1 Introduction

Saving energy has always been one of the greatest challenges for humanity and many efforts have been made over the past centuries. The effects of increasing energy consumption and the resulting intensification of environmental pollution began to be felt by mankind as early as the 19th century, as the first modern environmental law was enacted in the United Kingdom in 1863 [1].

Nevertheless, serious provisions were made only after World War II, and the first environmental movements emerged in the 1960s when climate change became more drastic. The first major conference on international environmental issues was the United Nations Conference on the Human Environment (Stockholm, Sweden, 1972) [2]. However, humanity needed an event so dramatic as the oil crisis in the 1970s to recognize the importance of reducing energy consumption and greenhouse gas emissions. From this period, studying the energy performance of buildings and the use of building materials specifically for thermal insulation became important [3].

Unfortunately, despite the many actions, the world's energy consumption is growing because energy resources should satisfy the increasing needs of a growing population. According to 2018 world statistics, the buildings, and construction industry accounts for the largest share of both global final energy use 36% and energy-related CO₂ emissions 39%; therefore, the buildings and construction sector is a primary target for greenhouse gas emission mitigation efforts [4].

According to 2020 statistics, in Hungary, households are responsible for the largest percentage of primary energy consumption 29.96% of the country, while industry accounts for 22.30%, transport for 22.34%, trade, public services, and national defense for 10.64%, agriculture for 3.54% and non-energy applications (e.g., using them as raw material) for 11.64% (Table 1). The most significant proportion of 70.69% of household energy consumption is heating, followed by domestic hot water 13.14%, lighting and electric devices 10.95%, cooking 4.98%, and cooling 0.24% [5].

It can be concluded that reducing the energy consumption of buildings is a priority task and the greatest saving potential lies in the energy savings used for heating [6]. With the help of thermal insulation, the heat loss of building envelopes can be significantly reduced, so choosing the right and effective thermal insulation material has become a key issue [7].

2 Laboratory tests

Ensuring adequate thermal comfort in buildings has always been an important task for designers [8]. With the development of building materials and construction technologies, human needs have also changed. The global climate change and the extreme intensity of environmental pollution that has taken place in recent decades have diverted these demands towards increasingly stringent building physical, especially thermo-physical and energetic requirements for buildings.

Legislations determine that all building structures should satisfy certain energetic and thermo-physical requirements, and these strict prescriptions can only be performed using a variety of thermal insulation materials [9].

Over time, more environmentally conscious building construction solutions have been developed and more efficient thermal insulation materials have appeared on the market of building materials [10]. Producers and researchers have continuously tested thermal insulation materials to develop their material properties, and have developed several standard testing methods (thermal conductivity, tensile strength, compressive strength, water absorption, fire resistance, etc.).

However, there are only a few reliable measurement results that would provide information on the impact of in-site installation conditions on certain thermal insulation products or on the changes in their original physical and chemical properties over time [11, 12].

In 2021, the Hungarian Expanded Polystyrene Thermal Insulation Manufacturers Association gave a commission to the Department of Architecture and Building Structures at Széchenyi István University Győr, Hungary to study the changes in the properties of different thermal insulation materials under various environmental conditions. The three most commonly used thermal insulation materials were highlighted as the subject of the study: Expanded PolyStyrene (EPS) foam and Rock Wool (RW), which are mainly used for façade insulation and PolyIsocyanuRate (PIR) foam with aluminum foil mounting which is mainly used for floor and roof insulation.

Based on the consultations with the customer, the special test conditions and the conditioning time have been determined, which correspond to certain real environmental conditions and can be modeled appropriately under laboratory circumstances. For laboratory measurements, the two most commonly insulated building structure types were selected, the flat roof and the façade.

In the case of flat roofs, it often happens that moisture leaks into the thermal insulation over time due to improperly installed waterproofing, causing a significant damaging effect. Moreover, water vapor can infiltrate into the thermal insulation from the internal space due to the incorrect or missing water vapor barrier layer and flat roof insulation can suffer from thermal stress caused by the direct and strong summer sunshine. In addition, it often happens, especially when insulating larger roof surfaces (e.g., industrial buildings), that the total amount of thermal insulation needed to insulate the roof is stored on the roof and stored there for a long time, in an open space. If the thermal insulation material is not adequately protected against humidity and high temperatures during storage, it is not possible to install it immediately in the event of a sudden rain, so the direct effects of humidity and temperature are often unavoidable. During laboratory tests, these effects were simulated one by one separately and also together using appropriate conditioning instruments.

