Crawl Space Vapor Barriers: Why You Need One & How to Install It

You may be familiar with more obvious moisture problems due to damaged roofs and walls or rainwater runoff that seeps into foundations or up through concrete foundation floors. However, crawl spaces are also prone to moisture problems from water vapor that rises from the ground and gets trapped in the enclosed space. Fortunately, you can add plastic sheeting as a water vapor barrier to correct the problem or prevent it from occurring.

A vapor barrier slows down ground moisture from affecting a house sitting above it. The barrier is made of rolls of ordinary sheet plastic that are laid down on the ground in a crawl space to trap water in the ground from evaporating or condensing. The U.S. Department of Energy calls a vapor barrier a "vapor diffusion retarder" since plastic sheeting, even with seams sealed, cannot seal off 100 percent of the moisture.

The plastic sheeting is made of reinforced polyethylene plastic sheeting (poly) in various thicknesses and strengths, usually from 6 mils to 23 mils, depending on your budget and needs. A mil is equal to 0.001 inches of thickness, so a 6-mil sheet is (0.006-inch thick). A 6-mil sheet is considered a Class 1 vapor retarder, which is the minimum recommended thickness when installing a reinforced poly vapor barrier.聽

Vapor barrier sheeting can cost 15 cents to $1.50 per square foot; the cost variation is based on the thickness of the sheet. For an expert to install, it can run $2,000 to $4,000, depending on the space size and sheet used.

A vapor barrier is a relatively inexpensive remedy to a situation that can get out of control if left unchecked. With so many benefits, installing a vapor barrier in a crawl space seems a great option for homeowners, and it has many distinct advantages:

Moisture is one of the main foes of a home's structure鈥攑otentially causing structural problems. Uncontrolled moisture can cause rot and decay in wood framing and other materials, fostering mold and mildew. Moisture is most damaging in a crawl space because you often can't see it.

Moisture determines if a home, room addition, or below-grade flooring is even possible. If moisture is a known problem, vapor barriers can be integrated into wall, ceiling, and floor construction to combat moisture affecting the home.

One of the most serious problems caused by moisture is mold, mildew, and other fungi. Eliminating mold is complex, and fully remediating mold-related problems is expensive. Mold can blacken floor cavity insulation and structural elements. Because there is often no light and minimal ventilation in a crawl space, the problem only goes away with preventive measures or remediation.

Structural elements of your house, such as joists, sills, posts, and beams, are wood. Wood will rot when it comes into contact with water for prolonged periods.

Many types of animals are drawn to water and can invade your home. These pests include carpenter ants, termites, rats, mice, snakes鈥攅ven skunks and armadillos.

While a vapor barrier is not required, it is highly recommended as many crawl spaces are prone to moisture. A vapor barrier can also reduce pests, insects, and potential structural damage to your home that comes from too much moisture.

If done on your own, installing a vapor barrier is not a complicated or expensive project, but it can be hard work because it requires a lot of crawling on hands and knees in a dark, often damp space. A good work light is essential to working safely and efficiently in the cramped space below the house.

While it is possible to do this job by yourself, it helps to have at least one helper to pass items through the crawl space door and to help with rolling out the plastic and taping the seams. It may also be beneficial to wear a face mask so you don't inhale debris or damp moisture that may contain bacteria and mold.