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 risksrelated to rainwater fall and the subtraction of CO2 from the atmosphere. To improve the energy performance ofexisting buildings, the retrofit with green roof is an effective solution, but it is mandatory to carefully consider theircompatibility with pre-existing structures due to the additional loads. This article through dynamic thermal simulationevaluates the magnitude of energy savings originating by the use of two different types of green roofs applied on theflat roof of a multi-storey residential building. The conducted simulations allow to calculate, for the two differentlayers of vegetation used, sage or sedum, the outer surface temperatures of the roof and the indoor surfacetemperatures of the local positioned under the green roof. Moreover, the annual energy consumptions spent for spaceheating and cooling are evaluated. Through the data obtained, the improvement of the energy performance of thecover is evident for both the layers of vegetation used. To test the different effectiveness of green roof retrofit, thesame simulations are developed at different latitudes representing different European climates, particularly inCatania, Amsterdam and Stockholm. Finally, it is determined whether green roof is cost-effective and how manyyears are necessary to recover the investment. The economic analysis proves to be quite costly and involves a longpayback period despite the tax breaks. However, looking beyond the merely economic interests of the investingprivate, the retrofit of buildings through green roof maintains its attractiveness considering the improvement of theenvironmental quality, with positive implications for the whole community.