Technical and economic analysis of green roofs to reduce building cooling needs

Several public/private institutions, at international and national levels, consider green roofs effective energy efficiency measures. A large feasibility study of their technical and economical profitability is proposed here. The main heat transfer phenomena and a brief literature state of the art are presented. Congruent with the cost-optimal approach, the economic indicators are described, as well as the physical set of equations for evaluating the energy performances of green roofs. Moreover, the case studies are shown by describing the various climates considered, energy and water costs, efficiency of power systems, emissions factors, and all parameters necessary for suitable feasibility studies. Various climates and building technologies are taken into account, as well as common typologies of extensive green roofs. The results show a relative feasibility of green roofs, when the watering costs are limited, and if the buildings are not properly insulated, so that the installation of greenery, combined with the added thermal insulation, provides energy savings for both heating and cooling. A final section proposes some considerations for improving the profitability of green roofs by adopting rainwater harvesting systems.