Enhancing values of roofs albedo for lowering cities’ air temperature and electric demand of buildings: a simple economic evaluation

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.