Is a Moisture Vapor Barrier Necessary Behind Siding?

Vapor Barriers

• The parts of the home in which vapor barriers are most commonly installed include basements, crawlspaces, attics and any exterior walls inside the home. But vapor barriers can be beneficial when installed anywhere that may be prone to moisture problems. Kitchens and bathrooms, for example, often benefit from the installation of a vapor barrier alongside other moisture control techniques such as exhaust fans and weather-stripping.

Siding

• As noted, vapor barriers can be installed on the inside of any wall that faces the outside. But vapor barriers are effective only if they are installed on the warm side of the wall (i.e., the side of the wall that faces the inside of the home). To install a vapor barrier on the cold side of the wall (behind the siding on the outdoor-facing side of the wall, for instance) not only will not offer effective moisture control but also can make indoor moisture problems worse.

Installation Tips

• Rather than installing a vapor barrier behind the siding of an exterior wall, therefore, place the vapor barrier material between the framing and the interior sheathing or wall finish of the side of the wall that faces indoors. Placing any material with high vapor resistance in this part of the wall can go a long way in preventing the buildup of indoor moisture. The effectiveness of the vapor barrier can even be augmented by covering the outside of the warm side of the wall with oil base or aluminum paint or vinyl wallpaper.

Suggested Materials

• Every construction material has some degree of resistance to vapor flow, but only those with a particularly high resistance should be used as a vapor barrier. These materials include polyethylene film, asphalt-coated or laminated papers, kraft-backed aluminum foil, rigid foam insulation, reinforced plastics, aluminum, stainless steel, aluminum- or paper-faced fiberglass roll insulation or foil-backed wallboard. The degree of vapor resistance of a given material is expressed in "perms," and the lower a material's perm value, the better that material is at resisting vapor flow. Therefore, look for materials with a perm value of 1.0 or lower.