 0%, 5%, 10%, and 15% high-density polyethylene (HDPE). Thermal performance assessments, conducted using a thermocouple-equipped chamber, revealed significant insulation improvements in bricks with 10% and 15% HDPE content, showing an average difference in internal–external air temperature of 6 $°\mathrm{C}$.

Table 2. Thermal insulation properties of sustainable insulation materials based on synthetic materials.

Fiber	Matrix	Fiber Weight Ratio (%)	Bulk Density (kg/m3)	Thermal Conductivity (W/mK)	Test Method	Ref.
Cellulose acetate plastic (cigarette butt waste)	gypsum	1.5	994–1006	0.017–0.024	A heat insulation house	[25]
Car shredded tire residue	Polyurethane	50	40	0.034	ISO 8302:1991 and BS 1902-5.8 standards	[26,127,128]
Carpet tiles shred	polyurethane	90	366	0.06
Loose-fill plastic waste		100	56	0.022–0.032		[27]
Microplastics	Alginate, Glycerol, CaCO3, D-gluconic acid $\delta$-lactone (GDL)		69.6–148.8	0.043–0.048	Heat flow meter according to ASTM C518	[119,129]
Polypropylene face masks	Serishoom glue		60–100	0.029	Guarded hot plate ASTM C177-85	[112,130]
Nylon/Spandex and polyurethane		60	1060	0.0953	Guarded hot ASTM C177	[122,131]
Blast furnace slag (BSF), waste photovoltaic glass (WPG), rice husk ash (RHA), and plant ash (PA)				0.0497–0.0581		[31]
Phosphate washing sludge and metakaolin		50	1627	0.076	ASTM C177	[122,132]
Fiberglass scraps	Alginate, CaCO3, D-gluconic acid $\delta$-lactone (GDL)	12.5	240–320	0.041–0.069	ASTM C518	[28,119]
Marble waste powder and rice husk ash				0.477–0.571	Transient heat conduction	[24]
Acrylic spinning waste		100	25	0.04541		[29]
Acrylic knitting waste		100	30	0.04581
Washed wool waste		100	62	0.03745
Carpet waste wool		100	45	0.04076
Ceramic shell waste	Styrofoam	30		0.061	ASTM D7984-16	[30,133]

summarizes the thermal properties of various sustainable insulation materials based on synthetic materials.

5.2. Acoustic Insulation Materials

Conventional acoustic insulators are usually produced from synthetic insulators such as polyurethane foam, polyester fiber, and melamine foam or inorganic materials such as mineral wool and glass wool [134]. Recently, more emphasis has been placed on using natural and recycled materials, as summarized in Figure 1, for acoustic insulation [135,136].

5.2.1. Natural Fibers

Plant Fibers

As explained in the thermal insulation section, plant fibers are natural materials derived from various parts of plants and are classified into categories such as fruit, grass, bast, stalk, wood, and leaf fibers. Each type of fiber has unique attributes, including flexibility, resilience, texture, and sound-dampening properties, making them effective in acoustic insulation applications by reducing noise and improving sound absorption. The SAC of natural plant fibers depends on various factors, including fiber thickness, fiber diameter, frequency, and fiber type [19].

• Fruit fiber

Fruit fibers are widely used in acoustic insulation, although they are less common than other types of plant fiber [68]. The use of oil palm waste (OPEFB) as a natural acoustic insulation material has been discussed in [137]. The research was conducted by creating samples of varying densities and thicknesses of OPEFB fibers to examine their influences on the efficacy of acoustical absorption. A mechanical retting process was used to extract fibers from OPEFB under high pressure and temperature. Then, samples of OPEFB fibers with different weights and thicknesses were prepared using a hot compressing mold, as shown in Figure 11 [137].

The impedance tube method was used to measure the sound absorption coefficients of samples from a frequency range of 500 Hz to 4.5 kHz. For samples with a fixed 10 mm thickness but varying densities, a direct correlation was found between increased density and improved sound absorption. Densities above 468 kg/m³ (4 g of fibers) achieved an SAC greater than 0.5 at frequencies greater than 2 kHz, while higher (702 and 818 kg/m³) and lower densities (117 kg/m³) showed reduced sound absorption. For samples with the same density but different thicknesses, increased thickness resulted in better absorption. The highest average SAC of 0.9 was achieved using 40 mm and 50 mm thick samples with a density of 292 kg/m³, particularly above 1 kHz, as shown in Figure 12 [137].

