## 1 Introduction  
The main obstacle to the commercialization of electric vehicles (EVs)--environmentally friendly cars--is that they offer lower mileage for a single charge when compared with the mileage of internal combustion engines [1; 2; 3; 4]. For vehicles with internal combustion engines, the vehicle cooling operation reduces fuel consumption by 20-30%, thereby reducing the mileage by 2%-30%; on the other hand, in the case of heating, fuel consumption is not affected as the waste heat of the engine is used [5; 6; 7]. However, in the case of an EV in which the waste heat source is relatively small, electrical power consumption caused by electrical heating decreases the mileage by about 30-50% [8; 9; 10]. Thus, small EVs are currently being designed for urban driving to achieve a target driving distance of 140-160 km per charge. As of today, city-based small EVs are not commercially viable owing to their long charging time and lack of charging infrastructure, and thus, car manufacturers are striving to secure the EV technical capabilities in this context [11; 12; 13]. Therefore, the comprehensive development of power-saving technology for heating and cooling operations in lieu of increasing the mileage has become essential for EV development. Currently, there is a need for efficient EV development strategies to reduce heating and cooling power consumption and increase the mileage per charge [14; 15; 16]. In order to increase the distance that the vehicle can travel during the heating/cooling operation and to improve passenger comfort, heat from outside the vehicle must be blocked effectively. Therefore, there is a need for using suitable thermal insulation management components that can minimize cooling and heating energy loss within the vehicle interiors [17; 18; 19]. Here, we note that the aesthetic quality, as well as thermal loading capacity, of automotive parts in a vehicle's interiors can directly impact passenger comfort. The headliner is an internal part on the roof of the vehicle, which affords good sound absorption and sound insulation performance to prevent outside noise or engine noise from entering the passenger cabin, thus enhancing passenger comfort. Further, the headliner also acts as a safety barrier for passengers in the case of a collision. However, thus far, the headliner has not been recognized as a product that can significantly contribute to the driving performance of the vehicle. In the stage of product development, the headliner is normally considered only in terms of its appearance, design sensibility, and stability; its insulation performance is not considered [20; 21; 22]. Consequently, this study examines the thermal insulation capacity of the headliner. The headliner is attached to the body panel of the vehicle. However, owing to its thickness and shape limitations, it is very difficult to improve its insulation performance (and that of internal components in general) with the use of common insulation materials. Further, regarding the enhancement of the heat insulating performance, the use of conventional heat insulating materials as the built-in component increases the overall volume and weight. In addition, the temperature within the vehicle cannot be easily managed, due to the heat released from the heat insulating material after a certain duration. Therefore, it is necessary to use materials with suitable thermal properties, such as vacuum insulation materials. Unlike conventional insulation, vacuum insulation creates a vacuum inside the headliner, and heat transfer is blocked because of the vacuum structure. The resulting heat insulating performance is better than that of conventional heat insulating materials, and even for low material thicknesses, the heat insulating performance is equivalent to or greater than that of existing heat insulating materials [23; 24; 25; 26]. Against this backdrop, in this study, we used a vacuum insulator to improve the surface roughness and thermal conductivity of the headliner and the overall insulation performance.