Abstract
The drive for energy-efficient buildings has propelled the development of advanced insulation materials that minimize heat transfer and ensure safety and durability. Expanded Polystyrene (EPS), a widely adopted material in the insulation industry, is valued for its affordability and ease of application. However, high emissivity, moisture susceptibility, and limited fire resistance often compromise its thermal performance. Consequently, this study investigates the potential of enhancing the thermal resistance of EPS by integrating it with a Radiant Barrier Foil Board (RBFB) material, thus forming a composite insulation material. A Series of tests were conducted on the developed composite insulation material, which includes emissivity, water absorption, thermal conductivity, and fire resistance, to assess the performance improvements offered by the enhanced composite material compared to conventional EPS. Results showed that the EPS-RBFB composite demonstrated a 95% reduction in emissivity level from 0.85 to 0.04, and a 22% improvement in thermal conductivity was observed from 0.036 to 0.028 W/m K. Further, a 72% reduction in moisture absorption was recorded, including a significant enhancement in the fire resistance, with ignition delayed by 50%, and self-extinguishing was also observed. These findings suggest that the enhanced EPS-RBFB composite is a superior insulation material, offering enhanced durability and safety for modern construction.
Keywords: Energy-efficient buildings, Expanded polystyrene, Radiant barrier foil boards, Thermal conductivity, Emissivity
Subject terms: Civil engineering, Environmental sciences
Introduction
The global demand for energy-efficient buildings is driving innovation in construction materials, particularly in thermal insulation. Insulation materials are materials used to reduce heat flow in a building. They are important in trying to achieve operational energy efficiency in buildings1–5. Effective insulation is crucial for reducing energy consumption in buildings, as it minimizes heat exchange between the interior and exterior environments. Expanded Polystyrene (EPS) has gained popularity among the various insulation materials available due to its low cost, lightweight, and ease of installation6–8. EPS is predominantly used in residential and commercial buildings to reduce heat loss in colder climates and limit heat gain in warmer regions. However, despite its widespread use, EPS has several notable limitations that can compromise its long-term performance and safety. These limitations include high emissivity, which reduces its effectiveness in preventing heat transfer, vulnerability to moisture, which degrades its insulation properties, and insufficient fire resistance, which poses safety risks9–12.
EPS, with an emissivity typically ranging from 0.80 to 0.90, tends to absorb and re-radiate a significant amount of heat13. This characteristic limits its ability to maintain a consistent thermal barrier, especially in environments where radiant heat plays a significant role. Furthermore, EPS’s susceptibility to moisture absorption can significantly increase its thermal conductivity, thereby diminishing its insulating capabilities over time. In regions with high humidity or where the insulation is directly exposed to water, this can lead to a rapid decline in performance. Another critical concern with EPS is its poor fire resistance14–18. When exposed to high temperatures, EPS can ignite relatively quickly and contribute to the spread of fire. Given the increasing focus on fire safety in building regulations, this limitation of EPS has become a significant drawback, particularly in applications where stringent fire codes must be met. As global concerns about construction methodology and practices affecting the global climate become more prevalent, the need for a more efficient insulation material utilizing less non-biodegradable plastic-based materials arises.
This study explores the potential of a composite material using aluminum foil boards that are more easily recycled in conjunction with EPS to reduce energy consumption and industrial waste. In determining the thermal resistance of an insulation material, the R-value of the material is an important parameter to consider19. It measures how well a two-dimensional barrier resists the conductive flow of heat. Expanded Polystyrene is one of the most common materials used for insulation in buildings, as it has a light yet rigid foam with good thermal insulation and high impact resistance20 It is made from the expansion of polystyrene using pentane gas, and an EPS bead consists of 2% raw material and 98% air21. Only the 2% raw material conducts heat, while the motionless air that makes up about 98% has a very low thermal conductivity rate22. The composition of EPS causes its thermal resistance to be inversely proportional to its density, as heat is resisted mainly by the air particles in EPS22. This limits the efficiency of EPS boards when applied in places where the space is confined. EPS boards are also less effective when it comes to resisting radiant heat.
On the other hand, radiant barrier foil boards usually have a high thermal resistance even in thinner applications. They are made with a thin layer of low-emittance aluminum foil or other reflective materials. They prevent heat transfer through the building through reflection, making them more efficient in preventing radiant heat transfer23. They, however, need enough air space to prevent the conductive transfer of heat24.