• Grass fiber

Grass fibers are utilized as sound absorption materials due to their porosity, flexibility, sustainability, and eco-friendliness. These fibers can be integrated into acoustic panels, soundproofing materials, and noise barriers to effectively mitigate noise levels [82]. Fattahi et al. [113] investigated the acoustic absorption properties of corn husk fiber (CHF). The CHF was collected, cleaned, dried, and cut into 2 mm pieces; then, it was mixed with a PVA binder and molded into cylindrical samples. These samples were pressed for 30 min at 200 bars to create different thicknesses and densities. The results revealed that increasing sample thickness from 20 mm to 30 mm enhanced sound absorption, particularly at low and high frequencies, although a slight reduction in absorption was observed at mid-frequencies. The peak absorption shifted to lower frequencies with increased thickness, indicating that increasing thickness is an effective strategy to improve low-frequency performance [113].

Compared with other natural fibers such as sugarcane, coconut husk, and sisal, CHF showed competitive sound absorption properties. All samples exhibited an SAC of greater than 0.5 at frequencies above 1 Hz, and increasing the bulk density from 200 to 250 kg/m³ further improved the acoustic performance [113].

• Bast fiber

Bast fibers, derived from the inner bark of plants, are known for their strength and durability. Flax, hemp, jute, kenaf, and ramie are bast fibers commonly used in acoustic insulation products [138]. The sound absorption of kenaf natural fiber samples was studied for normal and random sound incidence [139]. The study examined the impact of thickness on both full-fiber and air-fiber specimens and the influence of bulk density. For normal incidence, the kenaf fibers were fabricated using a cylindrical mold with a diameter of 33 mm and modeled in a certain thickness using a hot compression machine. However, fiber sheets were cut from industrial-prepared kenaf rolls for random incidence, as illustrated in Figure 13 [139].

The impedance tube setup was used to measure the normal incidence sound absorption coefficient, while a reverberation chamber was employed to assess random incidence absorption [139]. The study explored the relationship between thickness and sound absorption, maintaining a constant bulk density of 93.5 kg/m³ for all samples. The results for normal incidence showed that thicker materials absorb sound more effectively, especially at low frequencies. Samples thicker than 40 mm demonstrated exceptional absorption coefficients, often exceeding 0.8 or reaching unity at frequencies above 1 kHz [139]. The study also examined the effect of air gaps on sound absorption by testing kenaf fiber specimens of 20 mm and 30 mm thickness with air gaps of 10 mm, 20 mm, and 30 mm. Introducing an air gap shifted the peak absorption frequency to lower frequencies, suggesting that adding air gaps can enhance low-frequency absorption without increasing material thickness, thereby reducing the need for more fibrous material [139].

To study the effect of bulk density, samples with varying fiber weights were fabricated [139]. The results showed that increasing bulk density improved the SAC across a broader frequency range, from low to high frequencies. Figure 14 illustrates the impact of thickness, air gaps, and bulk density on normal incidence sound absorption [139]. The study also examined random-incidence absorption, with kenaf fibers achieving an SAC above 0.5 at 400 Hz and an average of 0.8 for 25 mm thick samples. Adding an air gap further improved low-frequency absorption, similar to normal incidence results. Figure 15 shows the effect of varying thickness and air gaps on random-incidence absorption [139].

• Stalk fiber

Stalk fibers, derived from plant stems like rice, wheat, oats, and barley, offer a sustainable and effective solution for acoustic insulation. Their strength, durability, and sound-absorbing properties make them valuable in acoustic panels, insulation materials, and other products designed to improve sound quality.

Wang et al. [140] examined the durability and sound absorption properties of geopolymer-based wheat straw (WG), rice husk (RG), and sawdust (SG) insulation materials. The geopolymer slurry was prepared by mixing an alkali activator with metakaolin, followed by the addition of a surfactant and H₂O₂. The prewetted WG, RG, and SG were then incorporated, and the mixture was molded using silicon molds. The samples were cured at 25 $°\mathrm{C}$ for 1 day, at 70 $°\mathrm{C}$ for 2 days, and dried at 40 $°\mathrm{C}$ for 1 week. The results indicated a stable skeleton structure with no visible cracks or material disaggregation after freeze-thaw, wet-dry, and cool-heat cycles, as shown in Figure 16 [140].

