A multi-criteria methodology for comparing the energy and environmental behavior of cool, green and traditional roofs

• Several roof typologies are compared, based on energy, environmental, comfort issues.
• Cool roofs have beneficial effects in summer, but not necessarily in winter.
• Excessive roof insulation may increase thermal stress and outer surface overheating.
• The performance of a green roof is almost insensitive to the thickness of insulation.
• On the whole, a green roof with moderate insulation is the more performing solution.

The reduction of energy demand for space cooling, as well as the mitigation of the Urban Heat Island effect (UHI), require adequate solutions at building and urban scale. In particular, the roofs of buildings have been identified as a possible field of intervention that could contribute to provide significant energy savings and environmental benefits. In this context, cool and green roofs are two very interesting solutions, which may allow obtaining both the reduction of the energy consumption and the improvement of the comfort sensation in the outdoor and indoor environment.This paper presents a numerical comparative analysis of the energy and environmental performance of three typologies of roof, namely a standard roof (SR), a cool roof (CR) and a green roof (GR). This analysis, developed through dynamic energy simulations under temperate climate, highlights the different thermal behavior for the investigated roof scenarios, also as a function of the level of thermal insulation. Moreover, with the aim to obtain a classification between SR, CR and GR in Mediterranean climate, a scoring system is presented, which considers the whole performance of the investigated roof scenarios under a broad perspective.As a result, it is found that green and cool roofs provide higher energy savings and environmental benefits than highly insulated standard roofs. In particular, scarcely insulated green roofs showed the best performance in relation to the UHI mitigation under the climatic conditions typical of the Mediterranean area.

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