Eco-Friendly Sandwich Panels for Energy-Efficient Façades

Abstract

To meet the European Green Deal targets, the construction sector must improve building thermal performance via advanced insulation systems. Eco-friendly sandwich panels offer a promising solution. Therefore, this work aims to develop and validate a new eco-friendly composite sandwich panel (basalt fibres and recycled extruded polystyrene) with enhanced multifunctionality for lightweight and energy-efficient building façades. Two panels were produced via vacuum infusion—a reference panel and a multifunctional panel incorporating phase change materials (PCMs) and silica aerogels (AGs). Their performance was evaluated through lab-based thermal and acoustic tests, numerical simulations, and on-site monitoring in a living laboratory. The test results from all methods were consistent. The PCM-AG panel showed 16% lower periodic thermal transmittance (0.16 W/(m²K) vs. 0.19 W/(m²K)) and a 92% longer time shift (4.26 h vs. 2.22 h), indicating improved thermal inertia. It also achieved a single-number sound insulation rating of 38 dB. These findings confirm the panel’s potential to reduce operational energy demand and support long-term climate goals.

1. Introduction

As the global population and living standards continue to rise, the proportion of global electricity consumption attributed to buildings has increased significantly. The building sector is highly energy-intensive, primarily due to the demand for thermal comfort, where the energy is mainly used for space heating and cooling systems that maintain desirable indoor temperatures. In particular, residential buildings account for over 60% of total energy use in this sector, and consumption is expected to rise further. This rising demand for thermal energy underscores the urgent need to adopt more sustainable and energy-efficient practices in the building sector, particularly in alignment with the European Green Deal’s objective of achieving climate neutrality by 2050 [1,2,3,4,5].

A key driver of energy use in buildings is the performance of thermal insulation materials, which directly affects heating and cooling loads. Thus, innovations in both the design and material composition of building envelopes are central to reducing energy consumption and associated emissions. However, conventional insulation solutions typically offer single-functionality and often necessitate additional structural components, which can introduce redundancy, increase construction costs, and complicate assembly logistics [4,6].

In contrast, multifunctional materials integrated directly into structural components present opportunities for enhanced performance, while reducing construction complexity. Instead of applying standalone insulation materials to existing building envelopes, multifunctional materials offer additional properties, such as improved thermal insulation, acoustic absorption, water repellence, passive cooling, environmental sensing, and energy harvesting. However, separating the functions of insulation and the building envelope often leads to redundant structural layers, higher construction costs, increased maintenance requirements, and performance control challenges [1,2,3,7].

One approach that addresses these challenges is the use of composite sandwich panels, which have emerged as a promising solution due to their low density and compatibility with multifunctional materials. These panels consist of two exterior face sheets (laminates) bonded to a thicker core, which enables the spatial separation and integration of various functional layers. Their unique configuration supports a broad range of applications, including thermal insulation and soundproofing. Sandwich panels are particularly suitable for dry-layer prefabricated construction techniques, as they do not require wet processes (e.g., traditional masonry) or insitu panel construction, thereby accelerating assembly and minimizing construction waste [8,9,10]. Despite their potential, current developments in multifunctional composite sandwich panels face several challenges. Compatibility issues between material layers may cause delamination, necessitating careful selection of materials to ensure structural integrity and ease of handling during construction [6,9,11,12,13]. Additionally, large discrepancies in thermal expansion coefficients, especially between sun-exposed exterior layers and interior components, can lead to mechanical failures such as core swelling and delamination [1,2,3].

Although growing attention has been given to sustainable sandwich panel systems, systematic assessments of their real-world performance remain limited. Recent efforts have focused on incorporating phase change materials (PCMs) and aerogels (AGs) into wall panels due to their excellent thermal performance and superior insulation properties. However, only a limited number of studies have explored the combined use of PCMs and AGs in a single wall system [14,15,16,17]. Findings suggest that sandwich panels made from materials such as fiberglass, recycled extruded polystyrene (XPS), epoxy resin, and cement mortar offer greater sustainability compared to conventional brick masonry. Among these, cement mortar often contributes the most to the panel’s overall environmental impact [6,18]. Recycled XPS is increasingly being used in sustainable construction, addressing both environmental concerns and the demand for high-performance building materials. Recent research has demonstrated the feasibility of manufacturing composite sandwich panels using recycled XPS [19]. Additionally, basalt fibres have emerged as a more sustainable alternative to traditional glass fibres. Derived directly from basalt rock without the need for chemical additives or precursors, basalt fibres offer enhanced mechanical performance and environmental benefits [20].

Additionally, these fibres have enhanced performance when compared to the traditional glass fibres. Accordingly, combining basalt fibres with complementary materials such as PCMs and AGs may enable a more sustainable, multifunctional panel with synergistic benefits for thermal management applications [21,22,23].

