Effect of Vapor Diffusion Port on Drying of Wood-Frame Walls

Abstract

Vapor diffusion ports (VDP) 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 the diffusion ports provided substantial improvement in the drying rates of Oriented Strand Board (OSB) sheathed walls, but not for plywood sheathed walls (Hazleden and Morris, 2001). A more recent laboratory test (Wang, 2018) 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 the test results provided by Wang (2018), which tested the effect of VDPs under laboratory conditions by using a wetted wood block installed inside each wall assembly as a simulated moisture source. The simulation results agree well with the measurements. The Model is then used for sensitivity analysis with different levels of rain leakage with and without VDPs using yearly weather data. The variables included are types of exterior insulation, types of sheathing (OSB versus Plywood), and types of sheathing membrane. This paper presents the validation of the model and simulation results under transient conditions with rain leakage. General conclusions on the effect of VDPs are provided.