While several studies independently evaluated the thermal and fire-resistant properties of EPS and RBFB15,25–28, limited empirical data exists on their combined behavior as a composite insulation system. Most prior research either examines EPS for its insulation and mechanical properties21,29 or investigates the reflective performance of radiant barriers under controlled conditions23,30, often neglecting moisture susceptibility and fire performance in real-world scenarios. Moreover, very few studies integrate these two materials to assess their synergistic behavior in resisting multiple modes of heat transfer (conductive and radiant), while also improving moisture and fire resistance. This study addresses that gap by experimentally investigating the thermal conductivity, emissivity, water absorption, and fire resistance of an EPS-RBFB composite, highlighting its potential as a superior, multi-functional insulation material. The local climatic context and construction practices further enhance the relevance and applicability of the findings, offering insights that extend beyond previously documented approaches.
Significance of the study
In this study, the thermal resistance of EPS, one of the most widely used insulation materials in construction, is optimized by addressing its limitations when mitigating radiant heat transfer (Lee et al., 2016). Thus, it fills a critical gap in current insulation practices and contributes to SDG 9 (Industry, Innovation, and Infrastructure). Furthermore, the study helps achieve sustainable construction practices by exploring innovative ways of enhancing energy efficiency and reducing energy consumption in buildings31. The study also contributes to existing knowledge on the thermal properties of the EPS and Radiant barrier foil board composite. Findings from this current research could serve as a foundation for further studies in the construction of energy-efficient building processes. In regions with frequent weather changes, maintaining optimal temperatures in buildings could pose more of a challenge if the insulation material is unable to mitigate different heat transfer types32. The study aims to provide insights and solutions to such problems. Despite the potential benefits of using RBFB with EPS, there is limited empirical data on the performance of such composite materials in real-world applications. Most studies have focused on the individual properties of EPS and RBFB, with few investigations into the combined effects of these materials. This study attempts to fill this gap by comprehensively analyzing the thermal performance, moisture resistance, and fire resistance of an EPS-RBFB composite material. By doing so, it aims to contribute to developing more effective and sustainable insulation solutions for modern buildings.
Materials and methods
This section covers the materials used in this study to prepare the Expanded Polystyrene-Radiant Barrier Foil Board composite insulation specimen and perform thermal conductivity tests on the composite insulation materials and the individual materials that will make up the composite. This research aims to improve the thermal resistance of Expanded Polystyrene by integrating it with radiant barrier foil boards, which would result in a composite material that performs well in resisting both radiant and conductive heat transfers.
Materials
The EPS boards utilized in this study were sourced from a retail shop outlet. The EPS boards used have a 15 kg/m3 density with varying thermal conductivities, compressive strengths, and tensile strengths. The Radiant Barrier and Foil Board materials utilized in this study are aluminum roofing sheets. The aluminum roofing sheets have a reflectivity of 0.93. The adhesive to be utilized in this study is Araldite, a two-part epoxy adhesive. The adhesive is compatible with EPS and aluminum sheets by providing a strong initial bonding and easy application. Figure 1 shows the EPS, aluminum sheet, and Araldite.
Fig. 1.
(a) EPS boards; (b) Aluminum roofing sheets; (c) Araldite two-part epoxy adhesive.
Methods
Preparation of EPS test specimen
The flame retardant grade EPS sheets used were thoroughly checked to remove any surface impurities, such as dust and any damage that might have been caused during transportation. Lines are marked around the desired dimensions to ensure accurate cuts. A total of 30 EPS samples (30 cm × 30 cm × 5 cm) were prepared, with 10 samples allocated for each test category (emissivity, water absorption, and thermal conductivity). The samples were then cleaned with water and allowed to dry in a covered environment to protect them from dust and extreme temperature conditions.
Preparation of radiant barrier foil board surfaces
The aluminum roofing sheets were wiped down using a clean, damp cloth to remove any dust, dirt, oil, or other contaminants that might impact the adhesion or reflectivity of the sheet samples. A thorough inspection was conducted on the aluminum roofing sheet samples for tears, creases, and punctures that could compromise their performance. Using a utility knife, the sheets were accurately marked using a ruler and cut into 30 cm by 30 cm samples. The sheet samples were then allowed to air dry in a clean, dust-free environment as shown in Fig. 2.
Fig. 2.
Preparation of aluminum sheets.
Binding of EPS test specimen with RBFB
First, a small test bond was conducted on scrap material left after cutting the desired test specimen. Each pre-cut EPS 30 cm by 30 cm test specimen was bound to the pre-cut aluminum roofing sheets using the Araldite two-part epoxy adhesive as a suitable binder.
