During winter, much of the energy contained within the battery of an electric vehicle is consumed by the air heating system. This energy consumption reduces the cruising distance of an electric vehicle. Additionally, stop-and-go driving, i.e., having many stops while driving, and frequent door opening/closing are typical of the operation of lightweight trucks used by home delivery services in residential areas; these behaviors lead to air exchange between the outside environment and the vehicle interior. This air exchange puts additional burden on the heating system and influences the thermal comfort of the driver. The effect of door opening needs to be investigated to develop a more effective heating system for electric vehicles. In this study, numerical simulations of a right-hand drive vehicle were performed to evaluate the variation in the thermal environment of the vehicle cabin when the door is opened and closed in relatively severe winter temperatures. The result showed that the average vehicle interior air temperature decreased at a rate of 2 °C/s during door movement. The warm interior air rose as it moved outward because of buoyancy. Simultaneously, cold outdoor air flowed into the lower region of the cabin. The total heat loss was approximately 57 kJ when the door was left open for 3 s and 37 kJ when the door was left open for 1 s. The standard new effective temperature (SET^＊) of the driver decreased at almost the same rate as the air temperature. The equivalent temperature on the right side of the driver’s body decreased drastically and rapidly as a result of the door opening. In contrast, the equivalent temperature on the left side of the driver’s body decreased more gradually. The equivalent temperature of the driver’s head remained consistent throughout the opening and closing of the door. The equivalent temperature of the driver’s hand, which is a thermally sensitive part of the body, was affected by the air temperature change caused by the door opening. The door movement itself had less to do with these results than the temperature difference between the vehicle interior and the environment. Thus, this discussion is applicable to a wide range of winter situations. The temperature difference is the trigger for the air exchange. The results of this study suggest that heat radiators may be more effective than air heating in improving the thermal comfort experienced by the driver because they do not cause an air temperature difference, which would reduce the amount of air exchange.