Green Roofs as passive cooling strategies under Temperate Climates

The research aims to the comprehension of the energy performance of a green roof, also compared to other passive cooling strategies, when realized on slabs with very low thermal transmittance, as imposed by recent laws on the energy performance of buildings also in Temperate Climates. Although many studies in literature highlight the potential of green roofs on the improvement of internal conditions (reduction of incoming heat fluxes, thermal wave phase shift), knowledge regarding the use of these technologies in highly insulated structures is quite limited. All EU countries in particular, following the issuing of two important Directives (“Save” and “Nearly Zero Energy Buildings”), are enacting decrees which have placed regulatory requirements and strict limits to the thermal transmittance of building components. If the “overinsulation” strategy in Temperate Climates (i.e. Italy, Greece, Spain), is useful for the winter energy savings, it risks to limits the effectiveness of alternative passive cooling strategies, as the use of green roofs. We carried out a thermal monitoring of seven low U-value roof systems, differentiated by the type of covering, the type of slab, the presence of a ventilated cavity, the presence of a vegetated substrate. Experimental results show that the green roof and the other passive cooling technologies benefits, as the reduction of the incoming heat flux and internal surface temperatures, are quite limited by the roofs high insulation imposed by actual laws, which consider the overinsulation as the whole strategy for energy saving even in Temperate Climates.