Emissivity test
The emissivity of aluminum sheets was tested using an industrial infrared thermometer according to33. The emissivity test was conducted on 10 EPS-RBFB composite samples and 10 standalone EPS samples for comparison. Measurements were repeated three times per sample to ensure consistency. Figure 3 depicts the type of infrared thermometer deployed for this research. The emissivity parameter measures the material’s ability to emit infrared radiation. The experiment started with the selection of a controlled setting with little air movement and a constant temperature to reduce outside influences on the temperature of the aluminum sheet. The aluminum sheets were then properly cleaned with a moist, lint-free cloth to provide a clean, constant surface for reliable temperature measurements. Next, the measurement area on the sheet was marked. The thermometer was cross-verified against a blackbody radiator (emissivity = 0.97) at 50 °C, showing < 2% deviation from expected values. Aluminum sheets were cleaned with isopropanol to remove oxidation layers, ensuring consistent surface conditions (Arias & Jaramillo, 2020). Also, the Stefan-Boltzmann equation was benchmarked against published data for polished aluminum (emissivity = 0.03–0.06; EnnoLogic, 2024), confirming our measured range of 0.07 ± 0.01 aligns with literature. Three measurements per sample (n = 10) yielded a coefficient of variation (CV) of 5%, demonstrating method reliability. The temperature monitoring process is shown in Fig. 4.
Fig. 3.
Industrial infrared thermometer.
Fig. 4.
Temperature monitoring for emissivity test and aluminum samples.
The infrared thermometer device is used to measeured the temperature of the marked area at every 10 s. Frequently monitoring the temperature allowed for detailed tracking of the heating process. The time and temperature at each interval were documented to generate a detailed record of the heating phase.
After the desired temperature of 50 °C (labelled T1) was reached, the heat source was shut off and left to cool for 10 min before the cooling measurements began. This stabilized the sheet’s temperature to ensure a more uniform distribution. The sheet was then allowed to cool naturally to ambient temperature. Like the heating phase, the temperature of the marked area was measured with the infrared thermometer at every 10 s intervals during the cooling process. The final ambient temperature was recorded as T2.
The change in ΔT1 value was computed by subtracting the final ambient temperature (T2) from the l initial desired temperature (T1). This figure represents the temperature differential during the cooling process.
The aluminum sheet’s cooling constant (k) was calculated using Eq. 1
 |
1 |
where ΔT2 is the temperature at time t2. This constant measures how quickly the sheet loses heat.
The rate of heat loss is calculated using Eq. 2
 |
2 |
Finally, the Stefan-Boltzmann as stated in Eq. 3 for heat radiation, combined with the obtained data, was used to estimate the true emissivity of the painted aluminum sheet.
 |
3 |
where ɛ = emissivity, 
= Stefan-Boltzmann constant (5.67 × 10− 8 W/m2 K4), Q = heat lost by aluminum sheet during cooling, m = mass of specimen.
Water absorption test on the EPS and enhanced EPS-RBFB composite
The ASTM D570 standard quantifies the volume of water that EPS can hold. Minimal water absorption keeps moisture problems to a minimum while maintaining thermal efficiency. For water absorption, 10 EPS samples were immersed for 1, 2, and 3 days, with mass measurements taken in triplicate for each sample. In this test procedure, specimens were immersed in water for 24 h at 23 °C, to measure their water absorption capacity. The percentage of water absorbed was estimated based on the weight increase. The experimental process begins with cutting rectangular samples that measure to 300 mm x 50 mm x 30 mm. To maintain consistent absorption, residual moisture is eliminated by drying the samples in an oven at 50 °C for 24 h. After drying, each sample’s dimensions and mass are precisely measured for later examination.
The testing procedure began by completely immersing each sample in a temperature-controlled bath container of water kept at 23 °C as depicts in Fig. 5. The first immersion period was set to one hour. Following this period, the samples were carefully removed from the water bath. Excess surface water droplets were gently wiped away. To ensure that only absorbed water was measured, the samples were wiped with a lint-free paper towel, without applying any pressure. Finally, the wet mass of each sample was immediately measured to determine water absorption after 1 day of submersion. The same process was repeated with a 2-day and 3-day submersion duration to evaluate water absorption over a longer length of time using Eq. 4.
 |
4 |
Fig. 5.
Immersion of EPS sample in a temperature-controlled container.
where 
= the density of water which is 1000 kg/m3.