To address this research gap, the present study focuses on the design, development, and performance evaluation of an eco-friendly composite sandwich panel with enhanced multifunctionality. The proposed panel integrates carefully selected materials, each contributing to a specific performance domain, as follows: basalt fibres, as a low-energy alternative to synthetic fibres, improve both the mechanical strength and thermal stability of the panel due to their high tensile modulus and resistance to thermal degradation; recycled XPS provides core insulation with low environmental impact, helping to reduce heat transfer and overall energy demand; PCMs absorb and release latent heat, thereby regulating indoor temperatures and improving thermal inertia; AGs, known for their ultra-low thermal conductivity, further reduce heat flux and enhance insulation efficiency; and an omniphobic surface coating improves durability by repelling water and contaminants, thus protecting the integrity and longevity of the façade system. This holistic material integration supports the development of next-generation façade systems aligned with nearly zero-energy building (nZEB) requirements and broader climate adaptation strategies. This study aims to design, manufacture, and evaluate an innovative composite sandwich panel optimised for thermal and acoustic performance in façade applications at both the material and component levels, with a focus on the intrinsic behaviour of a homogeneous panel configuration under idealized conditions. To achieve the main goal of this study, the following specific research objectives were established: (i) develop a sustainable composite sandwich panel; (ii) assess the panel’s thermal performance under stationary and dynamic conditions through laboratory tests, numerical simulations, and on-site measurements; (iii) evaluate the acoustic insulation capacity of the panel through laboratory tests; and (iv) compare performance against a conventional baseline panel to quantify the benefits of PCM and AG integration. This approach enables an in-depth assessment of its potential for façade integration as an initial step toward full-system implementation.

2. Materials and Methods

2.1. Materials

Composite sandwich panels were fabricated using a combination of the following components: recycled XPS foam core (Soprema^®, Soprema Portugal, Porto, Portugal), epoxy resin (Biresin^® CR83/CH 83-6, SIKA Portugal, Porto, Portugal), basalt fibres (450 g/m², Castro Composites, Pontevedra, Spain), compact storage modules (aluminium cases with 15 mm) filled with PCMs (RT21HC, Rubitherm, Berlin, Germany), structural glue (HB AS 89/HB AW 89, HB Química, Porto, Portugal), mortar made with white cement (CEM I 52.5 N, kindly offered by Cimpor, Porto, Portugal) modify with silica AGs (0.14 g/cm³, TUHH, Hamburg, Germany), mortar made with grey cement (CEM II 32.5 N, Cimpor, Porto, Portugal), omniphobic coating (Cidetec, Donostia-San Sebastián, Spain). RT21HC is an organic PCM, selected due to its effectiveness in a vertically installed compact storage module system within the sandwich panel and to be aligned with indoor comfort temperature ranges (18 to 26 °C). Figure 1 illustrates the partial enthalpy distribution of RT21HC.

2.2. Manufacturing Procedures

The production of composite sandwich panels was achieved through the vacuum infusion process, employing two layers of fibres oriented at ±45° on both sides of the core material. The epoxy resin (A) was blended with the hardener (B) at ambient temperature in a ratio of 100A:30B, followed by a degassing phase (~10 min) to reduce the presence of air bubbles and potential defects in the final panels. Subsequently, a release agent was applied, and the peel ply, infusion mesh, spiral tubes, vacuum bag, and sealant tape were tactically positioned. To produce the sandwich panel in just one infusion step in the working table designed at INEGI (Figure 2), four spirals were placed to make the panel laminates simultaneously (Figure 3). The spiral fixation at the edge (top and bottom) of the core, made with adhesive tape and temporary anchors, aims to ensure the stability of the resin feed channels during vacuum application and facilitate uniform resin distribution across the panel surface. The spiral tubes allowed for an even resin front progression during infusion, critical for avoiding dry zones or air pockets, which can lead to localized mechanical weaknesses and reduced long-term durability [24,25]. The control of vacuum pressure or curing temperature/time parameters are also critical for replicating panel quality and performance. Therefore, a vacuum pressure of 500 mbar was established, followed by the execution of a leak test. After the inlet valve was opened to allow the resin to flow through the fibre layers. The panels were then cured at 500 mbar for 10 h at a temperature of 70 °C throughout the entire production procedure to ensure optimal resin flow and the saturation of fibre reinforcements and to achieve complete resin polymerization and enhanced interfacial bonding between layers. The curing temperature was selected based on the resin manufacturer’s datasheet and validated in previous studies through differential scanning calorimetry (DSC) analysis [26].

After demoulding the composite sandwich panel, its surface was sanded and cleaned to remove superficial imperfections, excess resin, and any artefacts left by the spiral tubing. This step is essential for preparing the substrate for subsequent layer applications, since sanding increases surface roughness, promoting mechanical interlocking and improving the adhesion of subsequently bonded materials [27,28]. Then, a structural glue was applied, and the PCM plates were added to the side corresponding to the inside of the building. To ensure good adhesion, weights were placed above the compact storage modules. Next, PCM plates were also sanded and cleaned, and a layer of basalt fabric was added and glued using a brush with epoxy resin to enhance the adhesion with the mortar. Finally, this surface was coated with a cementitious mortar. On the outside face, a white mortar modified with 15% (by replacing the sand) of silica AGs was used as a finish layer. This layer was then coated with an omniphobic coating to repel liquids and dirt. Figure 4 represents the multifunctional composite sandwich prototype that was developed and tested.

An additional panel was produced without the PCMs and the silica AGs to serve as a baseline to analyse the impact on the thermal properties of incorporating these two materials in the sandwich panel.

After demoulding the composite sandwich panel, its surface was sanded and cleaned to remove superficial imperfections, excess resin, and any artefacts left by the spiral tubing. This step is essential for preparing the substrate for subsequent layer applications, since sanding increases surface roughness, promoting mechanical interlocking and improving the adhesion of subsequently bonded materials [28,29]. Then, a structural glue was applied and the PCM plates were added to the side corresponding to the inside of the building. To ensure good adhesion, weights were placed above the compact storage modules. Next, PCM plates were also sanded and cleaned, and a layer of basalt fabric was added and glued using a brush with epoxy resin to enhance the adhesion with the mortar. Finally, this surface was coated with a cementitious mortar. On the outside face, a white mortar modified with 15% (by replacing the sand) of silica AGs was used as a finish layer. This layer was then coated with an omniphobic coating to repel liquids and dirt. Figure 4 represents the multifunctional composite sandwich prototype that was developed and tested.

