By Pat Logan

June 25, 2010 4 min read

Musty indoor odors not only are unpleasant to your olfactory receptors but also can be harmful to your health. If a moldy scent is emanating from your home, it is often indicative of a bigger issue. It's imperative to locate the source, decipher the cause and eliminate the problem quickly. Your health and nose will thank you.

A moldy odor generally is caused by persistent dampness somewhere in your home. This not only is annoying but also can create serious health risks. Dampness also causes the structural lumber supporting the house to decay and attracts carpenter ants and termites.

There are two possible sources of dampness in a home. It can be generated internally from bathing, cooking or even a humidifier set at an unreasonably high level. It also can come from outdoors through a leaky foundation, slab or roof. High indoor humidity levels are most common in newer houses, which are more airtight.

In older homes, the source of dampness is usually from outdoors. When a house is built over several different types of foundations, it likely consists of several additions built at different times. There often can be leaks where the various additions were attached to one another.

Just to be certain the dampness problem is not caused by internal factors, check for any water leaks around the toilets. A common place for leaks is where the tank mounts to the bowl. If the toilet feels loose on the floor, the wax ring at the base may be deformed and leaking. Both of these items are easy to repair yourself.

Also, try being more diligent about running kitchen and bathroom vent fans when those rooms are being used. Run them even a little longer than you feel necessary for a while to determine whether this helps relieve the moldy odor.

If the odor is still there, the moisture source is from outdoors. A crawl space floor, especially a dirt floor without proper drainage, is a common source. The concrete floor and slab can allow moisture to pass through them. Even though concrete feels hard, it is a porous material.

Make sure the crawl space floor is covered with plastic film to block moisture from coming up from the ground. A 0.006-inch-thick plain polyethylene film is inexpensive and commonly used for this application. Run the film all the way up the crawl space walls, and staple it to the band joist.

A better film to use is made from cross-laminated polyethylene. Several layers of film are bonded together to make it much stronger and more airtight. Plain polyethylene can have some tiny pinholes in it. Expect to pay about three to four times more for the cross-laminated material, but it probably is a wise investment.

Tape a 1-foot square piece of plastic film over the concrete basement and slab floors. Seal it well all around its perimeter. After one day, remove the film. If moisture is coming up from the basement or the slab, you will see it on the underside of the film.

If you can remove any carpeting from the concrete basement floor or slab floor, put a layer of plastic film under the carpeting. Another option is to apply a liquid sealer over the floor. Water-based ones are the easiest to apply.

Columnist Pat Logan answers readers' questions about building, remodeling and home-maintenance problems. Read his column at creators.com.

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