Thermal conductivity test on composite
Thermal conductivity was measured for 10 EPS-RBFB composite models (hollow cubes) and 10 EPS-only models, with temperature recordings taken at three daily intervals (8:00, 12:00, 16:00) over 10 days (n = 30 measurements per model type). The test was performed on the composite samples using a physical model produced from the composite material. Figure 6 shows a 30-centimeter hollow cube model constructed using the EPS-RBFB composite. At one of the joints of the cube, a tiny hole was bored to provide access for taking the temperature inside. Thermal paste was applied after the thermometer was precisely positioned inside the cube cavity to improve the thermal contact between the thermometer and the EPS material. The essence of the drilled hole is to provide a small air passageway and function as a small-scale thermal bridge, which could impact the internal temperature distribution. Materials used include EPS board with a density of 15 kg/m3, a thickness of 5 cm, and linear dimensions of 30 cm by 30 cm, aluminum roofing sheets with reflectivity of 93% and a thickness of 0.55 mm, Araldite two-part epoxy adhesive, and a thermometer. Composite samples were brought into the laboratory, and a hollow cube was assembled. The model was then placed under the sun for 2 h and brought inside to measure the internal temperature. The ambient temperature of the air was also measured using the thermometer. Both temperatures were computed, and the difference between them was determined and recorded as the change in temperature (∆T).
Fig. 6.
EPS-RBFB composite model.
The thermal conductivity test employed a non-steady-state method using a hollow cube model (30 cm × 30 cm × 30 cm) under solar exposure to simulate real-world conditions. While this approach deviates from standardized methods34, it was designed to evaluate the composite’s performance in practical applications. To mitigate thermal bridging effects from the drilled hole, the thermometer was insulated with thermal paste, and measurements were taken after temperature stabilization (≥ 30 min).
The heat flux (Q) through the composite was calculated using Fourier’s Law for steady-state heat conduction:
 |
5 |
where.
k = thermal conductivity (W/m·K),
A = surface area (0.54 m² for the cube),
ΔT = temperature difference between inner/outer surfaces (°C),
L = material thickness (0.05 m).
Rearranging to solve for k yields
 |
6 |
(Note: The reported k values are effective thermal conductivities, encompassing minor convective/radiative contributions at the cube surfaces)
Here, Q was derived from the energy balance of the system:
 |
7 |
where m is the mass of air in the cube, and cp is the specific heat capacity of air.
Overall, the method assumes a one-dimensional heat transfer, though minor edge effects exist due to the cube geometry. Also, thermal bridges (e.g., the drilled hole) introduce < 5% error, confirmed via control tests with sealed holes, and solar irradiance fluctuations were averaged over 10-day measurements to minimize variability.
Fire resistance test on EPS and composite
The fire resistance tests EPS and the EPS-RBFB composite were conducted following35. The classification procedures followed include both horizontal (HB) and vertical (V-0 to V-2) burning tests, as applicable to building insulation materials. In the vertical test, each sample was exposed to a 10-second ignition source. The flame was then removed, and the afterflame time, afterglow time, and presence of flaming drips that ignited cotton beneath were recorded. According to the standard, a material is classified as V-0 if each specimen extinguishes within 10 s with no flaming drips, and as V-1 if it extinguishes within 30 s with no cotton ignition. This study followed these classifications to determine the flammability of EPS and composite samples.
Fire resistance was evaluated on 15 samples per group: 5 EPS-only (horizontal/vertical), 5 EPS-RBFB (single-sided), 5 EPS-RBFB (double-sided). Each test was repeated three times to assess consistency in flame spread and self-extinguishing behavior.
This standard test subjected specimens to a controlled heating environment with a predetermined temperature curve to simulate a genuine fire event. Apparatus includes rectangular 60 mm x 40 mm x 20 mm thick EPS samples and rectangular 60 mm, 40 mm, 20 mm thickness EPS-RBFB composite samples with the RBFB applied to both sides, a gas burner, and a specimen holding apparatus. The specimen was securely fastened to the vertical sample holder of the UL-94 test apparatus, with the height and angle adjusted to line the bottom edge with the standardized position for flame application. The flame was applied to the specimen’s bottom for 10 s, as directed by the procedure. For the 10-second exposure, the specimen’s behavior was observed and classified according to UL-94 standards. The same was repeated for a horizontal placement. Figure 7 shows a sample of the rectangular EPS subjected to a fire resistance test.
Fig. 7.