In the case of facade insulations, improper installation of window sills can most often cause problems, as a result of which moisture can leak into the thermal insulation. In addition, moisture can enter the thermal insulation when the covering layer is damaged by external exposures (e.g., mechanical effects); moreover, façade walls are also exposed to thermal stress caused by direct and strong summer sunshine.

In the case of both building structures, it is, therefore, necessary to examine the effect of rising relative humidity caused by moisture and the rising temperature caused by direct and strong summer sunshine on the material properties of thermal insulations.

During the study, the effect of moisture was simulated with 90% relative humidity (φ), the effect of direct sunshine with a temperature of 50 °C, and the effect of direct strong summer sunshine with a temperature of 70 °C. The duration of conditioning was 48 h. Conditioning (Table 2) was performed using a desiccator (lockable, glass vessel) and a standard climate chamber. The temperature and the relative humidity were continuously controlled by the thermometer and the hygrometer of the climate chamber. All test results were obtained by averaging the material properties measured on three samples. Where the standard deviation of the test results was too high, additional specimens were tested (standard deviations are shown in parentheses next to the average values) in the following.

Laboratory tests focused on changes in material properties that are the most relevant for usability: tensile strength (σ_t), compressive strength (σ_c), and thermal conductivity (λ).

The change in material properties (Table 3) was analyzed from three perspectives. On the one hand, the comparative analysis was aimed at discovering how the properties of the material changed compared to the original values, i.e. with the combined effect of humidity and temperature changes. On the other hand, material properties were examined, how they are affected by the same temperature but different humidity values, that is, what is the effect of the humidity change in itself. Finally, it was also analyzed, how material properties develop with the same humidity but different temperature values, i.e. what is the effect of the temperature change in itself.

It is important to mention that regardless of the installation method, all modeled conditions (combination of temperature and humidity) can occur in the tested materials, even though, based on the manufacturer's recommendations, two products (EPS, RW) are only suitable façade insulation, and PIR product is only suitable for floor and roof insulation. However, the examined material properties are not necessarily relevant for both installation methods. No tensile strength requirement is imposed on materials suitable for insulating floors and flat roofs, so only trend-like conclusions can be concluded from the results (although it should be noted that the tensile strength of the tested PIR product according to the performance declaration is the same as the PIR products recommended for facade thermal insulation). A similar trend can be concluded from the test results of the thermal conductivity factor because different thermal insulation requirements apply to the different building boundary structures (facade, floor, flat roof).

3 Results

3.2 Tensile resistance

Tensile resistance tests were performed according to EN 1607:2013 [15]. Sample sizes were 5 × 5 cm, with their original thickness. According to the requirements of the standard, the samples were adhered between the clamping tools of the test equipment with one-component polyurethane-based adhesive. The tests were made immediately after conditioning to ensure that the material was in a conditioned state during the measurement (Table 5).

Table 5.

Changes of tensile resistance concerning the test conditions

Test condition	EPS		PIR		MW
	σt	Δσt	σt	Δσt	σt	Δσt
	(kPa)	(%)	(kPa)	(%)	(kPa)	(%)
1.	205.2 (7.2)a	–	79.6 (2.1)a	–	7.7 (0.6)a	–
2.	205.1 (1.8.)	−0.1	82.1 (5.0)	+3.1	5.8 (0.1)	−24.5
3.	200.6 (3.3)	−2.3	127.1 (8.2)	+59.7	5.3 (0.4)	−31.2
4.	214.8 (9.0)	+4.7	111.2 (7.3)	+39.7	7.7 (0.9)	±0.0
5.	202.4 (2.6)	−1.4b	64.3 (2.0)	−19.3b	4.5 (0.4)	−40.9b
		−5.8c		−42.2c		−40.9c
		−1.3d		−21.7d		−21.6d
6.	201.4 (0.2)	−1.9b	83.7 (3.7)	+5.1b	7.3 (0.3)	−4.7b
		−6.3c		−24.8c		−4.8c
		+0.4e		−34.2e		+38.5e

Source: [14].

a: reference values, b: relative to the reference values (T = 23 ± 2 °C, φ = 50%), combined effect of temperature and relative humidity, c: relative to the values of test condition no. 4 (T = 23 ± 2 °C, φ = 90%), only the effect of temperature, d: relative to the values of test condition no. 2 (T = 50 °C, φ = 50%), only the effect of relative humidity, e: relative to the values of test condition no. 3 (T = 70 °C, φ = 50%), only the effect of relative humidity.