An additional panel was produced without the PCMs and the silica AGs to serve as a baseline to analyse the impact on the thermal properties of incorporating these two materials in the sandwich panel.

2.3. Test Procedures

The composite sandwich panels were characterised to evaluate their thermal and acoustic properties. Accordingly, several techniques were employed at different levels, including tests at the laboratory scale, monitorisation in an onsite laboratory, and numerical simulation analyses.

2.3.1. Thermal Analysis

Both static and dynamic tests were carried out on the developed sandwich panels (baseline and multifunctional prototype) to analyse their thermal properties. The information on the diverse layers is presented in

Table 1. Stratigraphy and dimensions of the tested samples.

Sample	Layer/Material	Thickness [mm]
Sandwich Panel with PCM and Silica AG	Grey mortar + PCM	20
	Extruded polystyrene with basalt laminate	160
	White mortar + AG	10
Baseline Sandwich Panel	Grey mortar	15
	Extruded polystyrene with basalt laminate	160
	Grey mortar	15

. It is important to note that the dimensions needed to be adjusted according to limitations on the test equipment.

The thermal transmittance (U-value) of the components was measured according to ASTM C518 standard [29], and a heat flow meter apparatus (HFM) FOX600 (TA Instruments, New Castle, DE, USA) was used. The HFM is an instrument commonly used to assess the thermal properties of a material under steady-state heat flux conditions, such as thermal conductivity or thermal resistance. The system consists of a heating/cooling unit, heat flux sensors, and E-type thermocouples (temperature resolution ± 0.01 °C), which are placed on the specimen’s surfaces (upper and lower plates). The working principle relies on maintaining a constant temperature difference between the two sides of the sample, while measuring the heat flux density. To ensure accurate measurements and minimize edge effects, sensors are typically positioned in a limited measuring area (sizes 254 × 254 mm) relative to the total surface of the heated/cooled plates [29]. Figure 5a illustrates the main components of the HFM device and its operating principle.

Different temperatures were set in the upper and lower plates of the HFM, representing the inside and outside conditions of a typical building envelope component. The specific setpoints of the test can be found in

Table 2. Test conditions for measuring the thermal transmittance of the component in stationary conditions.

Sample	T_up [°C]	T_low [°C]	T_avg [°C]	ΔT [°C]	T_room [°C]
Sandwich Panel with PCM and Silica AG	18.02	28.01	23.00	10.00	28.86
Baseline Sandwich Panel	18.02	28.02	23.00	10.00	25.97

.

Additionally, to simulate the surface thermal resistance of the air layer on both surfaces of the sandwich panels, different numbers of layers of PEX (cross-linked polyethylene) were placed. One layer of PEX was placed on the exterior surface to attain an external surface resistance of ~0.04 (m²K)/W, and three layers were used for the interior to achieve an internal surface resistance of ~0.13 m²K/W. Additional temperature sensors, E–type thermocouples (temperature resolution ± 0.01 °C), were positioned between the samples and the PEX layers to monitor the surface temperatures reached by the component during the test and calculate the panel’s R-value (Figure 5a,b) [29]. The room conditions, temperature, and relative humidity were monitored using a Logger Testo 175-H2 (Figure 5c).

The dynamic thermal measurements were conducted using a heat flow meter apparatus, in dynamic mode, by setting dynamic sinusoidal temperature conditions [29]. Traditional HFM systems are designed to operate under steady-state conditions, meaning that the temperature difference remains constant over time. However, dynamic heat flow meter (DHFM) (TA Instruments, New Castle, DE, USA) devices have been developed as an advanced version of an HFM, integrating more sophisticated software and a control unit capable of managing time-dependent boundary conditions. These systems allow for temperature ramps, where the temperature gradually increases or decreases over time, and sinusoidal temperature fluctuations, which simulate periodic temperature variations that better reflect real conditions [29]. In this study, the second approach was adopted to evaluate how the samples responded to a sinusoidal temperature change on one side. The heat flow density of the opposite side, whose temperature is maintained constant, was tracked throughout time to do this evaluation.

Then, the periodic thermal transmittance (Y₁₂-Value) and the time shift were calculated following the EN ISO 13786:2017 standard [30], in which some definitions are given, as follows: In the first place, the periodic thermal transmittance (Y₁₂) is defined as the complex amplitude of the density of heat flow rate [W/m²] through the surface of the component adjacent to zone 1 ($q_1$), divided by the complex amplitude of the temperature [K] in zone 2 (${\theta }_2$) when the temperature in zone 1 is held constant (Equation (1)).

$$Y_{12}=-{\displaystyle \frac{q_1}{{\theta }_2}}$$

(1)

In second place, the time shift is defined as the phase difference between the maximum amplitude of the stress oscillation (i.e., temperature) and the maximum amplitude of the response (effect) oscillation, i.e., of the specific heat flux, in other words, the time that passes between the peak point of the external stress (temperature) and the peak point of the response (heat flux).

