In summer 2002, investigations on the temperature and the heat flux diffusion process of rooftop greening were implemented to examine the effect of vegetation density and irrigation on local thermal conditions. The sample sites with unplanted soil, low-density plantings, high-density plantings, and irrigation plantings and no-irrigation plantings with low-density plantings were set up. The temperatures were measured at the surface of planting area and beneath the soil mat of each plot, and then compared to each other. It was found that, when the soil was wet by rain and irrigation, these daily maximum temperatures were low, and the differences of temperatures with vegetation density were not significant. Besides, when the soil was dry in dry spell under no irrigation, these daily maximum temperatures were high, although these temperatures of higher-density plantings could be lower. A detailed analysis for the heat flux diffusion process revealed that heat flow directs from soil to building in the daytime and from soil to atmosphere in the nighttime. Both the values of daytime heat outflow to building and nighttime one to atmosphere were bigger in low-density plantings and smaller in high-density plantings. It is considered to be due to differences in evapotranspiration value in daytime and radiative cooling in nighttime.
