Hygrothermal effects in lightweight roofs shaded by PV-elements

Sealing roof constructions against water and vapour on both sides only works well as long as the seal is perfectly tight and no water was built in, because any water trapped in the construction can not dry out. Therefore, a common way to build wooden lightweight roofs is to compensate for the vapour tightness of the roofing membrane by installing a moderately permeable membrane at the inside to provide some drying potential for safety reasons. However, drying to the indoor space will only happen if the solar heat gains of the roof are high enough to reverse the vapour flow for a sufficient time period. The application of PV elements on a roof leads to permanently shaded areas on the roof surface. To account for the attenuation of solar radiation below such panels a model has been developed which computes the influence of radiation on a point shaded by a quadrilateral object obscuring part of the sky. Using this data the balance of interstitial condensation in such constructions and subsequent drying is investigated using an adapted hygrothermal simulation tool for heat and moisture transport in building components. The investigations show that attaching PV elements on wooden lightweight roof constructions may cause problems by moisture accumulation in some assemblies.