## Materials and Methods  
### Specim and Measurement Conditions  
The products used as specimens were collected within seven days of their production date and stored in a constant temperature and humidity chamber at 20 \(\pm\) 5 \({}^{\circ}\)C with 50 \(\pm\) 5% relative humidity. For the accelerated test, 50 mm products were sliced into five 10 mm specimens and then stacked. For the specimen slicing method, ISO 11561 specifies using a butcher knife, fine-toothed band saw, or a thin heat line cutter for slicing. For a preliminary test, as seen in Figure 1, we tested various slicing methods such as a cutter knife, butcher knife, mechanical saw blade, woodworking saw blade, and a heat line, and examined the geometry and degree of damage of the sliced surface using a magnifying glass to choose a suitable slicing method for cutting specimens in this study. The specimen slice cut using the cutter and bucther knives showed well-maintained independent, extruded polystyrene cells, and the shape of independent bubbles on the back surface was clearly observed. However, the use of saw blades damaged a considerable portion of the cells on the cut surface. The heat from the heat line, which is most commonly used for cutting foam plastics, destroyed the cells on the cut surface and damaged the shape of the cells beyond identification considering they were melted and tangled like a spider's web. While most methods damaged the surface layer, it was observed in our test that a cutter knife was the appropriate tool to maintain the cells on the cut surface and exhibit the lowest degree of damage. Therefore, all specimens were prepared by using a cutter knife, which has a blade thickness of less than 0.5 mm. Figure 1: (**a**) Slicing tools; (**b**) 40\(\times\) magnified cutting cross-section of specimen corresponding above tools. For measuring the thermal conductivity, heat flow meters from Netzsch (Heat Flow Meters HFM 436 Lambda Series) were used based on the measurement methods of ASTM C 518 (Standard Test Method for Steady-State Thermal Transmission Properties by means of the heat flow meter apparatus) and ISO 8301 (Thermal Insulation--Determination of Steady-State Thermal Resistance and Related Properties). When heat is applied to the specimen, heat is transferred by the specimen in a steady state. Heat flux, thermal conductivity and test procedure including calibration and measurement method were followed by ASTM 518. In ISO 8301, three major components of heat transfer such as radiation, conduction in both solid matrix and foaming gas phase, and convection were covered in dealing with the heat flow meter to measure the steady-state heat transfer by the specimen. Parameters such as material composition, moisture, mean temperature, and prior thermal history regarding conditioning of material, which may infect the thermal transmission properties were referred [12; 13]. The high and low temperatures for the experiment were 33 \({}^{\circ}\)C and 7 \({}^{\circ}\)C, respectively, with an average temperature as of 20 \(\pm\) 1 \({}^{\circ}\)C [14]. HEM 436 Lambda deals with specimens from 5 mm up to 100 mm in thickness and 305 mm in length. Thermal conductivity in between 0.005-0.50 W/(m\(\cdot\)K) can be measured with an accuracy range of \(\pm\)1-\(\pm\)3% and 0.5% of reproducibility. Four different samples were prepared as summarized in Table 1. Insulation material was the same as XPS special grade in Korea. Specimen 1 and 2 represented the results of specimens on the 1 st and 2 nd floor from our previous study about real-time ageing tests for 50 mm thick XPS board [15]. To compare with previous results, additional samples were denoted as specimen 3 and 4, respectively, for the sliced acceleration test. All samples were stored under constant temperature and relative humidity as of 23 \({}^{\circ}\)C and 50%, respectively. Difference between specimen 1 and 2 was solar radiation exposure condition. In Figure 2, a schematic diagram of the heat flow meter was drawn to simply describe the principle of heat flow measurement.