It is important to understand that vapor diffusion can introduce moisture into a building assembly, and it can also allow moisture to exit the assembly; diffusion is both a wetting force and a drying force depending on the conditions of the vapor drive. In fact, when we refer to walls or roofs being able to “dry out,” we mean the mechanism of liquid water evaporating into vapor and diffusing through permeable materials, either to the inside or outside.
Perms Different materials allow diffusion due to vapor drive at different rates, and the ability of materials to retard diffusion is measured in perms. There are four classes of permeability, as seen in Table 6.1.
While vapor-permeable materials have the potential to allow water vapor into an assembly through diffusion when the vapor drive is sufficient, far greater concentrations of water vapor can be carried into the assembly through air infiltration; shown in this illustration is the aggregate amount of moisture that moves through these two mechanisms over the course of an average heating season in a cold climate. (Source: Lstiburek, Joe: “Insulations, Sheathings, and Vapor Retarders,”Research Report 0412,11/04, buildingscience.com)
Some common enclosure materials are shown in this chart. You should know the class of permeability of all materials in your building enclosure, and they must be examined for their collective effect on vapor diffusion in both directions.
Vapor barriers Materials that are Class I vapor retarders (typically called vapor barriers) can be used as the vapor control layer. A vapor barrier will prevent diffusion into an assembly, but will also prevent drying by diffusion out of the assembly.
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Vapor permeable assemblies Assemblies that are composed of vapor permeable materials will allow diffusion from either side of the assembly and will also allow drying from either side. For permeable assemblies, the air control layer will function as the vapor control
These three variables wetting, drying, and storage vary depending on climate conditions and material choices. A vapor control layer must be designed to meet the particular needs of your building in your climate.
Hygrothermal modeling Software exists to model the movement and potential accumulation of moisture and heat through an assembly over a period of time in particular climatic conditions. This type of modeling can help to ensure that the vapor control layer will function adequately for high-performance assemblies in cold or hot-humid climates, or where innovative or experimental materials or systems are being used. Hygrothermal modeling is more complicated than energy modeling, and should be conducted by an experienced professional.