Requirements of the Vapour Barrier in Wood-Frame Walls

This paper examines the water-vapour diffusion resistance (Z-value) of vapour versus wind barriers by determining their Z-value ratio in exterior wood-frame walls thermally insulated with six different materials to prevent mould growth. Using WUFI Pro, the water-vapour diffusion resistance requirements were determined for thermal insulation using mineral wool and biogenic materials: wood fibre, straw, flax, grass, and hemp. Hygrothermal simulations determine the minimum Z-value ratio between these materials with vapour versus wind barriers in temperate and cold climates. Wind barriers with Z-values between 1 and 8 GPa s m2/kg were used in walls with U-values of 0.15 and 0.10 W/m2 K. The indoor moisture load was defined from classes of 1 to 5 with a U-value of 0.15 W/m2 K and classes of 2 and 3 were used for a U-value of 0.10 W/m2 K. The Z-value ratio depends on the Z-values of the wind barrier and thermal insulation material, moisture load class, and U-value. The required Z-value ratio declines with an increased wind-barrier Z-value. The vapour-barrier Z-value approaches a fixed threshold for wind-barrier Z-values approaching lower values (1 GPa s m2/kg) and those approaching higher values (8 GPa s m2/kg), depending on the thermal insulation material. This parameter study examines wind barriers with a Z-value ranging between 1 and 8 GPa s m2/kg, which characterises typical wind barriers used in Denmark For the water-vapour diffusion resistance requirement of the vapour barrier, the Z-value increases for increased moisture load classes and thermally insulated walls with lower U-values. The conversion between the Z-value, the Sd-value, and the water-vapour resistance factor µ can be found in DS/EN ISO 12572:2016.

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