Influence of driving rain and vapour diffusion on the hygrothermal performance of a hygroscopic and permeable building envelope

Most natural building materials are hygroscopic and permeable to water vapour. These two characteristics have the potential to improve the longevity and indoor air quality of buildings. However, the potential of winter condensation due to vapour diffusion and the risk of mold growth should be assessed for safeguarding the longevity of building assemblies. This study investigates the relative importance of driving rain, plaster capillarity and the presence of a vapour barrier on the moisture content of building materials and the risk of mold growth for a hygroscopic and permeable building envelope (HPBE). Hygrothermal simulations of a single-family house in Denmark mainly made of wood and clay are performed with WUFI. Results indicate that the presence of an overhang is essential to ensure the durability of a HPBE rendered with a capillary active lime-based plaster while the presence of an overhang has a negligible impact for a mineral cement-based plaster. Including a vapour barrier did not introduce significant changes on the moisture content of this wall assembly. Simulation results indicate that the type of plaster and the wind-driven rain exposure are the most critical variables affecting the hygrothermal performance of this wall assembly.