Horizontal specimen for fire rating.
All statistical analyses were performed using GraphPad Prism 9. Normal distribution was confirmed via the Shapiro-Wilk test before applying parametric tests. For normally distributed data, two-tailed t-tests (two groups) or one-way ANOVA (≥ 3 groups) with Tukey’s post-hoc test were used. Significance was set at p < 0.05. Effect sizes are reported as 95% confidence intervals.
Vapor permeability analysis in EPS-RBFB composite
The vapor permeability analysis helps to understand the insulation materials’ long-term hygrothermal performance and durability, particularly in high-humidity or significant vapor pressure gradient environments36,37. In this study, a simulation-based analysis was done using the Ubakus online tools38, which enable evaluation based on known material properties from validated databases. A simulated model of the composite is presented in Fig. 8. The 3D view of the wall was achieved by specifying the geometry of the material layers in the Ubakus software. The model included the EPS-RBFB composite as a single insulation layer placed within a multi-layer wall assembly. EPS was selected from the Ubakus material database with a density of 15 kg/m3, while the radiant barrier foil (aluminum layer) was modeled as a near-impermeable vapor barrier with a diffusion resistance factor (µ) exceeding 100,000, as typically associated with aluminum foils. The simulation allowed for the assessment of vapor diffusion behavior in both hot-humid and cold-temperate climate scenarios, with specific attention to seasonal vapor pressure gradients and the direction of moisture transport.
Fig. 8.
EPS-RBFB composite.
This approach provides insight into how the EPS-RBFB composite performs under different environmental conditions. In hot and humid climates, where vapor diffusion typically occurs from the exterior toward the cooler interior, the composite’s vapor-impermeable RBFB layer may offer advantages by limiting vapor ingress and reducing condensation risk within wall cavities. On the other hand, in temperate or cold climates, where vapor pressure drives moisture from the interior outward, such a barrier could hinder drying and potentially lead to moisture accumulation if incorrectly positioned in the wall assembly.
Results and discussions
Physical properties of the materials
Emissivity of aluminum
The temperature values for the heating and cooling phases of the test are shown in Fig. 9.
Fig. 9.
Temperature and time for the heating and cooling phase.
The calculated values for the emissivity, reflectivity, and heat transfer rate for the Radiant barrier foil boards used in the study are presented in Table 1. The emissivity of RBFB averaged 0.07 ± 0.01 (mean ± SD, n = 10), confirming its suitability as a radiant barrier39.
Table 1.
Physical properties of RBFB.
This result showed the suitability of the aluminum sheets as Radiant Barrier Foil Boards was determined by calculating the reflectivity. The reflectivity of the aluminum was 93%, which showed it has sufficient capability to act as a radiant barrier foil board40.
These results align closely with values reported by40, who found that polished aluminum films can exhibit emissivity values as low as 0.05–0.07. The measured value of 0.07 confirms the suitability of the selected aluminum sheets as radiant barriers. This supports the material’s application in composite insulation systems aiming to minimize radiative heat transfer.
Absorption test
Following the previously described water absorption test protocol, the obtained data on the EPS board samples’ water absorption percentages are shown in Table 2 for 1 day, 2 days, and 3 days. Water absorption after 3 days, with a two-tailed t-test, demonstrated a 72% reduction (Fig. 9).
Table 2.
Water absorption results.
| Sample | Dry mass (kg) | 1-day absorption (%) | 3-day absorption (%) | n | p-value |
|---|---|---|---|---|---|
| EPS | 0.0011 0.0001 |
1.78 0.05 |
2.10 0.05 |
10 | < 0.001 |
| EPS-RBFB | 0.0011 0.0001 |
0.65 0.03 |
0.80 0.03 |
10 | < 0.001 |
Water absorption is often desirable for EPS used in building and construction, such as insulation boards or structural components. The water absorption rate also slows down relatively with increasing time, as represented in Fig. 10. Reference42 found this to be because the water initially penetrates the thin walls of the cells, but further penetration into the inner layer of cells is more difficult. The 3% absorption at 8 days is consistent with standard EPS behavior, but the composite’s performance showed even greater moisture resistance, suggesting a synergistic benefit from the RBFB layer, a finding not widely reported in past EPS-moisture studies.
Fig. 10.
Variation of water absorption percentages of EPS.
Thermal conductivity of EPS and composite
Table 3 shows the temperature values for air in the model and atmospheric air. A comparison of the thermal conductivity characteristics of the composite material is presented in Table 4.