The analysis provides a clear understanding of the sinusoidal response in terms of thermal flux of a component, to a specific external stress such as temperature. The procedure followed in this test is described by Fantucci et al. (2019) [31]. A constant temperature of 20 °C was maintained on the lower plate of the DHFM, while a sinusoidal temperature variation was imposed on the upper plate to simulate a temperature solicitation, with a semi-amplitude of 15 °C. A cycle has a duration of 24 h in which the temperature varies from 5 °C to 35 °C (20 ± 15 °C). Two sequential cycles were performed, and only the results retrieved from the second cycle were considered in the calculations, since the first cycle was used to reach temperature equilibrium in the panel (independent from the initial condition). The test conditions can be found in

Table 3. Test conditions for measuring the equivalent periodic thermal transmittance for both samples.

Test Conditions	Values
Fixed Plate (Lower) [°C]	20
Dynamic Plate (Upper) [°C]	20
Amplitude (Dynamic Plate) [°C]	±15
Test Duration [h]	48
Cycle Time [h]	24

.

To analyse the sandwich panel static thermal properties, the R- and U-values were calculated, following EN ISO 6946 [32]. The different thermal conductivity values considered for each layer of the sandwich panel were obtained from datasheets and the bibliography.

Moreover, a finite element method (FEM) has been developed to reproduce the analytical approach. The simulation output provided the total heat flux, and a conversion was necessary to determine the U-value. FEM simulation was also conducted to obtain and study the dynamic heat transfer and determine the Y₁₂. To reproduce the dynamic thermal response of the panel, an FE model with a transient thermal calculation was defined. A scheme of the applied boundary conditions is shown in Figure 6 for the sandwich panel with PCMs and AGs. A similar methodology was performed for the baseline sample. The main output obtained was the directional heat flux. These models were developed with Ansys thermal software (Ansys 2023 R1).

To validate this new type of sandwich panel that combines PCMs, XPS insulating material, and silica AGs, a functional prototype was produced, installed, and monitored for a full year at the Cube Living Lab in Dresden (Figure 7 shows the measurement setup). The panel, measuring only 60 cm × 60 cm, was integrated into a window opening of a post-and-beam façade construction. Despite the small size, the absence of significant thermal bridges ensured reliable performance assessment under real conditions. To test the thermal properties, a heat flux plate (Type ALMEMO FQA019C) and temperature sensors (Type ALMEMO OT9040AS) were installed on both the interior and exterior surfaces of the panel. Additionally, humidity sensors (Type ALMEMO FHAD 46-Cx) were placed inside, outside, and within the component itself in the test setup to capture potential influences from moisture variations. The sensors were factory-calibrated by the manufacturer for this measurement task to minimize errors. Using a simplified approach according to ISO 9869 [33], the U-value was calculated based on the temperature difference and heat flux, deliberately excluding influences from solar radiation effects. Data from the winter period 2023/24 were used for the U-value calculation to capture conditions with higher thermal gradients. However, this method does not fully account for the dynamic temperature conditions within the panel. Therefore, each calculation was based on a dataset comprising an entire week of measurements to better reflect the panel’s thermal behaviour. Furthermore, to verify measurement consistency, redundant sensors were installed on the panel to cross-check recorded values and detect potential anomalies. For this cross-check, interior and exterior surface temperatures as well as interior and exterior air temperatures were evaluated and compared according to the approach. Due to the small dimensions of the panel, a consistently stable indoor summer climate, and the absence of a reference component, the onsite test was limited to evaluating the panel under winter conditions only.

2.3.2. Acoustic Analysis

The developed sandwich panel solution was installed as a prefabricated element in the test stand designed for windows following ISO 10140 [34]. The component measured approximately 1.5 m × 1.25 m, corresponding to the size of the test stand opening. Although it is unusual to test a wall component in a window test stand due to its limited size, the setup was used in this case. Despite the smaller dimensions, it was sufficient for estimating the acoustic insulation properties of the sandwich panel. This was particularly justified given the lightweight nature of the panel, which allows for a meaningful assessment despite the constraints of the test stand’s dimensions. The edges of the component were stuffed with mineral wool, and the outer 5 cm were filled with airtight construction foam (Figure 8).

3. Results

3.1. Thermal Properties

The values obtained from the laboratory tests for the thermal transmittance of the component in stationary conditions are reported in

Table 4. Measured values of heat flux through the components (Q), temperature difference between upper and lower plates (ΔT), and thermal transmittance (U).

Sample	Q [W/m2]	∆T [°C]	U [W/(m2K)]
Sandwich Panel with PCM and Silica AG	2.08	10.00	0.208 ± 0.06
Baseline Sandwich Panel	1.92	10.00	0.192 ± 0.06

. The U-value of the sandwich panel incorporating PCMs and AGs slightly exceeds that of the baseline sandwich panel, likely due to the following two factors: (1) the PCM is enclosed in an aluminium case, which increases heat flux through lateral dispersion that is not fully controlled; and (2) the limited amount of AG in the 10 mm white mortar layer does not significantly enhance the thermal performance of the component.

Table 5. Thermal results obtained in dynamic conditions for both panels.

