Green roof: benefits analysis and development of a simplified dynamic energy model

Green roofs (vegetated roof surfaces) are increasingly realised in cities, because they are an important strategy addressing some key urban environmental issues and allowing the achievement of different benefits. At first, the work analyzed the advantages related to the realisation of a green roof, as a retrofit technique of buildings, with the aim of testing it in the context of historic cities; particularly, energy benefits obtained in existing buildings are quantified in winter and summer seasons. With this purpose, a simple and specific tool has been developed, to provide a complete methodology for the evaluation of energy needs and simulate the thermal energy behaviour of a green roof, i.e., a simplified dynamic energy model. As practical example of an extensive green roof, an existing building of the University of Pisa has been chosen: it has features easily observable in surrounding properties and energy results are relevant to similar buildings. The aim is to quantify the energy saving that would be obtained by applying this building retrofit technique to an entire University area, in order to demonstrate the economic feasibility of the intervention and the achievement of the above-mentioned environmental benefits. Main benefits related to the realisation of a green roof within this University area are a reduction of the used energy (Delta E-thermal = -86.5 MWh and Delta E-electrical = -16.4 MWh, resulting in annual savings of around 10,200 (sic)) and a reduction of rainfall contributions to the sewer system (up to 369 m(3) of water for each rainfall event).