This study investigated the thermal performance and passive cooling effect of an extensive green roof based on an experiment conducted on sunny days in the summer of 2007, for a room with a green roof and a room with a bare roof under air-conditioned and nonair-conditioned states. Three different heat transfer scenarios in terms of average heat flux inside the green roof are defined: (1) heat transfer from indoor space to the green roof when the indoor-outdoor air temperature difference is relatively small, (2) heat transfer from the green roof to the indoor space when the indoor-outdoor air temperature difference is large, (3) ideal thermal equilibrium scenario that heat flux between green roof and indoor space equal to zero. “Cooling flux” is proposed to explain and qualify the passive cooling effect from evapotranspiration on sunny days. A simple method to approximate the “cooling flux” is purposed. The result shows at least 27.4% of the heat flux was absorbed by the passive “cooling flux“ under air conditioned status.
Keywords extensive green roof, critical air temperature, heat flux, cooling flux