Cool roofs with variable thermal insulation: UHI mitigation and energy savings for several italian cities

The progressive modification of the land surface in the urban area during the last decades has involved an increase in urban air temperature compared to surrounding rural areas. The significant replacement of green areas with built surfaces characterized by high solar radiation favored the energy storage during the day and heat release in the nocturnal hours. The Urban Heat Island (UHI) is an emblematic effect of the increase of urban temperature which in turn affects the energy consumption in the building sector. Several passive building technologies are available to mitigate the UHI phenomenon and reduce the energy consumptions in buildings. In the Mediterranean climate, where the solar radiation control is the most important passive strategy in summer, cool roof technology is able to reduce the solar radiation absorbed by materials and to limit the outer superficial temperature in the presence of high solar loads. This paper presents a numerical analysis of the energy and environmental performance of an office building equipped with a cool roof for three different climate zones. Dynamic simulations highlight the different thermal behavior of the investigated cool roof in comparison to the standard roof, also as a function of the roof thermal resistance. Moreover, the Solar Reflectance Index (SRI) of the different roof configurations has been evaluated. It was found that slight or no insulated cool roofs has the lowest superficial temperatures and hence mitigates the UHI.