Improving the drying efficiency of timber frame walls in cold climates by using exterior insulation

Safe moisture performance of a structure requires high drying efficiency. Even if a structure has acceptable moisture performance under normal design conditions, long exposure to moist conditions due to high initial moisture content or accidental moisture loads may start biological deterioration. Timber frame walls with rigid exterior sheathing, such as oriented strand board (OSB) or plywood, have relatively low vapor permeance. Use of exterior insulation, made of vapor permeable, open porous material and installed at the cold side of the sheathing, is an effective method to improve the drying efficiency of a wall. The objective of this study was to present how the moisture performance of timber frame walls can be improved with vapor permeable exterior insulation. Numerical simulations and laboratory-scale experiments were used in the analysis. Both moisture content levels and a numerically solved mold growth index were used as criteria in the analyses. Experiments with wall structures having initial moisture loads and temperature gradients were used to study the drying efficiency through sheathing boards with and without exterior insulation. The conditions corresponded to those typical for cold climates experiencing drying: condensation with below and above freezing temperatures at the exterior sheathing. In some conditions, exterior insulation could improve the drying efficiency by nearly a factor of 8. Numerical simulation results show the effect of climate, thickness of exterior insulation, and moisture loads caused by diffusion and initial moisture content on the drying efficiency and moisture performance of timber frame walls.