Thermal and economic green roof performance for building retrofit

Green roofs are technical solutions that are increasingly adopted to improve the quality of the urban environment. The major benefits are the improvement of the thermal performance of the building envelope, the reduction of risks related to rainwater fall and the subtraction of CO2 from the atmosphere. To improve the energy performance of existing buildings, the retrofit with green roof is an effective solution, but it is mandatory to carefully consider their compatibility with pre-existing structures due to the additional loads. This article through dynamic thermal simulation evaluates the magnitude of energy savings originating by the use of two different types of green roofs applied on the flat roof of a multi-storey residential building. The conducted simulations allow to calculate, for the two different layers of vegetation used,sage or sedum,the outer surface temperatures of the roof and the indoor surface temperatures of the local positioned under the green roof.Moreover,the annual energy consumptions spent for space heating and cooling are evaluated. Through the data obtained, the improvement of the energy performance of the cover is evident for both the layers of vegetation used. To test the different effectiveness of green roof retrofit, the same simulations are developedat different latitudes representing different European climates, particularly in Catania, Amsterdam and Stockholm. Finally, it is determined whether green roof is cost-effective and how many years are necessary to recover the investment. The economic analysis proves to be quite costly and involves a long payback period despite the tax breaks. However, looking beyond the merely economic interests of the investing private, the retrofit of buildings through green roof maintains its attractiveness considering the improvement of the environmental quality, with positive implications for the whole community.