Effect of vapor diffusion port on the hygrothermal performance of wood-frame walls

Vapor diffusion ports (VDPs) that are drilled in the exterior sheathing of wood-frame walls are commonly used in wood-frame buildings in the coastal region of British Columbia with the intention to improve the drying capacity of wood-frame exterior walls. This practice originated following the systematic building envelope failures due to rain penetration that occurred in this region around 1985-1995. A previous laboratory study 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 energy code found that VDPs did not significantly improve the drying rates. To provide a more comprehensive evaluation of the effect of VDPs, hygrothermal simulations using WUFI 2D are carried out based on the recent tests. The WUFI 2D model is firstly validated with measurements from the recent tests and the validated model is then used for a parametric study to evaluate the effect of VDPs under three wetting scenarios using yearly weather data for Vancouver. It is found that VDPs have the ability to improve the rate of drying, which is directly related to the moisture content level of the wall assemblies, although the improvement is moderate even for high moisture levels. The contribution of VDPs to reducing mold growth risk is insignificant under a moderate moisture load and becomes considerable only under a sustained high level of moisture load, i. e., east orientation with an ongoing rain infiltration. Therefore, efforts should be made towards properly designing and constructing building envelope details instead of relying on VDPs as a mitigation measure to improve durability performance.

Automated summary

VDPs can improve the drying rate of wood-frame walls to a certain extent, but the effect is limited. Proper design and construction of building envelope details are more important for wood-frame walls, rather than relying on VDPs.