Thermal conductivity of polyurethane insulation boards subjected to temporary compression

Proper thermal insulation of buildings is currently one of the most important issues related to broadly understood energy efficiency and the rational use of non-renewable resources (Lis & Lis, 2018; Ryndiuk et al., 2019). One element related to this protection is appropriate ground-based floor insulation (Murat, 2012). In recent decades, this aspect has been underestimated, with layers of solid materials dominating in earlier building designs (Watts & Tesh, 2014). One reason for this was the lack of sufficiently hard insulation boards on the market. The quality of mineral wool or polystyrene boards available at the time was poor, in terms of mechanical strength (Porównanie, 2025). Only the widespread use of hard insulation boards contributed to the popularization of ground-based floor insulation (Housing, 2025). Currently, in Poland, expanded polystyrene EPS boards are most commonly used, with a compressive strength of 100 kPa – 200 kPa (Grubość i rodzaj, 2025). However, for heavy loads, it is necessary to use the more expensive, but more durable extruded polystyrene XPS (Czym jest styropian XPS, 2026).

Nowadays, it is possible to cover insulation boards not only with several centimeters of cement screed, but also with a massive reinforced concrete foundation slab. A foundation slab is simple and quick to construct, especially if the house has a complex shape. Instead of an extensive system of foundation strips, a single, compact foundation structure is constructed under the entire house (Francke et al., 2025; Matusiak, 2022). An example of such a solution, branded by Izodom, is shown in Figure 1. The reinforced concrete foundation slab is insulated from below with shapes made of a material with the trade name “peripor”. This material has similar properties to hard XPS polystyrene (IZODOM, 2025). The slab insulation is connected to the wall insulation, eliminating thermal bridges at the floor-foundation junction. Therefore, this solution is recommended for energy-efficient and passive houses.

Fig. 1.

Insulation under the foundation slab in the model (Izodom 2000 Polska Sp. z o.o., Zduńska Wola) single-family house (photo: Zbigniew Respondek)

Extruded polystyrene has good thermal insulation properties – manufacturers declare a thermal conductivity λ of 0.033 W/(m·K) – 0.04 W/(m·K). However, recent years have seen increased interest in closed-cell polyurethane (PUR) or polyisocyanurate (PIR) boards. These materials offer even better thermal insulation performance. The declared λ-value is 0.19 W/(m·K) – 0.23 W/(m·K), which allows for the use of thinner insulation layers (Kubanek, 2025). Polyurethane insulation is applied as spray-on insulation, sandwich panels (with facings of profiled coated sheet metal), or lightweight insulation boards (Gravit et al. 2017; Thermal, 2006). In the case of boards, claddings are most often used to protect the material against moisture and air infiltration – aluminum foil, fiberglass, bitumen, etc. (Wolnik, 2025). In recent years, interest in using these boards for external wall and over-rafter insulation has increased (Respondek, 2021). However, the use of polyurethane products for ground-based floor insulation raises concerns among designers and contractors regarding their behavior under mechanical loads.

The article aims to assess the influence of simulated temporary loading of polyurethane insulation boards on their thermal insulation performance. The performance of the polyurethane samples was also compared with a reference sample of XPS polystyrene.

The thermal conductivity of the samples was measured using a Netzsch HMF 446 Heat Flow Meter, a device available at the Building Physics Laboratory, Faculty of Civil Engineering, Czestochowa University of Technology (Fig. 2a). Plate lambda meters are commonly used for laboratory testing: a material sample is placed between two plates. One plate is heated and the other cooled, creating a temperature gradient within the material. The heat transfer rate (heat flux), the temperature difference across the sample, and the distance between the plates are measured (HFM, 2025). This serves as the basis for determining the λ-value.

Fig. 2.

General view: a) Heat Flow Meter Netzsch HFM 446, b) sample PIR, c) sample PUR (own photos)

Samples measuring 30 cm² × 30 cm², cut from insulation boards, were tested. The sample thickness was 10 cm – this is the maximum thickness that could be accommodated in the device due to its technical parameters.

Samples of three materials were used for the test:

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  Polyisocyanurate (PIR) board, trade name: termPIR AL, manufacturer: GÓRSTAL; the aluminum cladding was removed from the board surface because preliminary tests showed that it interfered with the λ-value measurement (Fig. 2b),

• –

  Polyurethane (PUR) board, trade name: PUR BLOCK E33, manufacturer: Stello; board without cladding (Fig. 2c),

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  Expanded Polystyrene (XPS) board, trade name: PRO 30 SF, manufacturer: Styropmin; reference sample, tested to compare the performance of polyurethane and expanded polystyrene products.

