Permeable and Hygroscopic Building Envelopes: Hygrothermal Simulations of “Det Naturlige Hus”

Unlike most conventional building materials currently used nowadays, natural building materials tend to be hygroscopic and permeable to water vapour. These two characteristics have the potential to improve the longevity and indoor air quality of buildings. For instance, the use of hygroscopic materials such as clay plasters can significantly reduce indoor humidity fluctuations, which yields many other indirect health benefits. However, with many countries that commonly use vapour retarders, there is lack of knowledge and general design guidelines on how to design safe permeable and hygroscopic building assemblies. This paper presents hygrothermal simulations of “det Naturlige Hus”, a single-family house in Denmark mainly made of wood and clay. Simulation results indicate high levels of moisture on the exterior building layers, especially in the coldest months, mainly caused by driving rain and water vapour diffusion. The mold index calculated at the exterior surface of the exterior wood fibreboard according to ASHRAE 160 grew over the two years simulated period to reach a maximum of 2,7 and would likely reach higher values with a longer simulation period. When removing the exposure to driving rain, the maximum mold index reached a value of 0,2. With a recommended threshold of 3, it seems that the wall design investigated here could perform satisfactorily provided that great care is taken to minimize the wall exposure to driving rain.