Throughout the Country, even in desert areas, there is moisture in the ground from groundwater being absorbed. Even in an apparently dry crawl space, a large amount of water is entering. The moisture is drying out as fast as it is entering, which causes high moisture levels in the crawl space and elsewhere in the house. A solid vapor barrier is recommended in all crawl spaces and should be required if moisture problems exist. This vapor barrier, if properly installed, also reduces the infiltration of radon gas. Of course, if the moisture is coming from above ground, a vapor barrier will collect and hold the moisture. Therefore, any source of moisture must be found and eliminated. The source may be as obvious as sweating pipes, or may be more difficult to spot, such as condensation on surfaces. The solution can be as simple as applying insulation to exposed sections of the piping or complex enough to require power exhaust fans and the addition of insulation and vapor barriers.

The more common causes of moisture problems in a new home are moisture trapped within the structure during construction and a continuing source of excess moisture from the basement, crawl space, or slab. To resolve this potential problem, 6-mil plastic sheets should be laid as vapor barriers over the entire crawl space floor. The sheets should overlap each other by at least 6 inches and should be taped in place. The plastic should extend up the perimeter walls by about 6 inches. The plastic sheets should be attached to the interior walls of the crawl space with mastic or batten strips. All of the perimeter walls should be insulated, and insulation should be between the joists at the top of the walls. Vents, which may need to be opened in the late spring and closed in the fall, should not be blocked. If not properly managed, moisture originating in the crawl space can cause problems with wood flooring and create many biologic threats to health and property. A properly placed vapor barrier can prevent or reduce problem moisture from entering the home.

Vapor Barriers for Concrete Slab Homes

Strip flooring and related products should be protected from moisture migration by a slab. Proper on-grade or above-grade construction requires that a vapor barrier be placed beneath the slab. Moisture tests should be done to determine the suitability of the slab before installing wood products. A vapor barrier equivalent to 4- or 6-mil polyethylene should be installed on top of the slab to further protect the wood products and the residents of the home.

Wall and Ceiling Vapor Barriers

Wall and ceiling vapor barriers should go on the heated side of the insulation and are necessary in cold climates. Water vapor flows from areas of high pressure (indoors in winter) through the wall to an area of low pressure (outdoors in winter). People and their pets produce amazing quantities of water vapor by breathing. Additional moisture in considerable quantities is created in the home from everyday activities such as washing clothes, cooking, and personal hygiene. The purpose of the vapor barrier is to prevent this moisture from entering the wall and freezing, then draining, causing damage. In addition, wet insulation has very little insulating value. Insulation with the vapor barrier misplaced will allow the vapor to condense in the insulation and then freeze. In cold climates, this ice can actually build up all winter and run out on the floor in the spring. Such moisture buildup blisters paint, rots sheathing, and destroys the insulating value of insulation.