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Concrete slab needs vapor retarder

By Pat Logan
www.creators.com

Q. 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?

Most people are surprised how easily ground water can pass right through concrete and into a home. This can cause problems for the flooring and contribute to poor indoor air quality, mold and allergies.

It is necessary to install a vapor retarder beneath the concrete before it is poured. Notice I called it a vapor retarder, not a 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 vapor retarder under the slab, not just the cheapest polyfilm you can find at a home center store. It does not take many breaks and tears in the film to create an indoor moisture problem.

Select a vapor retarder labeled as ASTM E 1745, an international standard. 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 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. Create an overlap of 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 scissors, cut this piece so it slips over the pipe. Tape the collar around the pipe and down to the film on the ground. Make sure there are no gaps.

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