The sound absorption coefficients of WG, RG, and SG samples were analyzed across a frequency range of 200–1600 Hz. WG exhibited the highest absorption coefficient, reaching 0.71 at 1028 Hz, while RG and SG achieved 0.57 at 1210 Hz and 0.56 at 1120 Hz, respectively. The superior performance of WG is attributed to its higher number of interconnected pores, which enhance air permeability and facilitate sound wave dissipation. Additionally, the lower density and abundant tube bundles of WG contribute to the formation of an open, microporous structure, further improving sound absorption [140].

• Wood fiber

Wood fiber, a renewable material derived from trees, is widely used in acoustic insulation due to its porous structure, which traps sound waves and reduces noise transmission. Its adjustable density enhances acoustic performance, making it a sustainable and versatile solution for noise control in buildings and other environments.

Fornes et al. presented a new material consisting of a matrix reinforced with phosphogypsum (PG) and waste wood fibers (WFs), made from used boards or natural wood fibers (NFs) [141]. PG specimens were prepared with varying amounts of WF and NF fibers, ranging from 0 to 5 wt%. The addition of WF significantly improved the mechanical strength of the samples compared to NF. The optimal strength was achieved with a small amount of fiber, specifically 0.5 wt% WF or 1 wt% NF. The specimen with 5 wt% WF content demonstrated the best acoustic insulation properties but exhibited poor mechanical strength [141].

Animal Fibers

Animal fibers, such as wool [142], silk, feathers [143], and hair, are natural materials sourced from animals [144]. Their porous structure and resilience make them effective for acoustic insulation, offering benefits like sustainability and enhanced sound absorption [121,145,146]. However, factors such as cost, availability, and ethical concerns may limit their widespread use in this field.

In [147], the authors investigated the sound absorption properties of sheep wool using seven samples with varying compositions and densities. Airflow resistance and sound absorption coefficients were measured under both normal and diffuse incidence conditions. The results demonstrated that sheep wool exhibited effective sound absorption, particularly at mid and high frequencies. Furthermore, its acoustic performance was found to be comparable to that of mineral wool and recycled polyurethane foam, highlighting its potential as a sustainable insulation material.

The acoustic properties of sheep wool were investigated by Berardi et al. [148]. The sound absorption performance of sheep wool was assessed using the impedance tube method. As shown in Figure 17, increasing the sample thickness significantly improved absorption coefficients, especially at medium and high frequencies [148].

In another study, the acoustic properties of waste wool from Ghezel (Gh), Arkharmerino (Ar), and their crossbreeds (ArGh1 and ArGh2) were analyzed [121]. Sound absorption tests revealed that Ar wool exhibited the highest SAC, followed by ArGh2, ArGh1, and Gh. The improved acoustic performance of finer fibers was attributed to increased flow resistivity. Additionally, the Mechel model effectively predicted frequency-dependent SAC, showing strong agreement with experimental data. In [96], the acoustic performance of sheep wool insulation was systematically evaluated using laboratory-based sound absorption testing. Circular test samples were examined using the Kunte’s Tube method to measure the SAC across third-octave bands in the 100–3150 Hz frequency range. To analyze the impact of material thickness on acoustic behavior, four different sample thicknesses were tested: 20 mm, 30 mm, 40 mm, and 60 mm. The results indicated a clear relationship between increased thickness and improved sound absorption performance, as shown in Figure 18, particularly at lower frequencies. As the insulation thickness increased, the frequency at which maximum absorption occurred shifted lower, which is consistent with the behavior of porous acoustic materials.

Other Natural Fibers

In addition to plant and animal fibers, other natural materials are also utilized for acoustic insulation [123,149]. For instance, a sol-gel process was employed to create an acoustic insulating foam incorporating glass powder. The foam was formed using alginate, a polysaccharide, and mannuronic acid to produce stable gels [150]. The process involved preparing a glass-alginate composite hydrogel and freeze-drying it to achieve a porous foam. This method was advantageous, as it did not require high temperatures, and no chemical pollutants or toxic gases were released. Rock wool was used as a reference for acoustic comparison. The foam exhibited superior acoustic performance, particularly at medium to high frequencies. This improvement is attributed to the foam’s anisotropic and tortuous structure, along with its cell size, which is influenced by the alginate content.

