Most building materials are permeable to water vapor. In cold climates during cold weather, such vapor generated in the house from cooking, dish washing, laundry, bathing, humidifiers and other sources, may pass through wall and ceiling materials and condense in the wall or attic space, where it may subsequently do damage to exterior paint and interior finish, and may even cause decay in structural members.
As a protection, material highly resistant to vapor transmission, called a vapor barrier, should be used on the warm side of a wall or below the insulation in a roof. Among the effective vapor barrier materials are such products as kraft papers coated on both sides, single or double-faced aluminum foils reflective insulation, aluminum foil backed gypsum lath and polyethylene plastic sheeting.
Materials also include thicker sheet materials sometimes called “structural” vapor barriers. Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion.
Blanket Insulation Barriers
Some types of blanket insulation have a barrier material on one side. Such blankets should be attached with the tabs at their sides fastened on the faces of the studs, and the blanket should be cut long enough so that the cover sheet can lap over the face of the soleplate at the bottom and over the plate at the top of the stud space.
Where the membrane type of vapor barrier is used, it should be applied vertically over the face of the studs and tacked down. The lath or wall finish is then applied over the vapor barrier. Vapor barriers should be cut to fit tightly around electrical outlets and switch boxes.
Paint coatings cannot be considered a substitute for the membrane type of vapor barriers, but do provide a good degree of protection for houses where other types of vapor barriers were not installed during construction. For paint, one coat of aluminum primer followed by two decorative coats of flat oil based wall paint is effective.
For rough plaster or for buildings in very cold climates, two coats of primer may be required. Primer and sealer of the pigmented type followed by decorative finish coats or two coats of latex-based paint are also effective in retarding vapor transmission. While it was once believed that only coatings with low perm ratings constituted the only effective vapor barriers, it is now believed that any paint or coating is effective at restricting most water vapor diffusion in milder climates.
The older term “vapor barrier” is still used even though it may inaccurately imply that the material stops all of the moisture transfer. Since everything allows some water vapor to diffuse through it to some degree, the term “vapor diffusion retarder” is more accurate.
The ability of a material to retard the diffusion of water vapor is measured by units known as “perms” or permeability. A perm at 73.4 degrees F (23 degrees C) is a measure of the number of grains of water vapor passing through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury (1″ W.C.) Any material with a perm rating of less than 1.0 is considered a vapor retarder.
Knowledgeable builders typically use vapor diffusion retarders with ratings of 0.1 or less. However, if you carefully seal the warm-side vapor diffusion retarder and interior finish, you can also safely install a low-permeable material, such as rigid foam board insulation (a perm rating as high as 1.4), on the cold side of walls.
A good rule-of-thumb: to prevent trapping any moisture in a cavity, the cold-side material’s perm rating should be at least five times greater than the value of the warm-side. Use a vapor diffusion retarder with a perm value of less than 0.50 if you also have a high water table.
When installing a vapor diffusion retarder, it should be continuous and as close to perfect as possible. This is especially important in very cold climates and in hot and humid climates. Be sure to completely seal any tears, openings, or punctures that may occur during construction.
Cover all appropriate surfaces; otherwise, you risk moist air condensing within the cavity, which would lead to dampened insulation. The thermal resistance of wet insulation is dramatically decreased, and prolonged wet conditions will induce mold and wood rot.