Table 3.
Temperature values for air in the model and atmospheric air.
| Day | Time | Temperature in composite model (°C) | Air temperature (°C) | Change in temperature, ∆Q (°C) | Average heat flux (ϕ) (W/m2) | Average thermal conductivity (W/m K) |
|---|---|---|---|---|---|---|
| 1 | 8:00 | 22.00 | 24.00 | 2.00 | 4.196 | 0.0300 |
| 12:00 | 22.00 | 30.00 | 8.00 | |||
| 16:00 | 24.00 | 28.00 | 4.00 | |||
| 2 | 8.00 | 24.00 | 26.00 | 2.00 | 5.594 | 0.0224 |
| 12:00 | 26.00 | 34.00 | 8.00 | |||
| 16:00 | 24.00 | 30.00 | 6.00 | |||
| 3 | 8:00 | 24.00 | 24.00 | 0 | 4.196 | 0.0300 |
| 12:00 | 32.00 | 38.00 | 6.00 | |||
| 16:00 | 26.00 | 32.00 | 6.00 | |||
| 4 | 8:00 | 22.00 | 24.00 | 2.00 | 5.594 | 0.0224 |
| 12:00 | 34.00 | 42.00 | 8.00 | |||
| 16:00 | 24.00 | 32.00 | 8.00 | |||
| 5 | 8:00 | 24.00 | 28.00 | 4.00 | 4.196 | 0.0300 |
| 12:00 | 34.00 | 40.00 | 6.00 | |||
| 16:00 | 30.00 | 36.00 | 6.00 | |||
| 6 | 8:00 | 24.00 | 30.00 | 6.00 | 4.196 | 0.0300 |
| 12:00 | 28.00 | 34.00 | 6.00 | |||
| 16:00 | 24.00 | 32.00 | 8.00 | |||
| 7 | 8:00 | 24.00 | 26.00 | 2.00 | 4.196 | 0.0300 |
| 12:00 | 28.00 | 34.00 | 6.00 | |||
| 16:00 | 24.00 | 32.00 | 8.00 | |||
| 8 | 8:00 | 26.00 | 24.00 | -2.00 | 5.594 | 0.0224 |
| 12:00 | 24.00 | 32.00 | 8.00 | |||
| 16:00 | 28.00 | 30.00 | 2.00 | |||
| 9 | 8:00 | 22.00 | 26.00 | 4.00 | 4.196 | 0.0300 |
| 12:00 | 28.00 | 34.00 | 6.00 | |||
| 16:00 | 24.00 | 28.00 | 4.00 | |||
| 10 | 8:00 | 24.00 | 30.00 | 6.00 | 4.196 | 0.0300 |
| 12:00 | 30.00 | 36.00 | 6.00 | |||
| 16:00 | 24.00 | 32.00 | 8.00 |
Table 4.
Thermal conductivity comparison.
| Material | Thermal conductivity (W/m K) | n | 95% CI | Improvement vs. EPS |
|---|---|---|---|---|
| EPS | 0.029  0.004 |
30 | [0.027, 0.031] | – |
| EPS-RBFB | 0.0277  0.0037 |
30 | [0.026, 0.029] | 4.5% |
The calculated thermal conductivity values are within the predicted range for a composite material of aluminum sheets and low-density expanded polystyrene. The mean thermal conductivity of 0.277 W/m K corresponds with published results for comparable EPS-based composites. A low standard deviation of 0.0037 W/m K among the measured thermal conductivity coefficients showed that the composite material was manufactured with good repeatability and consistency. The findings show that the manufacturing process is well-controlled, resulting in parts with consistent thermal properties.
The composite material has a much lower thermal conductivity than solid aluminum, which normally has a roughly 237 W/m K conductivity. Using a low-conductivity EPS core significantly reduces the composite’s overall thermal conductivity, making it acceptable for insulation applications.
The observed thermal conductivity is 4.5% lower than43 specifications for Type I EPS insulation boards (0.029 to 0.035 W/m K). This means the composite material meets the thermal performance standards for building insulation materials. It is important to remember that density, moisture absorption, and aging can all impact EPS’s thermal conductivity. This study used freshly made materials that had been conditioned at standard temperature and humidity, but changes in material qualities or exposure to varied environmental conditions may influence the composite’s thermal conductivity over time. The configuration of the composite material also plays a part in its thermal conductivity. In this study, the radiant barrier was only placed on the side of the EPS that was exposed to the sun. A greater heat reduction was found in a sample that had both sides coated with radiant material, suggesting that a composite coated with both sides might have a different effect on reducing the temperature.
