## 2 Materials and Methods
### Modified Guarded Hot Box Design  
Thermal properties of tested samples in steady-state and unsteady-state conditions were determined in the modified guarded hot box. The guarded hot box method according to EN ISO 8990 [32] was slightly improved and adjusted in order to reduce heat loss through the hot box envelope. The hot box was composed of two chambers. A hot chamber was supplied with the heating system to maintain high temperatures; on the contrary, a cold chamber was supplied with the cooling system to maintain low temperatures. The tested sample was placed in between. The difference between a guarded hot box, designed according to the standardized method, and the modified guarded hot box used in this experiment is illustrated in Figure 4. The box was located in a laboratory with controlled temperature and humidity (HVAC). The temperature on the hot side of the experimental box was kept constant at 24 \({}^{\circ}\)C, consistent with the ambient temperature. This setting leads to the minimization of heat losses that potentially occurs through the hot chamber walls--heat flow \(\varphi 3\) (Figure 4b). This enables a use of the Hot Box constructed as a Calibrated Hot Box according to EN ISO 8990 [32]. No calibration is needed to minimize the system heat losses through the hot chamber walls. The second most significant advantage of this solution is a minimization of a three-dimensional heat transfer through the sample in a flanking area of the metering chamber--heat flow \(\varphi 2\) (Figure 4a) as standardized in the case of the Guarded Hot Box method. Figure 4: Scheme of the difference between (**a**) a Guarded Hot Box (according to EN ISO 8990 [32]) and the Modified Guarded Hot Box used in this study (**b**). The modified guarded hot box eliminates heat loss through the chamber wall; therefore, one can assume that the energy supply to the hot chamber flows only through the sample. The gap between the sample and the hot box was filled with an additional mineral wool insulation. The entire sample, as well as the hot box wall perimeter, was sealed with an airtight tape, causing the heat loss to be insignificant. The heat loss and airtightness were checked with a thermal camera after 12 h of sample conditioning. For this study, a seven-day temperature cycle was selected with the temperature setting of Table 3. The preset temperature program was based on real climate conditions recorded at the local meteorological station, Prague-Suchdol, Czech Republic, in winter 2017. Each sample was tested under identical conditions with room temperature (22-24 \({}^{\circ}\)C) in the hot chamber. The ambient temperature of 22-24 \({}^{\circ}\)C was maintained by a stable high performance air conditioning unit (deviation 0.2 \({}^{\circ}\)C). The temperature in the cold chamber varied between +6 \({}^{\circ}\)C and -13 \({}^{\circ}\)C. The current state of the art allows the design of the climate chambers to be precisely controlled and programmed with a temperature program. The modified guarded hot box construction together with the specific sample position is depicted in Figure 5A. At the beginning of the experiment, a hot chamber was opened and the sample was placed in a hot box to separate the chambers. The temperature difference between the chambers determined the heat flow through the metering area of the sample--\(\varphi_{1}\). There was a heating system with a maximum power of 500 W in the hot chamber which was regulated by a PID (proportional-integral-derivative) panel controller with an additional thermocouple in the hot chamber. A Power Analyser Rohde & Schwarz HMC 8015 (Rohde & Schwarz, Munchen, Germany) measured the heat flow, as an electric power flows straight to the heating system placed inside the hot chamber. The air temperature was measured using the Data Acquisition Base with humidity and temperature sensors. A set of temperature surface sensors was placed on each side of the measured sample according to EN ISO 8990. The temperature sensors' location is shown in Figure 5C.