Abstract

Vapor diffusion ports (VDPs) that are drilled in the exterior sheathing of wood-frame walls are commonly used in wood-frame construction in the coastal region of British Columbia with the intention to improve the drying capacity of wood-frame exterior walls. This practice was originated following the systematic building envelope failures due to rain penetration that occurred in this region around 1985-1995. A previous laboratory study carried out by FPInnovations found that VDPs provided substantial improvement in the drying rates of OSB sheathed walls, but not for plywood sheathed walls. A more recent laboratory test using wood- frame walls with higher insulation levels in compliance with the current more stringent energy code found that VDPs did not significantly improve the drying rates. On the other hand, the provision of VDPs may allow moisture ingress into the wall assembly instead in a damp environment. The difference in these two studies in terms of test wall sizes, moisture sources, and test conditions may have attributed to the different findings. To provide a more comprehensive and systematic evaluation of the effect of VDPs, hygrothermal simulations using WUFI 2D are carried out in this study. The WUFI 2D model is firstly validated by comparing simulation results to measurements from tests carried out under laboratory conditions by using a wetted wood block installed inside each test wall assembly as a simulated moisture source. The simulation results agree well with the measurements. The validated model is then used for parametric study with different levels of rain leakage deposited on wall assemblies with and without VDPs using yearly weather data. The variables studied include types of exterior insulation, types of sheathing (OSB versus Plywood), types of sheathing membrane, and the location of rain deposition. It is found that VDPs have the ability to improve the rate of drying that is directly related to the moisture content of the wall assembly, although the improvement is moderate even for high moisture levels. As a result, moisture content levels in walls with VDPs are lower during the wet season but remain the same in other times of the year. In addition, the times to dry from high moisture content to safe levels is reduced by approximately 50% under high initial MC assumed in sheathing. Mold-index calculation shows that the improvements on drying provided by VDPs have little contribution from the perspective of mold growth risk. VDPs are found to be more beneficial in OSB sheathed walls than in plywood sheathed walls, and less beneficial when coupled with an exterior insulation layer, due to the lower overall moisture in such assemblies.