How to Fix a Musty Basement
Tom Silva explains how to avoid musty smells in a dirt-floor basement
The basement of our 1917 house is finished, except where there’s an enclosed storage area with a dirt floor and an exposed foundation wall. The area is very damp and makes the entire basement smell musty. How do we fix this problem without losing the storage space? —Kathryn Roche, Indianapolis
A dirt floor in a basement dumps massive amounts of moisture into a house, and encourages the mold that causes musty smells. Your best bet for keeping the humidity down is to cap the dirt with a poured concrete slab at least 3 inches thick; 4 is better.
But concrete by itself won’t stop water vapor from migrating through the slab. To do that, you need a vapor barrier—a sheet of 5-mil plastic—placed over the dirt before the pour. Along the perimeter, run the sheet a few inches up the foundation and the existing slab. And if it takes more than one sheet, overlap the edges by at least 18 inches. Next, cover the plastic with a 1-inch-thick layer of sand. It provides an absorbent surface that allows the wet concrete to be more workable, and creates a cushion to keep the plastic from being punctured.
While the new slab will stop a big source of humid air, your storage room will probably still be damp in the summer. Running a fan and a dehumidifier in that room will help keep the moisture in check. Set the dehumidifier to turn on when the relative humidity goes above 50 percent; that’s too dry for mold to grow but not so dry that your dehumidifier runs all the time.
—This Old House general contractor Tom Silva