Green roofs in European climates. Are effective solutions for the energy savings in air-conditioning?

► Scarce literature investigates the technical–economical feasibility of green roofs. ► This paper provides a wide analysis of their energy/economical performances. ► The amount of required irrigation is a key node in order to achieve benefits. ► In cold climates, green-roofs allow good performances, both in summer and winter. ► In warm climates, green roofs are not advantageous for well-insulated buildings. Several studies show the potential benefits achievable by recurring to roof vegetation. Really, little literature investigates the economic feasibility of such solution. The paper verifies utility of green roofs, under environmental and energy point of views, by considering all the aspects that influence their performances. With reference to several climates, intensity of rainfalls, needs of irrigation and kind of building use, a large parametric analysis evaluates the technical and economical feasibility of green roofs applied to a modern office building, considering various vegetations and different external coatings. The scarce amount of rainfall – and thus the irrigation cost – can nullify the savings in energy demand for air-conditioning. Moreover, even if green roofs show satisfactory performance if monthly rainfalls do not imply significant additional watering – the economic investigation shows scarce convenience for well-insulated buildings, above all if the higher initial cost of a green roof, compared to traditional roofing coating, is computed. Finally, cool roofs, by means of high-reflective and high-emissive coatings, are suitable solutions in warm climates, strongly improving the summer performances, with low extra costs for installation and maintenance.