Purpose of a Vapor Barrier in a Crawlspace

Crawl Spaces and Moisture

• Moisture needs to be controlled virtually everywhere in the home, but some parts of the home are more prone to moisture problems than others. These parts include the attic and basement, as well as slab foundations and crawl spaces that directly contact moist soil below the home. Since people rarely frequent the crawl space, problems associated with excessive moisture there, including mold, mildew and wood rot, can sometimes go unnoticed until they have already critically damaged the home.

Vapor Barriers in the Crawl Spaces

• Because of the severity of these problems, it's advisable to install vapor barriers in the crawl space as a preventative measure against moisture problems. Vapor barriers prevent moisture in the soil from leaching upwards and condensing on the wooden components of the crawl space, where the moisture can create ideal growth conditions for molds and mildews that can rot the wood and cause health problems for your family.

Crawl Space Vapor Barrier Tips

• Installing a vapor barrier in a crawl space requires a slightly different method than installing one in another part of the home. The vapor barrier should ultimately cover 2/3 to 3/4 of the crawl space area. Do not cover the entire ground area, particularly if you have hardwood floors inside your home; in homes that consist entirely of carpet or vinyl flooring, the whole crawl space area can be covered. Often, a 2- to 3-inch layer of sand is placed over the top of the moisture barrier, both to weigh the barrier down and to absorb the small amounts of water that collect on the top side of the water vapor.

Suggested Materials

• Any construction material can resist vapor flow to some degree, but only materials that can very strongly resist vapor flow should be used for vapor barriers. In the case of crawl spaces, asphalt-laminated paper and polyethylene film are commonly used. If for whatever reason these materials are not available in your area, consult a contractor for recommendations on an alternate material.