Vapor retarders typically are available as membranes or coatings. Membranes generally are thin, flexible materials but also include thicker sheet material sometimes termed structural vapor diffusion retarders. Thin membrane vapor diffusion retarders come in rolls or as integral parts of building materials. One such integrated product is the single-side aluminum-faced or specially treated kraft paper-faced batt insulation. Foil-backed wallboard is another popular material incorporating a vapor diffusion retarder. Polyethylene, a plastic sheet material, is perhaps the most commonly used vapor diffusion retarder. It is available in various roll configurations. Thus 4-mil or 6-mil polyethylene sheeting, available in rolls, is rolled out horizontally and stapled to the face of the framing after installation of unfaced insulation. Materials such as rigid insulation, reinforced plastics, aluminum, and stainless steel are also relatively resistant to water vapor diffusion. These types of vapor diffusion retarders usually are fastened mechanically. When sealed at joints, they seriously restrict air leakage. [A vapor diffusion retarder is only effective when it is continuous, fully covering the exterior envelope (walls, floors, and ceilings). By contrast, a sheet of drywall only retards vapor diffusion over the area it covers, not the edge joints, holes for electrical outlets, or window openings.3]
Paints and other coatings, when applied to a finished wall or ceiling, also may retard vapor diffusion. These coatings are asphaltic, resinous, or polymeric. They are applied by brush, trowel, roller, spraying, or dipping, depending on the surface. “Vapor barrier" paints are an effective option, but always verify that the paint formula is low in pigment. The paint label usually indicates the percentage of pigment.
It is best to use paint labeled as a vapor retarder. Most paint experts agree that, for this purpose, glossy paints work better than flat paints, acrylic paints are generally better than latex paints, and the more coats applied, the better.