In the case of EPS, a slight negative effect of rising temperature and relative humidity was observed. Similarly, to compressive strength, a slight increase in tensile strength due to the rising relative humidity can be observed (+4.7%). It can also be seen that the effect of increasing temperature is becoming more significant at 90% relative humidity (−5.8% at 50 °C and −6.3% at 70 °C).

In the case of PIR, mixed results have been obtained. While increasing temperature resulted in increasing tensile resistance, the effect of relative humidity is not clear. Unfortunately, the reliability of the results is questionable by the fact that in several cases the failure did not occur in the material but along the adhesive used for fixing the samples in the testing machine (Fig. 1). One reason for this is to be found in the conditioning of the samples. The high temperature might have affected the solidification process of the adhesive, so it is believed that in some cases the adhesive did not solidify sufficiently. In several cases, mostly at high temperatures, it can be observed that the mounting (aluminum foil) has detached from the sample. To obtain more reliable results, the tests should be performed after the removal of the mounting.

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Inadequate failure of a PIR sample partially along the adhesive (photo by Mózes Nobert Bedő)

Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01039

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In the case of RW, the increasing temperature itself led to a strongly decreasing tensile strength (−24.5% at 50 °C and −31.2% at 70 °C), presumably due to the slight softening of the adhesive used to fix the rock wool fibers together during the production. The effect of increasing relative humidity also damaged the tensile strength, but its tendency and magnitude were not clear also taking into account that the measured values showed a rather large standard deviation. Increasing the humidity to 90% alone did not cause a significant change in tensile strength. At 50 °C (φ = 90%) the tensile strength decreased by 40.9% compared to the values measured at room temperature, and by 21.6% compared to the values measured at 50 °C and 50% relative humidity. At 70 °C only a slight increase (<5%) can be observed. Unfortunately, the tensile strength of the material is much lower than that of the other materials, so the material may be already under stress during conditioning and preparation (i.e. before the adhesive solidifies).

The resulting relatively small additional stresses can easily approach the ultimate tensile strength which can affect the final result. In addition, the results showed significant deviations. Where the standard deviation was too high, additional samples were tested, but in many cases, it was not possible to make clear conclusions about the trend of changing values.

3.3 Thermal conductivity

Thermal conductivity tests were performed according to EN 12667:2001 [16] using a Taurus TCA 300 heat flow meter. Sample sizes were 30 × 30 cm, with their original thickness. Tests were made within 4 h after conditioning (Table 6).

Table 6.

Changes in thermal conductivity concerning the test conditions

Test condition	EPS		PIR		RW
	λ	Δλ	λ	Δλ	λ	Δλ
	(W mK−1)	(%)	(W mK−1)	(%)	(W mK−1)	(%)
1.	0.03739 (0.00075)a	–	0.01710 (0.00034)a	–	0.03385 (0.00068)a	–
2.	0.03739 (0.00075)	±0.0	0.01782 (0.00034)	+4.2	0.03328 (0.00067)	−1.7
3.	0.03745 (0.00075)	+0.2	0.01820 (0.00036)	+6.4	0.03362 (0.00067)	−0.7
4.	0.03853 (0.00077)	+3.1	0.01666 (0.00077)	−2.6	0.03351 (0.00067)	−1.0
5.	0.03817 (0.00076)	+2.1b	0.01789 (0.00036)	+4.6b	0.03401 (0.00068)	+0.5b
		−0.9c		+7.4c		+1.5c
		+2.1d		+0.4d		+2.2d
6.	0.03735 (0.00075)	−0.1b	0.02102 (0.00042)	+22.9b	0.03384 (0.00068)	±0.0b
		−3.1c		+26.2c		+1.0c
		−0.3e		+15.5e		+0.7e

Source: [14].

a: reference values, b: relative to the reference values (T = 23 ± 2 °C, φ = 50%), combined effect of temperature and relative humidity, c: relative to the values of test condition no. 4 (T = 23 ± 2 °C, φ = 90%), only the effect of temperature, d: relative to the values of test condition no. 2 (T = 50 °C, φ = 50%), only the effect of relative humidity, e: relative to the values of test condition no. 3 (T = 70 °C, φ = 50%), only the effect of relative humidity.

