Properly controlling moisture in your home will improve the effectiveness of your air sealing and insulation efforts, and vice versa. Thus, moisture control contributes to a home's overall energy efficiency.
The best strategy for controlling moisture in your home depends on your climate and how your home is constructed. Before deciding on a moisture control strategy for your home, you may first want to understand:
To help understand the principles of moisture control, you need to understand the basics of how moisture can move through your home.
In most U.S. climates, vapor barriers or vapor diffusion retarders should be considered as part of a moisture control strategy for a home.
A vapor barrier or vapor diffusion retarder (VDR) is a material that reduces the rate at which water vapor can move through a material. 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°F (23°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.
Vapor diffusion retarders are typically available as membranes or coatings. Membranes are generally thin, flexible materials, but also include thicker sheet materials sometimes called "structural" vapor diffusion retarders. Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor diffusion retarders are usually mechanically fastened and sealed at the joints.
Thinner membrane types come in rolls or as integral parts of building materials. A common example of this is aluminum- or paper-faced fiberglass roll insulation. Foil-backed wallboard is another type commonly used. Polyethylene, a plastic sheet material, can be used as a vapor diffusion retarder for above-grade walls and ceilings (only) in very cold climates (in locations with 8,000 Heating Degree Days or higher).
Most paint-like coatings also retard vapor diffusion. While it was once believed that only coatings with low perm ratings constituted the only effective vapor diffusion retarders, it is now believed that any paint or coating is effective at restricting most water vapor diffusion in milder climates.
In climates with less than 4,000 Heating Degree Days, materials like painted gypsum wallboard and plaster wall coatings impede moisture diffusion to acceptable levels. Usually, no further vapor diffusion retarder is needed.
In more extreme climates, vapor diffusion retarders are advisable for new construction. They perform best when installed closest to the warm side of a structural assembly; in cold climates, this is towards the interior of the building. In hot/wet climates, this is towards the exterior of the building.
Except for extensive remodeling projects, it's difficult to add materials like sheet plastic as a vapor diffusion retarder to an existing home. However, many existing homes don't really need a more effective vapor diffusion retarder than the numerous layers of paint usually on their walls and ceilings. These multiple layers are quite effective as a vapor diffusion retarder in all but the most extreme northern climates.
"Vapor barrier" paints are also an effective option for colder climates. If the perm rating of the paint is not indicated on the label, find the paint formula. The paint formula usually indicates the percent of pigment. To be a good vapor diffusion retarder, it should consist of a relatively high percent of solids and thickness in application. Glossy paints are generally more effective vapor diffusion retarders than flat paints, and acrylic paints are generally better than latex paints. When in doubt, apply more coats of paint. It's best to use paint labeled as a vapor diffusion retarder and follow the directions for applying it.