Measurements	Sandwich Panel with PCM and Silica AG	Baseline Sandwich Panel
Heat Flux min [W/m2]	−2.68	−2.63
Heat Flux max [W/m2]	1.97	2.68
Heat Flux avg [W/m2]	−0.36	0.02
T max [°C]	34.47	33.84
T min [°C]	5.3	5.88
T avg [°C]	19.89	19.86
Periodic Thermal Transmittance
T Amplitude (Upper Plate) [°C]	14.59	13.98
Heat Flux Amplitude (Lower Plate) [W/m2]	2.32	2.66
Y12 [W/(m2K)]	0.16	0.19
Time Shift of the Heat Wave
T max (Upper Plate) [h]	29.97	30.29
Flux min (Lower Plate) [h]	34.23	32.51
Time Shift [h]	4.26	2.22

shows the maximum, minimum, and average values of temperature (stress) measured on the upper face of the sample by the thermocouples, and the heat flux (response) measured by the instrument on the other side of the sample. From these measurements, both the dynamic thermal transmittance Y₁₂-value of 0.16 W/(m²K) and the time shift of 4.26 h were determined for the sample with PCMs and AGs (Figure 9). For the baseline, a Y₁₂-value of 0.19 W/(m²K) and a time shift of 2.22 h were obtained. The results indicate that the sandwich panel incorporating PCMs and silica AGs outperforms the baseline panel in terms of dynamic thermal performance. Specifically, the PCM-enhanced sandwich panel achieves a periodic thermal transmittance 19% lower than the baseline and a time shift 92% higher when compared to the baseline. These improvements align with the expectations, demonstrating the effectiveness of PCMs and AGs in enhancing thermal inertia and reducing heat transfer under dynamic conditions.

The values obtained from the numerical analysis and simulation for the thermal transmittance of the component in stationary conditions are reported in

Table 6. Obtained results from EN ISO 6946 and FEM at stationary conditions.

Sample	U [W/(m2K)]—EN ISO 6946	U [W/(m2K)]—FEM
Sandwich Panel with PCM and Silica AG	0.21	0.18
Baseline Sandwich Panel	0.19	0.20

.

Table 7. Thermal results obtained from FEM analysis in dynamic conditions for both panels.

Measurements	Sandwich Panel with PCM and Silica AG	Baseline Sandwich Panel
Flux min [W/m2]	−2.04	−2.39
Flux max [W/m2]	2.40	2.81
Flux avg [W/m2]	0.18	0.21
Periodic Thermal Transmittance
T Amplitude (Upper Plate) [°C]	14.59	13.98
Flux amp (Lower Plate) [W/m2]	2.22	2.6
Y12 [W/(m2K)]	0.15	0.19
Time Shift of the Thermal wave
Time Shift [h]	4.32	2.11

shows the FEM results for periodic thermal transmittance obtained under dynamic conditions, and Figure 10 highlights the time shift of the thermal wave. Based on the results, the Y₁₂ was calculated to be 0.15 W/(m²K) for the panel containing PCMs and AGs, while the time shift obtained was 4.32 h. In contrast, the baseline sample showed a Y₁₂ of 0. 19 W/(m²K) and a time shift of 2.11 h. This represents a 20% decrease in thermal transmittance and a 105% increase in the time shift compared to the baseline panel.

Measurement data from the prototype installed in the CUBE Living Lab were collected at a frequency of 10 Hz and saved as 5-min averages, providing a high-resolution representation of the panel’s thermal behaviour throughout various seasonal conditions. Figure 11 illustrates selected examples of the collected data, showcasing how the thermal performance of the panel varied across different seasons.

For the calculation of the U-value, two continuous periods of two weeks each were chosen. The first period spanned from 22 December 2023 to 5 January 2024, and the second period from 2 February 2024 to 16 February 2024. During these periods, the temperature difference between the interior and exterior consistently exceeded 10 °C, providing the necessary boundary conditions for reliable measurements.

The U-value was calculated according to the simplified method outlined in ISO 9869. This approach involves using the measured temperature difference across the panel and the heat flux through the panel’s surface. By averaging these values over the selected periods, a cumulative average U-value was determined, which is shown in Figure 12. This method accounts for the steady-state heat transfer, while simplifying the effects of dynamic thermal influences, such as temperature fluctuations and transient conditions, to provide a robust estimate of the thermal insulation performance.

The calculated U-value from the measurement data for the first period in December 2023 was 0.19 W/(m²K), while for the second period in February 2024, it was 0.18 W/(m²K). The U-value derived from the panel’s layer composition is 0.20 W/(m²K). Thus, the measured results are close to the theoretical value.

3.2. Acoustic Properties

Figure 13 presents the evaluation of the sound insulation performance according to ISO 717-1 [35], with a calculated single-number rating of 38 dB. The figure also shows the results as a sound level difference, illustrating the panel’s sound reduction capability. Notably, there is a dip in the sound level difference at 500 Hz, indicating a frequency at which the panel’s insulation performance is less effective. Figure 14 displays the detailed measurements of the sound level across the one-third octave bands, offering a comprehensive view of the acoustic behaviour. Despite the dip at 500 Hz, the results generally show consistent sound insulation across the frequency spectrum.

4. Discussion

In this research work, the vacuum infusion process was successfully used on a versatile working table developed by INEGI. This method enables greater control over process parameters (i.e., temperature, pressure, time), while reducing both operator workload and process time compared to other common composite sandwich panel production methods, such as hand lamination [36,37,38]. The main challenge overcome during the manufacturing of the single-step sandwich panel infusion process was ensuring the position of the top spirals, which needed to be fixed to the XPS foam. After production, these fixings were removed. Some difficulties also arose when adding multiple layers of materials (especially the compact storage modules with PCMs and mortar to the composite sandwich panel), leading to incompatibilities that caused delamination issues. These issues have been addressed by sanding and cleaning some of the layers. However, alternative approaches, particularly chemical bonding (e.g., epoxy bonding, advanced adhesive, surface treatments) or mechanical interlocking could be explored in the future, to enhance even further the adhesion, durability, and performance and mitigate delamination between the different layers (since it is a factor that can affect significantly the heat and sound transfer of the sandwich panel) of the developed façade system [39,40,41,42,43]. However, it should be noted that while the vacuum infusion process used in this study provides precise control and yields high-quality composite structures, its scalability to industrial production presents several challenges. Fabricating larger panels may lead to issues such as uneven resin distribution, increased void formation, and extended cycle times compared to other production techniques [44,45]. Additionally, the incorporation of AG-modified mortars and PCM modules introduces complexity due to potential interfacial incompatibilities during assembly [46,47].

