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September, 2007

Ask The Expert

Reducing Mold in Finished Basements...Finished basements require care to stop wetness and possible mold growth.

By Steve Easley

A builder from Indianapolis asks, "Many of our customers are asking for finished basements. There is a lot of concern about mold these days. Do you have any tips for reducing the potential for mold in finished basements?”

I’ll answer this question by starting from the outside, and then I’ll discuss interior finishes.

The first step in preventing mold growth is eliminating moisture sources. Basements are naturally damp places since they are below grade and surrounded by wet soil. It’s important to make sure that exterior basement walls are waterproofed, and not with just a thin coating of damp proofing. This means using a waterproofing material and drainage board over the foundation wall and footing.

Drain tile and a positive drainage system are also critical. In wet areas, this may require drain tile on both the inside and outside of foundation footings. Be sure to encase the drain tile with filter fabric. Make sure the drain tile freely drains to daylight or to a sump pit where it can be pumped away. Apply sealant between the basement slab and the foundation wall, as well as other slab penetrations. Under slabs, use gravel fill as a capillary break with poly over the gravel and directly under the slab to act as a vapor retarder. This will help prevent sub-slab moisture from diffusing through the basement floor slab.

At ground level, be sure the grade has a positive slope away from the perimeter of the house and that gutters and downspouts adequately divert water away from the foundation. A 1-inch rain on a 2,000 sq. ft. area deposits 1,250 gallons of water, so draining the water away is your first priority.

Interior Finishes

After every effort has been made to control ground water and create a dry basement, be sure to select building materials that do not feed mold. Mold loves cellulose or plant-based materials. Wood framing should be treated with mold inhibitors that are not toxic. Another choice is to use steel studs for wall framing. I wouldn’t put the steel studs in direct contact with exterior foundation walls. Instead, I would leave a space to insulate between the stud and the masonry and slip or spray insulation in between.

For insulating exteriors walls, I like closed-cell spray foam. Closed-cell spray foam is an excellent insulator and it absorbs negligible amounts of water. It also acts as a good air barrier. It insulates the basement wall without a builder or homeowner having to worry about it getting damp or storing moisture.

Another option for insulation is extruded polystyrene sheets or Thermax. (Always follow the manufacturer’s installation instructions.)

Batts with poly vapor retarders can trap moisture from inward drying of ground water and capillary moisture uptake from the foundation. In addition, if air bypasses the batts, it could also condense on cold foundation walls.

It’s also important not to use standard drywall gypsum panels in this application. The paper facing on drywall is an excellent food source for mold. Moisture that migrates from a foundation can be absorbed by the drywall panels and grow mold. I would use Dens Armor Plus paperless drywall made by Georgia Pacific. It uses fiberglass matt technology instead of paper facing, which eliminates the major food source for the mold. It also finishes and paints just like standard drywall. (www.densarmorplus.com) Be sure there is at least a 1⁄2-inch space between the basement floor and the bottom edge of the drywall to prevent moisture from being drawn into the drywall by capillary action. And don’t use vinyl or impermeable wall coverings, as they can trap moisture in the stud cavity and drywall.

Another suggestion is to select trim materials that are made of plastic. One such supplier is Azek, which makes trim boards. These come in many popular styles and are not a food source for mold.

As far as flooring in a finished basement, I prefer ceramic tiles, stone, or materials that do not support mold growth. These fare a lot better in the long run if you ever do have standing water. If you are going to use carpet, select commercial-grade carpets that are designed to be applied over concrete slabs.

Be careful to caution homeowners to control indoor humidity, as well. Many homeowners heat their basements, and oftentimes they have humidifiers to add moisture to the air. Heating systems do not distinguish where this humidified air is going in the home, and that can lead to high basement moisture levels, which also leads to mold.

This response is only an overview of this subject, and more detailed information can be found at the Building America programs (www.eere.energy.gov/buildings/building_ america) and at the Building Science Corp. (www.buildingscience.com).

Steve Easley is a construction quality consultant who specializes in solving building science-related problems. He provides consulting and seminars on topics such as better building practices, energy efficient construction techniques, and cost-effective green building. Contact him at steve@steveeasley.com or visit www.steveeasley.com.

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