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Energy policy makers, architects and researchers, when designing new buildings or rehabilitating the existing ones, are engaged in the attempt of limiting the energy needs for climatization (NZEB buildings) and the environmental pressure exerted by buildings (EU Climate Action). The pursuit of this demanding assignment calls for innovative solutions in conceiving the building envelope and its energy systems. Recently, among the most effective tools for improving the energy and environmental performances of buildings, the technical interventions regarding the roofs are gaining a rising attention. Indeed, covers of buildings are responsible of a relevant part of their energy losses and, on the other hand, could contribute to increasing the UHI effect. In the paper, a simple methodology that compares the energy and environmental benefits of cool and green coverages with traditional ones is presented. The capability of limiting the UHI effect of these solutions is also analysed. The methodology is applied to four cities of the Sicilian Island, characterized by different building density and different microclimates. This in-field application shows the feasibility of the method to be used in different urban contexts, as a useful contribution to the design of new buildings or to the re-design of the existing ones.

In cities vegetated roofs are becoming more popular because they can mitigate Urban Heat Island phenomena by decreasing the outdoor air temperature in summer. This decrease reduces the electric energy demand for climatization of buildings, which, in front of a milder climate, will recur less to mechanical tools for guaranteeing thermal comfort conditions to occupants. Cities can registered another indirect positive effect: the reduced cooling energy demand, limits the heat released by the climatization systems’ external unities toward the urban open spaces, thus lowering the outdoor air temperature. Therefore, the outdoor surface temperature of green, as well as cool roofs, can be assumed as an important design parameter for guaranteeing the sustainability of buildings and their approach toward an nZEB path. Obviously, designers and city planners must have at their disposal simple and effective tools for evaluating the economic feasibility of these two choices. This paper proposes a simple method to assess the economic effectiveness of green or cool roofs. It relies on the appraisal of the number of hours during which a building requires a cooling mechanical support for maintaining the indoor comfort conditions. This duty period of the cooling system is then simply converted into the cost of the needed electric energy.

Energy policy makers, architects and researchers, when designing new buildings or rehabilitating the existing ones, are engaged in the attempt of limiting the energy needs for climatization (NZEB buildings) and the environmental pressure exerted by buildings (EU Climate Action). The pursuit of this demanding assignment calls for innovative solutions in conceiving the building envelope and its energy systems. Recently, among the most effective tools for improving the energy and environmental performances of buildings, the technical interventions regarding the roofs are gaining a rising attention. Indeed, covers of buildings are responsible of a relevant part of their energy losses and, on the other hand, could contribute to increasing the UHI effect. In the paper, a simple methodology that compares the energy and environmental benefits of cool and green coverages with traditional ones is presented. The capability of limiting the UHI effect of these solutions is also analysed. The methodology is applied to four cities of the Sicilian Island, characterized by different building density and different microclimates. This in-field application shows the feasibility of the method to be used in different urban contexts, as a useful contribution to the design of new buildings or to the re-design of the existing ones.