This study demonstrates the potential of integrating recycled XPS and basalt fibres to enhance the sustainability of multifunctional composite sandwich panels. Previous environmental analysis indicates that the use of recycled XPS significantly reduces embodied energy and carbon footprint compared to virgin polymer foams, by mitigating raw material extraction and waste [48,49]. Similarly, basalt fibres, derived directly from natural rock without chemical additives, offer lower environmental impacts relative to conventional glass or carbon fibres [20,50]. To further enhance thermal performance and reduce weight, AGs were added to a modified mortar matrix to improve thermal performance and reduce weight. AG mortars feature low thermal conductivity and high porosity, resulting in lightweight, thermally efficient materials. Optimised production methods like ambient pressure drying lead to lower embodied energy than traditional insulation materials [51,52]. Additionally, PCMs enable dynamic thermal energy storage, improving passive temperature regulation, which could reduce energy demands in buildings [53,54]. Although a full life cycle assessment (LCA) is not included, combining recycled XPS, basalt fibres, AG mortar, and PCMs indicates a sustainable approach for high-performance panels.

It is also worth noting that at the end of their service life, most of the raw materials composing the developed sandwich panels offer potential for reuse or recycling, supporting sustainability goals. The recycled XPS foam core can be effectively separated from the laminate using hot wire cutting techniques, enabling its reuse in the production of new panels or further recycling processes [55,56]. The composite laminate, although more challenging, can be repurposed in non-structural applications, while basalt fibres embedded within the laminate are increasingly recognized for their recyclability through mechanical, thermal, or chemical methods [57,58]. The thermoset epoxy resin and encapsulated PCM modules present recycling challenges; however, novel of recycling and recovery technologies are emerging to address these limitations [59,60,61]. Both modified and unmodified cementitious mortars can be crushed at end-of-life and reused as aggregate in new mortar formulations, thus minimizing material waste and supporting circular resource flows [62,63]. These strategies collectively enhance the environmental performance and circularity of multifunctional sandwich panels, aligning with broader sustainable construction practices.

However, the overall environmental performance depends on manufacturing processes, transportation logistics, and end-of-life management strategies.

While the current study presents a prototype installed in the Cube Living Lab that exhibited no observable degradation during the monitoring period, offering preliminary evidence of its durability within a composite sandwich panel system, a thorough assessment of long-term performance under realistic environmental conditions remains essential. In particular, repeated thermal cycling, representative of diurnal and seasonal temperature variations, may generate interfacial stresses arising from differential thermal expansion between constituent layers. These stresses have the potential to accelerate delamination and compromise mechanical stability [64,65]. Moreover, extended ultraviolet exposure deteriorates polymer matrices and coatings, reducing structural integrity and protection, and in humid conditions, moisture can cause degradation through hydrolysis, weakening fibre-matrix adhesion and impairing mechanical properties [66,67,68,69].

The sandwich panels were designed as a lighter, multifunctional solution, and the performance of the panels was addressed both thermally and acoustically.

It should be noted that the experimental and numerical specifically address a short-duration scenario (48 h, thermal dynamic test) under controlled laboratory conditions. Besides, it is important to highlight that the sandwich panel tested consists primarily of XPS, which accounts for approximately 95% of the total thermal resistance of the panel. XPS is a non-hygroscopic material, meaning that its thermal conductivity is minimally affected by variations in moisture content. As reported in the literature, even with an increase in relative humidity and water content, the thermal conductivity of XPS rises by a maximum of 10%, from 0.030 W/(m·K) in dry conditions (0% RH) to 0.034 W/(m·K) in moist conditions (100% RH) [70]. Furthermore, analysing the sorption isotherms and thermal conductivity curves available in the Fraunhofer IBP database via WUFI confirms that the impact of moisture absorption on XPS remains minimal [71]. Therefore, the influence of moisture on thermal and acoustic performance was considered negligible in the performed experimental and numerical studies.

From the thermal study, a definitive conclusion is not attainable as to whether or not the latent heat, implicitly driven by the dynamic response of the PCMs modelled by a temperature-dependent enthalpy function (Figure 1), was fully exploited in the developed sandwich panel with PCMs and silica AGs. However, it can be inferred that there was a passive activation of the PCMs based on the results of the time shift (almost double when compared to the baseline sample) and periodic thermal transmittance (lower than the baseline sample).

The latent heat capacity was utilized, since the temperature on the response side (near the PCM layer) varied between a minimum of 20.4 °C and a maximum of 20.8 °C (Figure 15). This confirms that the PCM operated within the transition range, which, according to Rubitherm (Figure 1), is between 19 °C and 23 °C for melting and between 21 °C and 18 °C for solidification.

To further analyse the thermal performance of the sandwich panels, a comparison between the measured values shown in Table 6 and Table 7, FEM simulation results, and those calculated according to EN ISO 6946 was conducted. The percentage difference between results is presented in

Table 8. Percentage difference of thermal performance parameters measured in DHFM tests and calculated by FEM thermal models for both panels.

