How to Fix a Moldy, Wet Crawl Space | Ask the Expert

Pete Westera is the owner of Basement Systems USA and one of the leading authorities on basement waterproofing, crawl space, and foundation repair in the Baltimore, MD area, also serving Philadelphia and Wilmington, DE.

In this episode, Pete explains how to fix a dirt-floored, vented crawl space to prevent mold growth, dry rot and structural decay, as well as eliminate bad crawl space odors.

The first step is to eliminate standing water with proper grading, drainage and a sump pump system. This will keep water from any source from pooling under the house. Yet, the sump pump alone will not take care of all the problems. The crawl space can still experience high levels of relative humidity (RH) during the summer, and anything above 60% RH will lead to mold growth.

The next step is to address the second source of crawl space moisture: the ground. In a dirt-floored crawl space, ground moisture is always evaporating into the crawl. To keep that from happening, a vapor barrier must be installed on the floor.

Pete and his team use and recommend lining not only the floor, but the walls as well, with the CleanSpace Liner: a product specially designed for crawl space encapsulation applications by the company that pioneered that technology. CleanSpace is a 20-mil, UV-treated and puncture resistant plastic liner with a built-in antimicrobial layer to prevent mold growth. The liner is then sealed and caulked at the seams, creating an air tight seal. The idea is to completely isolate the crawl from any moisture evaporating from ground or walls.

The next step is to seal all the crawl space vents. Crawl space vents, which were once mistakenly required by most building codes in the U.S as a way to help dry out moisture, were not only proven inefficient by modern building science, but were also found to aggravate the problems in the vast majority of the cases, allowing humid air from the outside into the crawl.

Due to a physics phenomenon known as the "stack effect", the humidity outside doesn't circulate in the crawl; it moves upwards increasing relative humidity in the crawl space, soaking floor joists and fiberglass insulation, and often moving into the living areas.

The last step in the process is to provide a good dehumidifier to monitor and control humidity in the crawl year-round, so mold will never grow back.