Experimental and numerical analysis of the energy performance of a large scale intensive green roof system installed on an office building in Athens

• 15 K lower surface temperature.
• Plants present low surface temperature and a high mitigation potential.
• Indoor temperature decreases up to 0.7 K.
• Important decrease of the cooling needs.

The aim of this study is to investigate the thermal behaviour and the energy efficiency of an intensive green roof system composed of indigenous aromatic plants with low irrigation needs, installed in the 10,000 m2 roof of a fully insulated, low energy office building in Athens, Greece. It consists of almost 16,000 Mediterranean plants of at least 14 different kinds and a running track of a stabilized ceramic floor. The urban heat island mitigation potential of the green roof as well as its energy contribution is investigated using experimental and theoretical methodologies. The surface temperature of the green roof is found to be up to 15 K lower than that of a conventional roof. Plants having a low absorptivity to solar radiation together with a dense foliage are found to present a much lower surface temperature and a higher mitigation potential. The surface temperature of the plants is found to be highly influenced by the ambient air temperature. Using simulation techniques it is calculated that such a type of green roofs can decrease the average indoor temperature of a non air conditioned building up to 0.7 K, while it may decrease substantially its annual cooling and heating needs.