	Sandwich Panel with PCM and Silica AG	Baseline Sandwich Panel
Stationary Thermal Transmittance, U
Percentage Difference:	14.29%	−5.26%
Periodic Thermal Transmittance, Y12
Percentage Difference:	+6.25%	0%
Time Shift of the Sinusoidal Heat Wave
Percentage Difference:	−1.4%	+4.95%

.

The experimental and simulated results show a good agreement overall. However, some discrepancies are observed in the U-value, especially for the PCM-enhanced panel and the time shift for both panels.

Besides, the stationary response of the sandwich panel has been calculated analytically using the available material parameter values, and their response has also been successfully reproduced by FEM thermal models for both panels. The simulations slightly underestimate the measured values (by less than 15%, Table 8), likely due to material properties sourced from the literature, which may not fully match the actual materials. Additionally, the FEM model does not account for variability in experimental conditions, contributing to these minor discrepancies. Despite these differences, the FEM transient thermal models prove to be an effective tool for satisfactorily reproducing the dynamic thermal response observed in DHFM tests for both sandwich panels in terms of the evolution of the flow versus time.

In addition to uncertainties in the estimated material properties and the boundary-condition assumptions, several design factors may account for the discrepancies observed between the simulation and the experimental tests. Specifically, inter-layer thermal contact resistance, microscopic geometric imperfections, and non-uniform material distribution, especially uneven PCM dispersion, influence the actual heat flow but are not fully captured by the idealized model.

The onsite test results demonstrated that the measured U-values of 0.19 W/(m²K) and 0.18 W/(m²K) closely align with the theoretical value of 0.20 W/(m²K), validating the panel’s thermal performance under real conditions. The slight discrepancies are likely due to the simplified evaluation method outlined in ISO 9869, which approximates dynamic thermal processes and does not account for radiation effects or other environmental variables.

When analysing the thermal performance of the developed sandwich panel, and comparing its U-value with the typical and legally mandatory U-values for exterior wall constructions in several European countries, it is found that it is within the spectrum, since according to Bienvenido-Huertas et al., (2019), U-values range from 0.12 to 1.25 W/(m² K) (depending on the specific country and climate zone), while the U-value of the panels produced in this study are between 0.18 and 0.21 W/(m² K), indicating compliance with strict insulation standards across Europe [72].

However, it should be also noted that the thermal tests and simulations were carried out on a smaller sample of panels to characterize the centre of the panel performance only, provided that numerous other researchers have looked at the impact of thermal bridges on composite panel building integration over different scales and considering different materials [73,74,75,76]. From the literature, it is evident that the heat loss at junctions between thermal elements (Ψ-value, W/mK) of thermal bridges depends on the type of joint (panel-to-panel, panel-window, panel-slab, etc.); therefore, to consider such effect, a detailed geometrical model of a building (or of a typical façade bay) is required, which is out of the scope of this paper. Nevertheless, for similar panels (adopting materials with equivalent thermal performance, constructions layering, possible joint configurations), the linear thermal bridge was quantified with values between 0.00 and 0.040 W/mK (depending on the joint type and thickness of the insulation layer) [73], while panel-to-panel joint thermal bridges specifically were quantified between 0.01 and 0.020 W/mK approximately.

It is also worth highlighting that those previous studies indicate that traditional insulation materials, such as mineral wool- or polystyrene-based systems, contribute to heating and cooling energy reductions. In contrast, multifunctional insulation systems that incorporate thermal storage, such as PCM-based panels, have demonstrated the potential to further decrease peak energy demands and enhance indoor temperature stability [77,78,79].

The literature highlights several challenges in the application of PCMs in building systems, which can affect their expected thermal performance. One of the main difficulties is the proper activation of the melting and solidification cycle. Often, indoor temperatures do not reach the required values to trigger the PCM phase transition, thereby limiting its effectiveness in storing and releasing latent heat. This issue has been observed in previous studies, where insufficient temperature fluctuations have restricted the full utilization of PCM potential. S. Fantucci et al. found that indoor temperatures were not always sufficient to ensure the complete activation of the PCMs [80].

To enhance PCM efficiency, integration with other passive strategies, such as night ventilation, has proven beneficial. These strategies can promote night-time cooling, facilitating PCM solidification and ensuring a full thermal cycle. In the aforementioned study, the adoption of night ventilation contributed to optimizing PCM performance, highlighting the importance of an integrated approach in building design [81].

Another significant issue in PCM use is the risk of leakage during the phase transition from solid to liquid. This can occur due to inadequate encapsulation, leading to thermal inefficiencies and potential structural damage in buildings. Jan Kośny discusses various challenges associated with PCM applications in building envelopes, including issues related to material leakage and the importance of proper encapsulation techniques to prevent such problems [82]. To mitigate such risks, effective encapsulation techniques, such as micro- or macro-encapsulation, should be employed to ensure PCM stability throughout thermal cycles. Additionally, the selection of suitable PCM-containing panels (e.g., Rubitherm CSM panels) should consider their long-term durability and reliability in preventing leakage-related issues. It should also be noted that PCMs can withstand 10,000 to 15,000 cycles of melting and solidifying with little thermal property loss, leading to a lifespan of approximately 20 years [83,84]. While the lifespan of the composite sandwich panel could be higher, this would lead to a possible need for PCM maintenance or replacement to maintain thermal efficiency during its entire lifecycle.

