Moisture- and Mold-free Basement Living

Many basements are damp, musty, leak air and have little natural light. As a lifetime drywall contractor, I have hung more than my share of drywall in basements. I would just hang and tape some moisture-resistant drywall and be on my way. In 2009, I became a building-performance analyst through the Malta, N.Y.-based Building Performance Institute and a year later a green-home verifier through the National Association of Home Builders, Washington, D.C. My building-science training helped me realize I needed to spend more time thinking about solving the moisture/water problem first and incorporating construction techniques that prevent moisture from moving into unwanted areas.

When dealing with a moisture/water problem, first eliminate the source; then, remove or divert the pathways; and finally, install resistant materials. Understanding building science is very important. The practical purpose of building science is to provide predictive capability to optimize building performance and understand or prevent building failures. When creating living space in a basement, the failures could involve moisture intrusion, air quality and energy efficiency.

Moisture Intrusion

If a moisture problem is discovered in a basement, you must solve the moisture problem before doing any construction work. First, identify the cause. To tell if walls are damp from exterior water or condensation from humid interior air, tape a 2-foot square sheet of plastic to the masonry on a below-grade area of the wall.

Condensation can appear as water droplets, wet spots or puddles on basement floors and walls. It happens when moist, warm air hits cool foundation walls or uninsulated cold-water pipes, dampening carpets, rusting appliances and making the basement clammy. If moisture collects on the front of the plastic, it is condensation.

There are several ways to solve condensation problems:

  • Insulate: Insulate surfaces where condensation is likely to occur, such as cold-water pipes and ductwork. Installing continuous insulation along the foundation wall will help prevent air from cooling and reaching its dew point, which is the temperature at which moisture changes to liquid form based on the relative humidity of the air before it cools. Warm air can hold more moisture than cool air; 80 F air with high humidity will reach its dew point at a higher temperature than 80 F air that has a lower humidity. Having a continuous air barrier at the same location as the insulation also will help.