Economic development limits the living quality of rural residents. In particular, the residential buildings in northern China generally have poor thermal comfort in winter, which affects the physical and mental health of residents. Because of the separation of rooms, residents who live in cave dwellings often have to enter and leave rooms in the course of their daily lives, which leads to worse thermal feelings in winter. Because of the low price and the wind insulation and heat storage, sunrooms are widely used in renovations of rural houses. The traditional purpose of the addition of a sunroom is to provide a buffer room between outdoor and indoor spaces. This manuscript focuses on improving the degree of thermal comfort by means of a sunroom connecting all rooms. This study selected two families with the same number of members and similar daily activities as the study cases. One of the families had a sunroom built to connect its bedroom, living room, and washroom. The household’s air temperature and human comfort were measured both on holidays and on workdays. It is demonstrated that adding a sunroom can significantly stabilize the thermal environment and increase the air temperature in both the bedroom and the living room. Adding a sunroom can increase the air temperature of a cave dwelling’s main room by 1.0 °C on workdays and 4.3 °C on holidays. A cave dwelling with a sunroom can also provide residents with a decent level of human comfort for 24.4% of their daily time on workdays and 39.1% of the time during holidays. This research demonstrates that a sunroom can not only increase the air temperature in cave dwellings but also enhance the stability of human comfort. The conclusion provides new renovation ideas for improving the living comfort of cave dwellings.