In the case of EPS, only specimens conditioned at 90% humidity showed relatively low changes in thermal conductivity (+3.1%). In the case of RW, no clear changes were detected, because all changing values were within the statistical error limit (<1%). Only in the case of PIR, it can be shown that both increasing temperature and relative humidity hurt the thermal insulation ability (rising values of thermal conductivity). The rising temperature itself caused a 4.2% increase in thermal conductivity at 50 ºC and a +6.4% increase at 70 °C. High relative humidity itself did not cause deterioration in thermal insulation ability, but when it was combined with the rising temperature, the effect was much more intense. The highest changes compared to the reference values were observed at 70 °C with 90% relative humidity (+22.9%). The above phenomenon can be explained by the test method and the test conditions. Although the tests were performed within the prescribed time after conditioning (within 4 h), the duration of the test was long (3–4 h). During this time, especially at higher temperatures, the moisture absorbed during the conditioning could escape from the material. The amount of escaping water depends on the material structure. RW is a fibrous material, which can remove the highest amount of moisture, so most of the absorbed moisture can escape from the material during the test, resulting in nearly constant values of thermal conductivity. EPS has an open cell structure; therefore, it is also able to remove relatively high amounts of moisture while PIR with its closed cell structure can remove its moisture content relatively slowly. This explains why the smallest changes occurred in the case of RW samples and the largest changes in PIR.

Based on these results, it can be concluded that the used measurement method can only show whether the conditioning for a given period causes a permanent change in the thermal conductivity of the insulating materials. It is not suitable for testing the material property in the current state of conditioning, as the test takes too long and the testing equipment is not capable of maintaining the conditioning conditions during the entire duration of the test. It would be worthwhile to repeat the test so that the absorbed moisture cannot escape during the test. Based on the experiences of other researchers, the solution could be the wrapping of test samples in plastic foil after the conditioning [11]. In the case of the polyurethane foam samples, it was also observed that the mounting can sometimes be the weak point of the system because in some cases it was observed that the mounting material blistered on samples conditioned at a temperature of 70 ºC and a relative humidity of 90% (Fig. 2).

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A blistered PIR sample after conditioning at a temperature of 70 ºC and a relative humidity of 90% (photo by Mózes Nobert Bedő)

Citation: Pollack Periodica 20, 1; 10.1556/606.2024.01039

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4 Conclusions

The research aimed to experimentally obtain information on how the most important material properties (tensile and compressive strength, thermal conductivity factor) of the three most commonly used thermal insulation materials (EPS, PIR, rock wool) change when they are installed on a flat roof or facade and during construction or operation (e.g., as a result of a structural defect or damage) they are exposed to moisture and/or heat effect different from the laboratory conditions. For this purpose, before the standard tests were performed, the test specimens were treated under conditions (humidity, temperature, and their combinations) by which they could model the environmental and exposure effects that can be suffered under in-site installation conditions.

The results of the compressive strength tests showed that this property of polyisocyanurate and rock wool is not sensitive to the rise in temperature, while expanded polystyrene foam suffers a significant decrease in compressive strength. The change in humidity affected the compressive strength of all three materials, the polyisocyanurate foam the least, and the rock wool the most.

The tests also revealed that neither temperature nor humidity changes significantly reduce the tensile strength of expanded polystyrene foam. In the case of rock wool, the effect of both temperature and humidity was detectable. In the case of polyisocyanurate foam, mixed results were obtained; therefore, it would be necessary to carry out some tests even after removing the mounting.

The results of the thermal conductivity factor tests were the clearest in the case of polyisocyanurate foam, where significant changes in both temperature and humidity were detectable. In the case of the other two materials, it was only revealed that drying out after conditioning for a given period does not cause a permanent change in the thermal conductivity in them. It would be worthwhile to repeat the test by preventing the moisture absorbed during conditioning from leaving the material (e.g. wrapping in plastic foil).