Q: We have been plagued by moisture in the crawl space of our home (built in 1978).

A French drain system surrounding our house was installed two years ago. A pest inspection at that time indicated there were no pest or mildew problems. However, when it rains, we get a terrible odor in our home. It smells of wet chemical earth.

Should we have heavy plastic installed in the crawl space? The pest inspector said the earth would never dry out then. But other home gurus on television and the Internet seem to advise doing this.

We are having our carpets replaced with wood floors this year, which will include a moisture barrier between the subfloor and the new floors, but we also want to protect our subfloor from any moisture.

A: We disagree with the pest control inspector. You would be well served to install a vapor barrier of 6-mil plastic in the crawl space of your home. The plastic will help reduce the moisture in the crawl space, which we suspect is the cause of a mold/mildew problem. We suspect the odor you describe when the soil is wet is mildew, which thrives in wet, underventilated areas and is dormant in dry conditions.

Mildew is the generic name for a few types of fungi that grow readily on moist surfaces. Once it invades your home, it spreads rapidly and reproduces itself by releasing airborne spores. Eliminate the moisture or reduce the mildew’s ability to release spores into the air and you’ll eliminate the odor.

Lack of ventilation is a major cause of mildew production. Although it is almost impossible to completely eliminate it from your home, you can take some positive steps to control it.

Even though the pest control inspector claims to have not found mildew in the crawl space, we’d bet a dollar to a doughnut that the inspection took place in the summer months when the ground had dried out and mildew spores were dormant.

Attacking your problem is a three-step process — one of which you’ve already completed by installing the French drain. The next two are installing the vapor barrier and making sure the crawl space is adequately ventilated. The goal is to reduce moisture infiltration and increase ventilation to effectively control mildew in the crawl space.

First, carefully inspect the crawl space to determine if there is an obvious source of water infiltration. Do this during or immediately after a rainstorm and search for evidence of water. If you find any surface water at all, you must eliminate the source. Check the gutters and downspouts for blockages. Make sure the downspouts are discharging the water away from the foundation. Finally, make sure the dirt adjacent to the foundation slopes away from the house.

Next, install the vapor barrier to lessen the overall amount of water vapor entering the crawl space from the ground. Lay a heavy plastic tarp — 6 mil or thicker — over the entire area. If you must use multiple sheets, overlap them by at least 2 feet. Use stones to hold the plastic down. The plastic acts as a vapor barrier, eliminating much of the migration of water vapor from the ground to the crawl space. Consider spraying a TSP/bleach mix before installing the vapor barrier to kill any existing mildew spores.

Finally examine the ventilation of the crawl space. This means the foundation vents. Make sure they aren’t blocked by plantings and that there are enough of them. Here, more is better. The minimum amount of ventilation required is determined by the square footage of the footprint of the foundation. The presence or absence of a vapor barrier affects this number.

Check with your local building department as local codes vary depending on your area’s particular moisture and weather conditions. Check the volume of ventilation provided by your foundation vents and ensure it meets or exceeds the minimums required for new construction in your area.

We’d also suggest that you install fiberglass batt insulation between the floor joists to isolate the crawl space from the subfloor before you have your new hardwood floor installed. Take these steps and we believe you’ll severely reduce or eliminate the moisture and odor problems.

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