In response to concerns over resource shortages and environmental impacts, biobased materials are increasing in popularity. This includes an interest in replacing traditional vapour control systems, including polyethene (PE) membranes. However, the susceptibility of these materials to moisture-related degradation poses challenges. This study examines the water vapour diffusion resistance of the vapour retarder and the wind shield as key properties. Examining wood frame walls designed with low water vapour diffusion resistance wind shields, this study analyses the necessary properties of the vapour retarder as a function of the properties of the wind shield. We evaluated exterior wood frame walls that were thermally insulated with materials including mineral wool and biobased options such as flax, grass, wood fibre, straw, and cellulose. Using WUFI Pro software, we determined the relations between properties necessary to prevent mould growth. Hygrothermal simulations determined the necessary properties of the vapour retarder as a function of the properties of the wind shield. Analyses were carried out in temperate cold climates. Wind shield diffusion tightnesses ranging from 0.01 to 1 (m2<middle dot>s<middle dot>GPa)/kg were evaluated. Assessments were performed for walls with a U-value of 0.15 and 0.10 W/(m2<middle dot>K). The indoor humidity classes 1 to 3, as defined in EN ISO 13788, were used for the simulations. The results indicate that the necessary properties of the vapour retarder depend on the properties of the wind shield, as well as the insulation material, the indoor humidity, and the U-value. As the wind shield diffusion tightness decreases, the necessary vapour retarder diffusion tightness also decreases, eventually reaching a fixed value determined by the insulation material, the indoor humidity, and the U-value.
