Install a Vapor Barrier Under Your Slab by Pat Logan on Creators.com

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Install a Vapor Barrier Under Your Slab

Dear Pat: I am having a house built partially on a slab and on a crawl space. Is it necessary to install a vapor barrier under the concrete, and is this something I can do myself to save some money? — Ellen K.

Dear Ellen: Even though concrete feels as hard as a rock, it is still permeable to moisture in the ground. Most people are quite surprised how easily ground water passes right through the concrete into a home. This causes problems not only for the finished flooring materials, but it can also contribute to poor indoor air quality, mold and allergies.

For this reason, it is necessary to install a vapor retarder beneath the concrete before it is poured. Notice I called it a vapor "retarder," not "barrier." Even the best material still allows some moisture to get through. A small amount of moisture is not a problem for the typical home.

In addition to installing the vapor retarder, designing adequate drainage around the house is important. Even the best vapor retarder materials will not adequately block moisture if there is strong hydrostatic water pressure in the ground around the slab and foundation. Follow your local building codes concerning proper drainage for your local ground conditions.

It is not difficult to lay the vapor retarder yourself. It just requires attention to detail and much hand work to be sure the slab will be well sealed. The hardest part of the job will be handling the large roll of vapor retarder film, so you will likely need a helper or two.

Install a high-quality underslab vapor retarder, not just the cheapest polyfilm you can find at a home center store.

This common film used in walls may not hold up well in an underslab installation. It does not take very many breaks and tears in the film to create an indoor moisture problem.

Select a vapor retarder with meets ASTM E 1745 standards for underslab installation. These films are extremely durable and, if carefully installed, should eliminate any floor moisture problems. If you have problems finding it, visit these manufacturers' websites: fortifiber.com, ravenind.com, mondousa.com and wrmeadows.com.

The E 1745 films are often available in widths of 12 feet and 15 feet and thicknesses of 15 mils and 10 mils, respectively. The rolls are several hundred feet long. A base of sand or gravel is not required by the manufacturer, but the soil should be level, tamped or compacted fill.

When the film is rolled out, you may walk on it, but try to be careful not to puncture it. If the slab area is wider than 15 feet, you will have to use two pieces of film. Lap them about 6 inches, and cover the joint with 4-inch-wide tape. Purchase the type of tape recommended by the specific film manufacturer. Cut the film large enough so it will also cover the vertical edge of the slab.

There will be penetrations through the film by plumbing pipes and drains. To seal around these, cut a rectangular piece of film 1 foot by 1.5 times the diameter of the pipe. Using a scissors, cut fingers radially in this piece so it slips over the pipe. Tape the collar up around the pipe and the fingers down to the film on the ground. Make sure there are no gaps.

Send your questions to Here's How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit www.dulley.com. To find out more about Pat Logan and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.