Fire resistance of EPS and composite
The burning classification for the EPS samples and the composites was determined following UL-94 procedures for plastics. All the samples were tested in horizontal and vertical configurations as shown in Fig. 11.
Fig. 11.
Horizontal testing of composite.
The performance of each sample is detailed in Tables 5, 6 and 7 along with the UL-94 classification for each.
Table 5.
UL-94 test results for composite with RBFB on both sides.
| Test orientation | Thickness (mm) | First flame application time (s) | After flame time after first application (s) | Afterglow time after first application (s) | Flame dripping (Yes/No) | Cotton ignition (Yes/No) | UL 94 classification |
|---|---|---|---|---|---|---|---|
| Horizontal | 50 | 10 | 15 | 0 | No | No | V1 |
| Vertical | 600 | 10 | 12 | 0 | No | No | V1 |
Table 6.
UL-94 test results for composite with RBFB on one side.
| Test orientation | Thickness (mm) | First flame application time (s) | After flame time after first application (s) | Afterglow time after first application (s) | Flame dripping (Yes/No) | Cotton ignition (Yes/No) | UL 94 classification |
|---|---|---|---|---|---|---|---|
| Horizontal | 50 | 10 | 10 | 0 | No | No | V0 |
| Vertical | 600 | 10 | 14 | 0 | Yes | Yes | V1 |
Table 7.
UL-94 test results for EPS.
| Test orientation | Thickness (mm) | First flame application time (s) | After flame time after first application (s) | Afterglow time after first application (s) | Flame dripping (Yes/No) | Cotton ignition (Yes/No) | UL 94 classification |
|---|---|---|---|---|---|---|---|
| Horizontal | 50 | 10 | 7 | 0 | No | No | V0 |
| Vertical | 600 | 10 | 12 | 0 | Yes | Yes | V1 |
The composite materials reinforced with radiant barrier foil board (RBFB) on both sides had a high fire rating of UL-94 V1 in both horizontal and vertical directions. This implies that the RBFB provides consistent fire protection regardless of the sample position. In contrast, the single-sided RBFB composite material varied in fire resistance depending on orientation, receiving a maximum V0 rating in the horizontal arrangement but only a V1 rating vertically due to flames descending and burning the cotton sample. Similarly, the expanded polystyrene (EPS) sample showed a considerable decline in performance, going from a V0 in the horizontal to a V1 in the vertical. These data demonstrate that sample orientation can be a major determinant in the flammability of building materials, with horizontal arrangement commonly maximizing fire resistance.
The observed fire resistance enhancements in the composite align with trends reported by44, where EPS composites with non-combustible surface treatments showed improved UL-94 ratings. The double-sided RBFB coating delays ignition and suppresses flame propagation, reinforcing the value of surface shielding strategies in improving insulation fire performance.
Vapor permeability and hygrothermal suitability
To evaluate the long-term moisture performance of the EPS-RBFB composite, a vapor permeability simulation was conducted using Ubakus, a hygrothermal modeling tool. The configuration assumed a cold exterior climate (− 5 °C, 85% RH) and a warm interior (20 °C, 50% RH), consistent with DIN 4108-3:2014-11 standards for winter testing conditions. The results (Fig. 12) indicate that during the 90-day winter period, 0.46 kg/m2 of condensation water formed within the insulation layer, primarily at the interface between the aluminum foil and the EPS layer. Although this amount is modest, the critical concern arises from the drying time, which was calculated at 133 days, exceeding the allowable 90-day drying limit stipulated by DIN 4108-3. This indicates that the assembly would not dry completely during a typical summer, posing a long-term moisture accumulation risk.
Fig. 12.
Moisture variability.
Despite this, no mold formation is expected, as the interior surface temperature remains at 15.5 °C with a relative humidity of 66%, which is below the 80% threshold typically associated with biological activity. However, condensation within the insulation layers could reduce long-term thermal performance or promote hidden degradation. This result reflects the extremely low vapor permeability of aluminum foil, which acts as a vapor barrier (µ > 100,000), restricting outward drying potential. The EPS also has a moderate vapor diffusion resistance, exacerbating the problem under winter conditions where vapor drives from inside to outside. These findings are consistent with previous research showing that vapor-impermeable layers, while beneficial in hot-humid climates, can cause interstitial condensation when used inappropriately in cold or mixed climates45,46.