The acoustic properties of sustainable insulation materials based on natural fibers are summarized in

Table 3. Comparison of acoustic properties of different natural fibers.

Natural Material Type	Material	Thickness (mm)	Density (Kg/m3)	Maximum SAC	Frequency (Hz)	Ref.
Fruit	Date palm empty fruit bunches (DPEFB)	40	200	0.7–0.8	Above 1500	[151]
	Luffa	40	225	0.95	1200	[63]
Bast Fiber	Hemp	20		0.99	2000	[152]
	Cotton fibers	50	40.5	0.96	3150
	Jute fibers	50	65.6	0.76	3150
	Sisal fibers	50	38.6	0.42	3150	[153]
	Flax fibers	50	78.4	0.96	3150
	Bamboo fibers	50	120	0.95	1600	[154]
	Flax-tows	50	180	0.8	3150	[155]
	Nettle fibers	56		0.98	3150	[156]
Animal	Chicken feathers	25, 50, and 75	48	0.99	1600, 950, 650	[143]
Other	Fiberglass	15	170& 320	Noise-reducing factor between 0.25–0.45	3000	[28]
	Olive tree pruning wastes	50		0.9		[157]
Salk Fiber	Rice straw	18	0.05–0.06 g/cm3	0.99	3000	[91]

.

5.2.2. Recycled (Synthetic) Fibers

Synthetic fibers are commonly used as acoustic insulation materials due to their durability, water resistance, and moisture resistance [16,158]. Their ability to provide effective sound absorption makes them suitable for various applications, including construction, automotive, and industrial settings. However, despite these advantages, synthetic fibers have notable limitations. They are highly susceptible to heat damage and have a low melting point, making them vulnerable in high-temperature environments [159]. When exposed to excessive heat, these materials can deform, lose their structural integrity, or even release harmful fumes. As a result, their application in fire-prone areas requires additional treatment with flame retardants or the incorporation of heat-resistant materials to enhance their thermal stability and safety.

Table 4. Acoustic properties of synthetic wasted material.

Material	Thickness (mm)	Density (Kg/m3)	Maximum SAC	Frequency (Hz)	Methodology of Sample Preparation	Ref.
Waste tire and textile-reinforced epoxy composite	-	-	0.61	3000–5000	Different ratios of waste tires and textile combined with epoxy composites were used to investigate sound absorption properties.	[160]
Cardboard waste and natural fibers	80	278.6–343.8	0.6	200–1400	Natural fibers (40%) were mixed with 60% of the dense board pulp for 5 min. The mixture was poured into cylindrical molds of 100 mm in diameter. The prepared samples were dried in ambient air and then in the oven.	[79]
Glass powder and slag mixtures			0.98	1200–1500	The waste glass and slag were ground up. NaOH pellets were dissolved in water and a solution mixed with glass and slag. The resulting pastes were cast in rectangular molds. The samples were cured for 24 h at 60 $°\mathrm{C}$ and then cured in the air for 7 days at 20 ± 2 $°\mathrm{C}$.	[161]
Wood ash was used as supplementary cementing material	-	1000–1400	High sound absorption at very low frequency and very high frequency (max absorption 0.7 at 125 Hz)	250–2000	Portland cement was partially replaced by wood ash in varying amounts (0%, 10%, and 20%) and mixed using a drum mixer. Glass microfibers were added at a 1% ratio. The mixture was molded into cylindrical molds.	[162]
Plastic waste	30		NRC = 0.45	2000–2500	Recycled polyethylene terephthalate (rPET) PVA and glutaraldehyde (GA) were prepared using a freeze-drying process.	[163]
Cigarette butts	9.5, 19, 28, 38, 57, 67, 75, and 85	110–160	0.99 for 19 mm	50–1450	Cigarette butts were dried for 24 h at 80 $°\mathrm{C}$. Samples were prepared by putting them in holders with 29 mm and 100 mm diameters.	[164]

summarizes the acoustic properties of a material based on synthetic waste material.

6. Challenges in the Use of Natural Fiber Composites

Natural fiber composites offer several advantages, including sustainability, biodegradability, and excellent insulation properties. Their ability to provide thermal and acoustic insulation makes them highly valuable in construction, automotive, and packaging applications. However, they also face challenges related to durability, fire resistance, and moisture absorption.

