Green roofs can often lead to moisture damage, especially with fully insulated wooden designs. One possible cause is the fact that green roofs – like all flat roofs – are tight on the “wrong” side from a building physics perspective: namely, the external side. This means that drying can only take place in an inward direction. Green roofs tend to heat up much less than flat roofs without a surface layer. As a result, they can fail to dry adequately during the summer months on account of the vegetation. For this reason, wooden green roofs need precise, expert planning. However, traditional moisture calculation techniques such as the Glaser method are unable to guarantee this precision.
To explore new avenues toward this goal, a research project on the hygrothermal behavior of vegetation-covered surfaces ran from November 2011 to April 2013. Funded by the German Federal Institute for Research on Building, Urban Affairs and Spatial Development’s “Zukunft Bau” research initiative, the project included comprehensive open-air and laboratory experiments. In the course of this project, Fraunhofer IBP scientists managed to successfully implement new assessment models for the evaluation of green roofs in the institute’s widely used WUFI® Pro simulation software. This breakthrough gives planners and building product manufacturers an extremely precise and reliable basis for planning moisture protection and also for evaluating the energy behavior of green roofs.