## 3 Experimental Verification  
### Material Evaluation  
The materials currently used for automotive interior parts have diverse physical properties. To accurately confirm the thermal insulation performance for the fabrication of high-thermal-insulation components, we first identified the exact thermal characteristics of each normally used material. Four materials were selected for the evaluation: dry polyurethane (DPU); wet polyurethane (WPU); high-stiffness felt (HSFELT, and polypropylene glass fiber (PPGF). Figure 4 compares the thermal conductivities and air permeabilities of each material; in our study, DPU was chosen as the core of the conventional insulation model (hereafter the \"conventional model\") due to its lower air permeability relative to WET. The thermal conductivity variation of DPU with temperature is shown in Figure 5. To improve the reliability of the results when measuring the insulation performance, we utilized the thermal conductivity values under the same temperature condition (25 \({}^{\circ}\)C) during evaluation. Figure 4: Comparison of thermal characteristics of each material considered: (**a**) Thermal conductivity; (**b**) Air permeability. Figure 3: Product development process according to the evaluation method of insulation performance standard of automotive interior parts. The vacuum insulation material used in the study was high-performance insulation with a thermal conductivity of \(<\)0.0045 W/mK, with vacuum evaporation realized within the insulation at less than 1 mbar. The resulting thermal insulation performance was \(-10\) times better than that of conventional building vacuum insulation material whose thermal conductivity ranges from 0.031 to 0.045 W/mK. Since this vacuum insulation material has a low thermal conductivity, it is mainly used to improve the energy efficiency of buildings and devices such as refrigerators. Likewise, the vacuum insulation material can exhibit excellent insulation performance when applied to vehicles. In addition, because it is an inorganic insulating material, harmful and toxic gases are not generated in the event of a fire. Meanwhile, in the case of automotive interior parts, the appearance quality is very important; surface irregularities in the material can lead to mismatch with the peripheral parts, and thus the vacuum insulation material cannot function properly. Here, we note that the conventional model using the DPU core has the disadvantage of having an irregular surface because of the use of the common nylon film on the outer wall. To solve this problem, in the improved model using the vacuum insulation, we used a laminated heat-resistant polyolefin-based adhesive film on both sides of an aluminum-foil-laminated film with a thickness of 50 \(\upmu\)m on a glass wool core material that was used as the vacuum insulation material. Figure 6 compares the surfaces of the conventional model with a DPU core and our improved model with the glass wool core. In the study, we tested the vacuum insulation materials based on IEA ECBCS Annex 39 [27; 28]. The material thermal conductivity was measured according to the ISO 8302 standard [29]. The test used two identical specimens, and the thermal conductivity was measured by studying the temperature difference between the hot and cold surfaces, resulting from the application of a heater between the two specimens. The specimens were 200 mm \(\times\) 200 mm in size and 10 mm thick. In the conventional model, the outer shell of the DPU core was composed of a nylon shell. In the proposed model  
Figure 5: Thermal conductivity of dry polyurethane (DPU) material as function of temperature. Figure 6: Comparison of insulation surface: (**a**) Conventional; (**b**) Improved. (hereafter the \"improved model\"), the glass wool core was composed of the vacuum insulation of aluminum-foil-laminated film. The thermal conductivities of the DPU and vacuum insulation materials were estimated to be 0.0364 W/mK and 0.0028 W/mK, respectively.