6.1. Durability

Natural fibers are susceptible to environmental degradation due to their organic composition. Factors such as exposure to radiation, moisture, temperature fluctuations, and microbial attacks can lead to the deterioration of the fiber structure, reducing the lifespan of the composite material. Testing the durability of natural fibers is essential to ensure their long-term performance and reliability. Various tests can be performed to assess the durability of natural fibers. Mechanical testing, such as tensile, flexural, compressive, and abrasion testing, measures the strength of the fibers. Environmental testing, including UV exposure, humidity, accelerated weathering, temperature, and chemical resistance tests, evaluates the fiber’s ability to withstand harsh conditions. Additional tests such as microscopic analysis, moisture regain testing, and colorfastness testing provide further insight into the fibers’ properties. By conducting these tests, manufacturers can determine the suitability of natural fibers for specific applications [165].

The durability of natural fiber composites can be enhanced through various approaches, including fiber treatments such as alkali or silane treatments, the use of coupling agents, and the incorporation of hybrid composites that blend natural fibers with synthetic ones. Moreover, the development of advanced resin systems that offer improved resistance to moisture absorption and environmental degradation can significantly extend the lifespan of the material, making it more suitable for long-term applications.

Table 5. Durability tests of different types of natural fibers.

Natural Fibers	Testing Type	Testing Result	Ref.
Sisal, coir, banana fibers	UV rays heat moisture	Sisal shows a resistance decrease of about 62% for UV aging and about 81% for soil aging in the first 15 days. Coir fibers present a strength loss of about 61% for UV aging and shallow strength loss for soil aging in the first 15 days. Banana fibers exhibit a strength loss of about 63% for UV testing and about 77% from soil aging in the first 15 days. After 3 months, the tensile strength loss for banana and sisal fibers represents about 90% of the initial strength.	[166]
Water hyacinth, reed, sisal, roselle	Moisture absorption	Sisal and roselle fibers had similar moisture sorption levels, lower than reed and water hyacinth. Moisture absorption increased with higher relative humidity (RH), with water hyacinth showing an eightfold increase at 97% RH compared to 75%, while other fibers increased about fourfold.	[165]
	Tensile strengths and elongations	A total of 50% in tensile strength reduction after 840 h exposure time for all fibers. The elongation at break for all yarns showed statistical differences, likely due to material heterogeneities and small defects, except for sisal.
	Accelerated weathering	The studied fibers (except sisal) would stay at least 1 year outdoors.
Ramie, Jute, hemp, sisal wires	Alkaline sensitivity for plant fibers into cementitious composites	The tensile strength of hemp, sisal, jute, and ramie wires reduces when embedded in alkaline conditions. After 60 days of immersion, the fibers lose about 50% of their initial strength.	[167]
Silica Fume (SF), metakaolin (MK), blast furnace slag (BFS)	Compression	The BFS significantly increased the compression strength of the material. The value of compressive strength reaches 82 MPa at 7 days.	[168]
	Tensile	The composite based on BFS showed a significant increase in the first crack, ultimate tensile strength, and strain capacity. The composite based on 100 % MK presented a small decrease in the first crack and ultimate strength. The composite based on SF showed the lowest result for tensile strength.
	Flexural loading	The composite containing BFS presented a better performance for the first and ultimate deflection and first and ultimate strength.
Wood flour	Moisture absorption	WPCs were prepared by blending wood flour (20, 80, 120 mesh) with polypropylene (PP). WPCs with finer wood particulates absorbed less moisture.	[169]
Pig hair	Compressive and flexural strength	Reinforced mortars to significantly improve impact strength, abrasion resistance, plastic shrinkage cracking, age at cracking, and crack widths as fiber volume increases. Density, porosity, and modulus of elasticity of reinforced mortars are not significantly affected by the addition of pig hair.	[170]
Horsehair (HH), polypropylene (PF), carbon (CF), basalt (BF), glass fibers (GFs)	Dry bulk density and water absorption	The fibers were added to the mixture at 0.3%, 0.6%, and 1.2 % by weight of natural hydraulic lime-based mortars (NHL) and compared with plain NHL mortar. Horsehair with 0.3% hair has the highest dry density (1794 Kg/m3) and smallest water absorption (16.2%) after 28 days.	[171]
	Compressive strength	The addition of 0.3% of HH improved the 28-day compressive strength by 10 %.
	Flexural strength	The addition of 0.3%, 0.6%, and 1.2% HH fiber improved the 7-day and 28-day flexural strength.

summarizes different durability tests of different types of natural fibers and their properties.

