Thermal effectiveness of green roofs on NZE buildings

The research deals with the experimental assessment of the thermal performance of a green roof compared to other passive cooling technologies (roofs with ventilated cavities or with a reflective covering), under temperate climate in Italy. All roofs are installed on a highly insulated building, in order to understand whether their performance is inhibited by the low thermal transmittance necessary to meet the building energy saving regulations in Europe. Green roofs are also recognized as a fundamental strategy that dense urban areas can deploy to mitigate the Urban Heat Island effect, thanks to their high albedo. Roofs albedo and thermal transmittance were experimentally evaluated. The measurement results show that the passive cooling property of a green roof is not only due to its albedo, but also to combined effects of soil insulation, evapotranspiration and radiative shading of the plant canopy. In summer, a reduction of the surface temperatures has been recorded on the upper part of the soil substrate where temperatures turn out to be lower than those of the external air, and also on the roof soffit because of the thermal inertia of the soil substrate. In winter, the soil substrate of the green roof acts as a natural insulating material, providing further thermal resistance to the roof.