Regarding the use of cement mortar modified with AG, Becker P.F.B. et al. indicate that for it to have an impact on thermal insulation, it should have a higher thickness (≥14 mm) than that used in this research (10 mm). However, it is worth noting that traditional mortars typically require greater thicknesses to have thermal performance similar to mortars with AGs [85].

On the other hand, the acoustic performance evaluation revealed that the sandwich panel achieved a single-number rating of 38 dB according to ISO 717-1, indicating a satisfactory level of sound reduction. Therefore, the developed sandwich panel presents acoustic insulation similar to traditional materials, such as mineral wool or acoustic foam-based systems, demonstrating its suitability for numerous applications where noise control is essential, such as in residential and commercial buildings [86]. However, the data also showed a noticeable dip in the sound level difference at 500 Hz, suggesting a specific frequency range where the panel’s acoustic performance is less effective. This is a critical point, because it falls within the range where human sensitivity to sound is heightened. This dip could be attributed to the inherent material properties (i.e., density, thickness, and porosity) or structural characteristics of the panel (i.e., the configuration of the core material and the bonding between the different layers of the sandwich panel), which may result in resonant behaviour or reduced damping efficiency at this frequency [86,87]. To improve sound insulation at this frequency, several approaches can be considered. One option is to modify the core material by adding viscoelastic or porous damping layers that increase internal friction and reduce resonance amplitudes [88,89,90]. Another strategy involves introducing additional functional layers, like constrained-layer damping or mass-loaded vinyl sheets, between the core and face sheets to increase mass and stiffness, thereby shifting resonance frequencies and improving sound attenuation over a broader range [91,92]. Adjusting the panel’s geometry, specifically the thickness and density of both core and face sheets, can also help by altering modal behaviour to reduce resonance effects near 500 Hz [93,94]. Moreover, ensuring airtight sealing around panel edges and improving junction connections can prevent sound leakage that worsens performance in this frequency range [95,96]. Finally, the arrangement and bonding of integrated components, such as PCM- and AG-modified mortars, may be refined to further reduce standing wave effects [97,98]. Combining these measures is expected to result in a more consistent acoustic performance and better sound insulation at critical frequencies, making the sandwich panels more effective in applications sensitive to noise.

5. Conclusions

This study presents a validated eco-friendly composite sandwich panel that demonstrates how multifunctional design principles can be effectively applied to structural, lightweight façade systems. The panel’s enhanced thermal and acoustic performance contributes directly to the goals of nearly zero-energy and low-emission building strategies. The integration of recycled and low-impact materials into a cohesive panel system illustrates a scalable pathway for reducing embodied energy in building envelopes, aligning with the sustainability principles outlined in the European Green Deal, making them a potentially viable product for the construction sector.

The use of a single-step vacuum infusion method demonstrates the feasibility of fabricating advanced composites using sustainable feedstocks, with potential for prefabricated panel production at a large scale. The developed panel includes materials with varying embodied energy levels, and its design aims to optimize the balance between thermal performance, feasibility, and durability. The use of basalt fibres, a natural and abundant volcanic mineral, offers a low-energy alternative to synthetic fibres, while ensuring high mechanical strength and thermal resistance. The recycled XPS core contributes significantly to the panel’s insulation capacity, comprising approximately 95% of the total thermal resistance, and its non-hygroscopic nature ensures thermal stability under varying moisture conditions. The incorporation of PCMs and AGs enhances thermal inertia and insulation efficiency, which can contribute to reducing operational energy consumption. The application of an omniphobic coating represents an innovative addition to the panel system, aiming to improve its durability by offering resistance to moisture, dust, and pollutants. This coating treatment not only protects the panel from environmental degradation but may also reduce maintenance requirements, extending the panel’s service life under real climatic conditions. Therefore, this work advances the field by validating the integration of passive thermal storage and low-conductivity AGs within a mechanically robust composite, offering a new class of high-performance façade materials.

In this research work, the thermal and acoustic performances were studied, and the following conclusions can be drawn:

• It was possible to verify that the thermal laboratory tests, simulations, and onsite measurements were aligned, validating the panel’s thermal performance under real conditions.
• It is not possible to determine with certainty whether or not the latent heat was fully exploited; however, based on the results of the time shift and periodic thermal transmittance, it can be inferred that there was a passive activation of the PCMs.
• The panel with PCMs and silica AGs outperforms the baseline panel, since it has better measured thermal performance corresponding to lower Y₁₂ (−16%) and higher time shift (+92%).
• The sandwich panel with PCMs and AGs achieved a satisfactory level of sound reduction; however, at 500 Hz, the panel’s acoustic performance was less effective.
• The results revealed that it is feasible to produce panels using different materials with promising thermal properties, with the sandwich panel with PCMs and AGs being an interesting solution for the construction industry, since it can contribute positively to the thermal management of buildings.

In conclusion, the developed sandwich panel presents a technically viable and scalable solution for sustainable building envelopes, integrating recycled components, passive energy systems, and surface treatments to advance building resilience and efficiency.

This study contributes to ongoing research in minimizing both operational and embodied energy in buildings through the development of multifunctional composite materials. The synergy between mechanical reinforcement, thermal management, and environmental resistance offers a clear route towards circular, climate-adaptive construction systems. Future investigations will expand on these findings by examining lifecycle impacts, mechanical and hygrothermal behaviour, durability, economic feasibility, and integration into full-scale structures, including joints and thermal bridging, to broader application and industry uptake. Ultimately, this research supports the transition toward sustainable architecture by advancing bio-based sandwich panel technologies that enable more energy-efficient and environmentally responsible construction practices.