After placing the sample in the device, a controlled pressure is applied to ensure proper contact between the working plates and the material. In this case, 2 kPa was used, constant throughout the test. After starting the device, the upper plate gradually reaches a temperature of 10 °C, while the lower plate reaches a temperature of −10 °C. The heat flux flowing through the material sample is measured using three calibrated heat flux transducers. The final result, converted to the λ-value, is obtained when thermal equilibrium (steady state of heat flow) is achieved. The distance between the plates (sample thickness) is also monitored. The measurement typically takes 2–4 hours. The test report is automatically generated in the form of spreadsheets and graphs presenting the measured parameters over time. A sample result graph illustrating changes in temperature and λ-value is shown in Figure 3.

Fig. 3.

Example graph generated by the device (own research)

Each of the three samples was tested three times in different positions (sample rotation) relative to the device plates. Before each test, the sample was stored at room temperature for 24 hours. Each sample was then seasoned for 30 days under a load of bricks weighing 17.5 kg (Fig. 4), which corresponds to approximately 2 kN. This approximately corresponds to the pressure exerted by the working plates in the device during testing. It was assumed that this load simulates the action of a compressive load on the insulation board, equivalent to the weight of a layer of cement screed or lean concrete approximately 9 cm thick.

Fig. 4.

XPS sample seasoning under load (own photo)

After seasoning under load, the λ-value measurements were repeated.

The test results for all three samples are summarized in Tables 1–3. The subscript “0” indicates the results before seasoning; the subscript “30” indicates the results after 30 days of seasoning under load.

Based on the obtained results, it was found that for all samples, 30-day seasoning under load led to a slight deterioration of thermal insulation performance. For the PIR, PUR, and XPS samples, the λ-value increased by 1.1 %, 0.9 %, and 2.4 %, respectively. It should be mentioned that according to the device manufacturer's information (HFM, 2025), the measurement accuracy is ±1 % – 3 %, the repeatability is ±0.5%. Therefore, the results of the study fall within the measurement error range.

The λ₃₀-value for both polyurethane samples is similar and roughly consistent with the manufacturers' declarations. For the XPS sample, the λ₃₀-value was 10.8 % better than declared by the manufacturer, but approximately 47 % worse than for the polyurethane samples.

To determine whether the deterioration in the insulation performance of the samples after seasoning was due to “compression” of the material under load, the results of measurements of the distance between the plates (sample thickness d) were compared in Tables 4–6. Since the plates press the sample material during the measurement, two values are given for each test: the subscript “(in)” denotes the initial value immediately after the start, the subscript “(fi)” the final value after reaching thermal equilibrium. The “0” and “30” subscripts were used as before. Diff indicates the percentage change in the sample thickness.

The tests did not reveal any significant effect of compressive loading on the samples on their thickness reduction. The pressure applied during the test, which lasts approximately 2–4 hours, causes only a small (up to approximately 0.3 %) compression of the material. No permanent reduction in the thickness of the samples subjected to 30 days of loading was observed. Small differences in thickness (hundredths of a millimeter) can be considered unavoidable measurement deviations.

The research presented in this article focused on assessing the effect of temporary compression of polyurethane boards on their thermal insulation performance in the context of use in ground-based floor structures. The performance of polyurethane materials (PIR and PUR) was also compared with extruded polystyrene (XPS) in this regard.

All tested samples were found to exhibit a slight deterioration in thermal insulation, approximately 1 % for polyurethane samples and 2.4 % for polystyrene, after 30 days of sample seasoning under a load of 2 kPa. However, this is not due to a reduction in board thickness (“ compression” ). The board thicknesses before and after sample seasoning aging were comparable. Also, during the test, a compressive load of 2 kN applied for several hours did not significantly change the thickness of the tested samples.

The observed slight deterioration in thermal insulation performance was therefore due to other factors. Small samples were cut from larger boards and exposed to weathering during seasoning. Therefore, increased exchange of foaming gases with atmospheric gases may have contributed to this, which reduces the thermal insulation performance of both polyurethanes (Harris et al., 2025; Makaveckas et al., 2021) and extruded polystyrene (Vėjelis et al., 2025). Therefore, it is important that these materials, once mounted into structures, do not come into direct contact with atmospheric air.

The study also demonstrated that concerns about using polyurethane boards in ground-based floor structures are unfounded. No significant differences were found in the behavior of the tested materials under simulated loads. Polyurethane boards have a significant advantage: the measured λ₃₀-value was approximately 32 % lower than for XPS, meaning that the thickness of the boards used can be reduced to achieve the specified thermal insulation performance. However, selecting a specific solution requires consideration of economic and environmental aspects, which were not analyzed in this study.

The subject of further research should be the analysis of the thermal insulation efficiency of the products in question under long-term load conditions.