Installing a Vapor Barrier in Your Crawlspace - dummies

Installing a Vapor Barrier in Your Crawlspace

By James Carey, Morris Carey

Because excessive dampness in a crawlspace or basement can condense, you can install a vapor barrier to protect floor framing from becoming damp and covered with fungus, efflorescence, and rot. A vapor barrier consists of one or more layers of sheet plastic (6-mil Visqueen) on top of the soil in the crawlspace or basement.


If you’d rather have someone else take on this less-than-desirable-job, an entirely new industry has cropped up consisting of firms that specialize in crawlspace finishing systems. The average system consists of a sheet plastic that is about ten times thicker than the DIY 6-mil stuff. They also use industrial-strength tape, caulking on the walls, and an assortment of other tools and techniques that will make your crawlspace almost livable. Check in your local Yellow Pages or search online for “crawlspace finishing systems.”

Lap the plastic a minimum of 6 inches and seal the seams with duct tape. Cut around piers and along the inside edges of the foundation. In severe cases, you can run the plastic up the sides of piers and the foundation and secure it with duct tape or anchor it with a line of soil at the perimeter.

Just because you installed a vapor barrier doesn’t mean that your work under the house is done. With the plastic in place, you have a nice, dry surface on which to work to remove efflorescence or mold that has propagated on wood framing. Wearing safety goggles and use a wire brush and a putty knife or paint scraper to clean the affected areas.

As you remove efflorescence or mold, have a blunt tool handy, such as a flathead screwdriver, to test for evidence of rot. If you can insert the blade of the screwdriver into the wood fibers using moderate pressure, it’s time to call in a pest-control specialist to make an inspection and suggest needed repairs.

Wood framing that was previously damp but not severely damaged should be treated with a wood preservative that contains a pesticide such as copper or zinc naphthenate. This wood preservative is a liquid that you can brush on with a disposable paintbrush.

Use extreme caution when working with wood preservatives that contain pesticides. They can be hazardous to your health — not to mention the fact that they have a fierce odor for the first week or so. Follow the manufacturer’s instructions on the label to the letter.