Simulation of heat exchange phenomena and water regime in green roof substrates

Abstract : Installation of intensive or extensive green roofs increases in Western European and North American cities. It is estimated that 12% of all flat roofs are green in Germany and the roof industry is growing at a rate of 10% per year in the EU. Apart from the aesthetic point of view, the main objective is to reduce heating (by increasing thermal resistance value) during winter and cooling (by evaporation) during summer. Most researches are conducted on specific plant species for green roof establishment. The major purpose of this work is to evaluate the theoretical approach of thermal properties (heat capacity and thermal conductivity) of wet substrates and to propose a simulation modelling of the interactions of the processes of water transport and heat flow and exchanges. Water flow properties using the van Genuchten approach and heat flow equation are applied in 1D simulation to different substrates and configurations. The main result is that the water retention properties of the substrate are the main parameter to consider but with different effects between winter and summer. The geometry of installation (height, support) is an important aspect of installation. Heat diffusion phenomena stay limited vs. latent heat exchanges in summer.