Hygrothermal performance and damage risk of green roofs

Green roofs become more and more popular especially in cities where they can help to improve the microclimate by reduced peak temperatures compared to conventional roof surfaces. They also reduce the gutter loads by rain water retention and save both heating energy in winter and cooling energy in summer. On the other hand, the cooler temperatures in summer may reduce the drying potential of the load bearing structure beneath the green roof. In inverted roofs the permanently humid conditions can lead to moisture accumulation in the foam insulation slabs. Due to a multitude of influencing factors like moisture storage, freezing, evaporation, shading and reduced radiation absorption through the plant cover etc. a simulation of the hygrothermal behaviour of green roofs represents a challenge. Therefore, the hygrothermal conditions of green roofs on wooden structures and on inverted concrete roofs at different locations has been measured and evaluated for several locations in Europe. Based on these experimental results a new approach was developed to calculate in a realistic way the hygrothermal performance of green roofs including the important influencing factors mentioned before. This contribution first investigates the conditions beneath the growth medium layers and the water content of the insulation boards and of the wooden sheathing by field tests in Holzkirchen, Vienna and Leipzig. In the second part a new approach to simulate green roofs including the growth medium layer is shown. In the case of inverted green roofs, the moisture film beneath the insulation boards is a dominant factor too that must be taken into account in the simulation. Finally, the results of simulations and experiments are compared in order to validate and fine-tune the new approaches that have been developed to model the behaviour of vegetated wooden structures and inverted concrete roofs in South, Central and North Europe.