Impact of Garden Greening on the Outdoor Thermal Environment of Buildings - a Case Study of Japanese Residential Houses

Garden greening not only enhances the aesthetic value of residential buildings but also plays a critical role in improving microclimates and regulating the outdoor thermal environment. In Japan, incorporating garden greenery has become a prevalent feature of residential architecture. However, limited empirical research exists that systematically compares how different types of vegetation influence outdoor thermal conditions in Japanese residential gardens. This study addresses that gap by examining the impact of varying greening configurations on air temperature and relative humidity. Field measurements were conducted during both summer and winter in two traditional residential gardens located in Kitakyushu, southern Japan, using four monitoring points representing different greening structures. Results revealed that air temperature and relative humidity showed consistent patterns across seasons, with Measurement Point A (combining herbaceous plants, shrubs, and trees) yielding the lowest average temperature and highest humidity, while Point D (no greening) exhibited the highest temperature and lowest humidity. The findings confirm that vegetation type significantly affects the microclimate. Notably, while vegetation improves outdoor comfort in summer by reducing heat stress, it may worsen thermal comfort in winter by retaining humidity and lowering temperature. This study provides evidence-based insights into the dual seasonal effects of garden greening, offering valuable guidance for sustainable residential garden design in Japan’s urban context.