Healthy Home Tips: If a Finished Basement is in Your New Year, Lend an Ear.

Your family is growing, and you need more room.

The real estate market is slow, though, so instead of buying a larger home, you may be considering finishing your basement to gain a playroom, office or exercise room.

Maybe that’s your New Year’s Resolution, and you plan to hit Home Depot for your supplies and get started on the project.

Let me issue you a warning. Most of the finished basements I’ve seen end up – or should have ended up – in a dumpster, because finished, partially or fully below-grade (below ground level) spaces can easily become contaminated with mold growth. Then children playing on moldy carpets, or parents exercising in the basement can disturb and breathe in huge concentrations of mold spores – an unhealthy situation.

Mold can grow within twenty-four to forty-eight hours wherever food sources (such as skin scales and insect body parts in dust, and cellulose in drywall, cardboard and paper) and moisture are present. It’s impossible to live in a dust-free environment, but you can work to control moisture, which will help prevent mold growth.

Many basements experience water intrusion during heavy rains, because the ground is sloped toward the house, the gutters and downspouts are clogged with debris, or the downspouts empty water next to the foundation. Inside sources of basement water can include a leaking hot-water tank and water pipes, as well as an overflowing washing machine.

Even a seemingly dry basement is vulnerable to mold, because many indoor molds, including some species of the highly friable and allergenic Aspergillus mold, can grow when the relative humidity is in excess of 75%. As air cools, its relative humidity rises. The temperature of the earth around a foundation is approximately 55°F (colder in winter), so a below-grade space is naturally cool and thus the air has a higher relative humidity. Dust captured in basement carpeting, as well as dust on basement furniture surfaces facing the cool floor, gets moldy. Mold can grow on the lower few feet of walls or even on acoustical ceiling tiles, if the relative humidity is high enough.

As an indoor air quality professional and the parent of two children with asthma, I’d like to offer you some advice about finished basements:

1. Rather than finish your basement, finish your attic, or buy a futon couch for your guest room. Then the room can serve as a bedroom when needed and a playroom at other times.

2. If you remain committed to finishing your basement, build finished walls about two feet away from the foundation, to leave a space you can access to watch for pest activity or water intrusion. If building codes allow, insulate the foundation walls with two-inch solid sheet foam insulation, rather than put fiberglass between the studs of the finished walls. Leave the top inch of the foundation wall exposed, however, so you can keep an eye out for termite activity. Never install exposed fiberglass in a basement; cover it with Tyvek, stapled to the joists. (If your basement has exposed fiberglass and you haven’t controlled basement relative humidity with dehumidification, I encourage you to have the insulation professionally removed under containment, and the ceiling framing HEPA vacuumed and lightly paint-sealed – especially if you’ve had mice in the basement.) If you decide to install an Owens-Corning basement system, seal the exposed fiberglass insulation at the back with an adhesive plastic that will stick permanently (try Pro-tect, available in many building supply stores; www.pro-tect.com). Avoid wall-to-wall carpeting below grade. Put ceramic tile on the concrete, or lay two-inch solid sheet-foam insulation down on the concrete, then plywood, and then vinyl flooring.

3. If you already have a finished basement, and anyone in your family suffers from allergies or asthma, have the space professionally evaluated for mold, especially if any part of the basement smells musty.

4. To prevent mold growth, finished below-grade spaces must be consistently dehumidified in the humid season. Keep windows to the exterior as well as the basement door closed while dehumidifying, and doors to basement closets and rooms open. Be sure to use a dehumidifier with sufficient capacity for the space (see www.therma-stor.com). Attach any dehumidifier to a condensate pump that discharges into a sink, a sump or to the exterior. Then if the dehumidifier has a bucket, you won’t have to empty the bucket all the time to prevent the dehumidifier from shutting off because the bucket is full. Measure the relative humidity (RH) with a hygrometer, available in many hardware and building supply stores. Therma-stor also offers a Humidity Alert, which records the relative humidity (RH) and sounds an alarm if the RH exceeds the set point. Increase dehumidification if the RH exceeds 50%. Consistent air conditioning can be used to replace dehumidification, but always measure the RH.

5. To control the RH in cool weather, consistently heat all finished basement spaces – with the thermostat set at a minimum of 60°F – whether you are using the basement or not.

Avoiding mold growth in a finished basement can be expensive, but in the interests of health, it’s worth the cost. And the money you spend will still be less than buying a new house.