Assessment of the effectiveness of cool and green roofs for the mitigation of the Heat Island effect and for the improvement of thermal comfort in Nearly Zero Energy Building

The effectiveness of cool and green roofs to improve thermal comfort could be strongly dependent on the U-value of the roof itself and on the way it has been constructed (ventilated or unventilated, lightweight or massive, etc.). Recent strict limits on the U-values of building envelopes run the risk of reducing the effectiveness of cooling strategies in roofs which could be employed in warm and temperate climates to reduce surface temperatures and consequently to cool internal environment. In this paper, we experimentally analyse some roof systems (a high-albedo membrane and a green roof) compared to traditional ones in a Nearly Zero Energy Building, in order to provide new information concerning their effect on the internal comfort and the air temperatures of the surrounding environment. Experimental results confirm that, while the effectiveness of green and cool roofs for the mitigation of the Urban Heat Island effect is well established, the use of high-albedo materials on roofing systems with very low U-value is of little effectiveness for internal comfort. The green roof is distinguished by its passive cooling ability due to the evapotranspiration phenomena of the vegetation and the storage capacity of the substrate.