What is a Crawl Space Vapor Barrier?

A crawl space vapor barrier is a layer of material laid on the floor and walls of a crawl space to protect a home against moisture damage. This material is generally installed beneath the home at the time of its construction, though homeowners may wish to replace this after several years or upgrade it to a thicker fabric. It is typically made of plastic, though some versions include a layer of insulation. These vapor barriers may be purchased at home improvement stores or online, and installed by either the homeowner or a certified contractor.

The areas around the foundation of a home are typically very susceptible to flooding. Builders must generally dig down several feet into the ground to place timbers and cement blocks for the bottom framework of a house. The space created within these timbers is known as the crawl space. Duct work for air conditioning and heating elements is often contained within the crawl space, as are most of the pipes for the plumbing on the ground level.

Contractors frequently grade the land around a new home so that rainwater will run away from the foundation. Erosion over time, however, may reverse the grading process and allow water to backfill beneath the home. As water begins to collect within the crawl space, mold may begin to form in the timbers and cement of the framework. This mold can then be circulated throughout the home through the air conditioning and heating duct work.

Many methods for waterproofing crawl spaces are available to homeowners. Installing a crawl space vapor barrier is one of these many methods. Homeowners may also close outside vents to the crawl space, allow air conditioning to vent into the space, and install a sump pump.

Most vapor barriers are made of plastic, though some types include a layer of insulating material, often aluminum. The crawl space vapor barrier may also be referred to as polyethylene film, and is generally sold in rolled sheets. These may be used for a variety of purposes, including providing moisture protection between the drywall and insulation of a home, and as a moisture barrier beneath hardwood floors. Barriers are available in different thicknesses, and those suitable for use within the crawl space are typically between four mil — one mil is a thousandth of an inch — and ten mil thick.

To install the crawl space vapor barrier, run it in strips along the dirt floor of the space. Overlap the seams up to six inches (15.24 cm) and tape them in place with duct tape. Homeowners may wish to use a plastic barrier on the floor and along the lowest six inches (15.24 cm) of the wall, as well as along the edges of upper walls in the crawl space. Using such a layer adds protection from frost for the pipes and duct work located beneath the home, and prevents further moisture from seeping through the ground and concrete blocks and into the crawl space.