6.2. Flame Resistance

Flame resistance is crucial for sustainable thermal and acoustic insulation materials, particularly in construction. It refers to a material’s ability to resist ignition, delay combustion, and limit fire spread, ensuring safety. Developing flame-resistant materials requires balancing functionality, environmental impact, and safety standards. The material composition plays a key role in flame resistance. Natural fibers like wool, cotton, and hemp are renewable and biodegradable but often need treatments to improve fire resistance. Recycled materials such as cellulose or PET and bio-based materials like cork offer eco-friendly options, though their fire resistance varies [172]. Flame retardant additives, like aluminum hydroxide and magnesium hydroxide, reduce flammability by releasing water vapor when exposed to heat, while intumescent additives form a protective char layer during combustion. Halogenated flame retardants, though effective, are increasingly avoided due to environmental concerns [173,174].

Surface treatments provide localized protection but may degrade over time, while bulk treatments offer more uniform, long-lasting flame resistance. Achieving flame resistance in sustainable materials requires minimizing the environmental impact of flame retardant production and disposal. The durability of these treatments is also essential, ensuring they do not compromise thermal or acoustic insulation properties or require frequent reapplication [2].

7. Discussion: Comparative Analysis of Thermal and Acoustic Insulation Materials

Numerous studies have extensively investigated the performance of both natural and synthetic insulation materials, and their findings have been tabulated in the previous tables (Table 1 and Table 2). In this section, we focus specifically on comparing conventional insulation materials with some of the most widely used and effective natural alternatives. To ensure a realistic and meaningful comparison, key parameters such as thermal conductivity, sound absorption, fire resistance, EC, and bulk density are taken into account. The fire-resistant properties of insulation materials are classified according to the European standard BS EN 13501–1 [175], which ranks materials from Class A (non-combustible) to Class F (highly combustible) based on their combustibility.

Conventional insulation materials such as glass wool, rock wool, PS foams (EPS and extruded PS), and polyurethane have long been utilized for their reliable thermal performance and availability. These materials generally exhibit low thermal conductivity values ranging from 0.022 to 0.055 W/mK and moderate-to-high sound absorption coefficients (0.2–0.9), as shown in

Table 6. Comparison of conventional and natural building insulation materials [176].

Insulation Type	Density (kg/m3)	Thermal Conductivity (W/m·K)	SAC	Reaction to Fire	EC (kg CO2-eq/kg)	Ref.
Glass wool	10–100	0.03–0.05	0.45–0.8	A1	1.24	[135,177,178,179]
Rock wool	40–200	0.033–0.04	0.29–0.9	A1–A2	1.05	[12,135,177,178]
EPS	18–50	0.029–0.041	0.22–0.365	E	6.3–7.3	[12,135,177,178,180]
XPS	32–40	0.032–0.037	0.2–0.65	E	7.55	[12,177,178]
Polyurethane	30–160	0.022–0.035	0.67 or 0.8	D–F	5.9	[12,177,181,182]
Foamed glass	100–200	0.038-0.055	–	A1	–	[183,184]
Vermiculite	64–130	0.04-0.064	0.8	A1	–	[185,186]
Phenolic Foam	40–160	0.018–0.024	0.3–0.5	B–C	4.15–7.21	[187,188,189]
Cork	100–120	0.037–0.043	0.39–0.85	E	0.82	[190,191]
Cellulose	30–80	0.037–0.042	0.53–0.9	B–C–E	0.31–1.83	[135,192]
Bamboo fibers	431–538	0.077–0.088	0.2–0.56	–	–	[193,194]
Flax	20–100	0.033–0.09	0.54–0.84	C	20	[195,196,197]
Hemp	25–100	0.039–0.123	0.52–0.6	E	0.14	[148,198,199]
Kenaf	30–180	0.026–0.044	0.3–0.95	E	0.59–2.09	[139,148,200,201,202,203]
sisal	200	0.042–0.044	0.16–0.5	-	-	[204]
Reeds	130–190	0.045–0.056	0.08–0.54	E	–	[205,206]
Sunflower	36–152	0.038–0.05	0.7	–	0.56	[177,207,208]
Rice husk	130–170	0.048–0.08	0.15–0.66	A	0.6	[209,210,211]
Coconut husk	250–350	0.045–0.069	0.16–0.52	-	–	[204]
Bagasse	250–350	0.049–0.055	0.46–0.71	–	–	[212,213]
Date palm	187–389	0.072–0.085	0.59–0.83	–	–	[214,215]
Jute fibre	–	0.033–0.046	0.54–0.72	–	-	[177]
Sheep wool	20–40	0.034–0.050	0.082–0.977	E	0.12	[96]
mycelium	87–112	0.047–0.05	0.87–0.9	D–E	-	[216]