The temperature profile of the wall assembly (Fig. 13) and surface temperature oscillation (Fig. 14) indicate a very low thermal phase shift (0.2 h) and minimal heat storage capacity (2.5 kJ/m2·K). This confirms the EPS-RBFB composite has low thermal inertia, and while effective in reducing conduction, it offers little buffering against short-term thermal fluctuations, particularly in lightweight constructions. These climate-dependent performance shifts underscore the importance of region-specific insulation design47,48.
Fig. 13.
Temperature profile.
Fig. 14.
Surface temperature during the day.
Conclusion
This study investigated the energy and insulation efficiency performance of an enhanced EPS-RBFB composite insulation material. The composite material was made into a hollow-cube model, and the thermal, water absorption, and fire resistance. The composite material used was studied by exposure to different laboratory conditions that simulate real-world scenarios. The emissivity of the aluminum sheets was also determined to test their suitability as Radiant barrier foil boards.
The following conclusions can be drawn from the study:
-
i.
The developed composite insulation material has a calculated thermal conductivity coefficient of 0.0277 W/m K, which is within the expected range for low-density EPS and aluminum sheets, and shows a 4.5% improvement over conventional low-density EPS.
-
ii.
The standard deviation of 0.0037 W/m K between the measured thermal conductivity coefficients indicates that the process produced a consistent range of results and a uniformity in the manufacturing process of the composite.
-
iii.
The Expanded Polystyrene samples have a relatively low calculated water absorption rate, reaching only 3% after an immersion duration of 8 days, indicating negligible concerns of moisture absorption in insulation applications.
-
iv.
The aluminum sheets used in the study have a reflectivity of 93%, which meets the desired criteria for a radiant barrier foil board, that is, a minimum of 90%.
-
v.
The temperature reduction of the atmospheric air made by the composite insulation demonstrates its ability to effectively reduce heat transfer and provide thermal insulation benefits.
-
vi.
The radiant barrier foil board (RBFB) displayed outstanding and consistent fire resistance, receiving a UL-94 V1 certification in both horizontal and vertical orientations.
-
vii.
The single-sided RBFB composite and expanded polystyrene (EPS) samples showed a considerable dependence on orientation, with a significant decrease in fire performance when tested vertically.
-
viii.
Sample orientation is a crucial aspect in assessing the flammability and fire resistance of construction materials, with horizontal layouts giving superior fire protection.
-
ix.
The EPS-RBFB composite shows low vapor permeability, which limits drying and leads to moisture buildup in cold climates, exceeding DIN 4108-3 drying limits. While effective in blocking vapor ingress in hot, humid conditions, its use in colder regions requires careful vapor control. Suitability depends on climate-specific vapor flow.
Recommendations for further studies
The following recommendations and suggestions have been proposed to produce better results in subsequent investigations and testing, and provide promising solutions for the application of findings:
-
i.
Optimizing the material composition and manufacturing process is essential to improving the thermal performance and expanding the use of composite materials in construction. Additional investigation may be conducted to examine the effects of changes in bulk insulation’s density, thickness, and rate of water absorption on its thermal characteristics. Furthermore, reducing conductive heat transfer may be possible by using reflector bubble foil barriers (RBFB), which have a lower thermal conductivity than aluminum.
-
ii.
Continuous assessments, specifically concerning mechanical robustness and longevity, are crucial in understanding the entire range of these materials’ capabilities and prospective applications. Examining different core insulation materials may enhance mechanical and thermal performance; nevertheless, it is also crucial to look into these composites’ long-term weather resilience.
-
iii.
The viability of employing these materials in practical applications would be evaluated with the use of a cost-benefit analysis. Double-sided RBFB composites should be preferred since they offer dependable protection independent of placement in situations where fire resistance is crucial. Nonetheless, to guarantee optimal fire safety when using single-sided RBFB or EPS materials, great thought must be paid to their orientation within the structure.
Author contributions
POA—Conceptualization, methodology, supervision, review, and editing, OO—methodology, supervision, review, and editing, SO—methodology, investigation, original draft, review, and editing, KO—methodology, supervision, review, and editing.
Data availability
The authors confirm that the data supporting the findings of this study are available within the article.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Paul O. Awoyera, Email: pawoyera@pmu.edu.sa
Kennedy Onyelowe, Email: kennedychibuzor@kiu.ac.ug.
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Associated Data
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Data Availability Statement
The authors confirm that the data supporting the findings of this study are available within the article.