3.1. Thermal performance comparison under free-floating condition
Meteorological and soil water ratio data between 08/08/2014 and 12/08/2014 are shown in Fig. 5,
when both test rooms were under free-floating condition. Fig. 6 reflects thermal performance
difference between green roof and common roof. It can be seen from Fig. 6(a), indoor air
temperature under green roof was 2 ◦C lower than that under common roof at noon when solar
radiation was strong, while they were almost the same in cloudy and rainy days. At night, air
temperature under green roof was 2.5 ◦C higher than that under common roof, which showed the
insulation effect of green roof. Fig. 6(b) indicates that green roof reduced temperature fluctuation
of structural layer significantly, the maximal tempera- ture amplitude was 6.5 ◦C for green roof
while 39 ◦C for common roof. Besides, there was no obvious lag between both temperatures. Effect of
green roof on local microclimate is shown in Fig. 6(c), the local air temperature above green roof
was about 5 ◦C lower than that above common roof when solar radiation was strong, and little
difference could be seen at night. Heat fluxes through both roofs are presented in Fig. 6(d). Heat
flux through green roof was neg- ative all the day, which suggests that heat always flowed out of the
room. By contrast, for common roof, a large amount of heat entered the room at noon, and more heat
escaped away from the room at night. Cooling effect of green roof was most remarkable on
sunny
sunny days. The maximal difference of heat flux through both roofs was up to 15 W/m2. Equivalent
thermal resistance was often used to assess thermal performance of green roof in some literatures
[29,30], which treated green roof as a common thermal insulation,
t suitable for free-floating condition in this experiment.