. However, they also come with notable environmental drawbacks, including high EC—such as in the case of EPS (6.3–7.3 kg CO₂-eq/kg) and polyurethane (5.9 kg CO₂-eq/kg)—and limited biodegradability. In comparison, natural insulation materials such as hemp, flax, cork, cellulose, rice husk, and sheep wool show comparable thermal conductivity values (typically, 0.030–0.050 W/mK) and similar or superior sound absorption capacities (up to 0.9 in some cases). In addition to their insulation properties, these natural materials exhibit lower environmental impacts, often with EC values below 1 kg CO₂-eq/kg, and the added advantage of being renewable and biodegradable. Although natural materials often show higher variability in density and typically lower fire resistance compared to conventional insulations, their renewable nature, low EC, and comparable thermal and acoustic performance make them strong and sustainable alternatives for use in both residential and commercial buildings.

8. Conclusions and Future Research

Sustainable thermal and acoustic insulation materials derived from waste sources offer a promising approach to improving energy efficiency while reducing environmental impact. This review has highlighted a diverse range of natural and synthetic waste materials that have been effectively transformed into insulation products, delivering key advantages such as low density, high thermal resistance, and efficient sound absorption at a relatively low cost. Evaluating production methods and testing protocols further supports the feasibility of these materials as strong competitors to traditional insulation options. Natural and renewable insulation materials typically have a much lower global warming potential (GWP) compared to traditional petrochemical-based insulation products because their production consumes less fossil fuel energy and often utilizes renewable or carbon-storing raw materials. In contrast, conventional insulation materials like glass wool and stone wool require the energy-intensive, high-temperature processing of raw materials such as sand, rock, or recycled glass, resulting in higher CO₂ emissions and greater environmental impact during manufacturing. Notably, studies on sheep wool highlight its potential as a superior alternative to mineral wool, demonstrating comparable or better insulation performance alongside significant environmental benefits and reduced health risks. Additionally, sheep wool’s non-toxic nature, ease of handling, and eco-friendly profile make it an attractive and sustainable choice for modern insulation needs.

However, there remain challenges in ensuring the long-term durability and performance consistency of waste-based insulation materials, particularly under fluctuating environmental conditions. An important aspect of this review is the correlation between research findings on sustainable insulation materials and their applicability in construction. Beyond evaluating thermal and acoustic properties, the review analyzes key construction-relevant criteria such as durability, flame retardancy, and various manufacturing and testing methods. These factors are essential in assessing the potential of natural and recycled insulation materials to meet building performance requirements and safety standards. Addressing these challenges is essential for scaling up and mainstreaming sustainable insulation across diverse construction applications.

Future research should focus on optimizing formulations and manufacturing processes to enhance the mechanical strength and stability of waste-based insulation materials over time. Investigating hybrid insulation materials that combine multiple waste types could lead to significant improvements in thermal and acoustic properties. Additionally, real-world performance testing under diverse environmental conditions is crucial to validate the resilience and longevity of these materials. Establishing standardized testing protocols will be essential for ensuring consistency and comparability, facilitating the widespread adoption of waste-derived insulation materials in construction and other sectors. Furthermore, the use of bio-based binders in natural insulation materials will be critical in the future, offering the added benefit of reducing greenhouse gas emissions compared to conventional petroleum-sourced PS and binders. These bio-based binders will further enhance the sustainability of insulation materials by lowering the environmental impact associated with traditional production processes. Together, these research efforts will support the broader implementation of sustainable insulation solutions, contributing to global energy efficiency and sustainability goals.