Crawl space moisture

Where the moisture comes from

Advanced Energy's closed crawl space guide lists four sources and a control for each: liquid water from outside (exterior water management), humid outside air (air-sealed walls), evaporation from the ground and walls (vapor retarders), and what's left, which a mechanical drying system removes (Advanced Energy).

Vents don't solve it in humid climates. DOE's Building America program notes that the outside air coming through the vents is often wetter than the air already in the crawl space, which leads to structural, mold and pest problems (DOE Building America).

The humidity numbers

Crawl space researchers use 70% relative humidity as the level where mold grows, and report that even well-built wall-vented crawl spaces in the Southeast can't stay below it (Advanced Energy). EPA's 60% target is for living space.

What goes under the house

• A ground vapor retarder at least 6 mil thick, covering as close to 100% of the ground as possible and running at least 6 inches up walls, columns and footings (DOE Weatherization job aid).
• In an unvented crawl space, the 2024 International Residential Code accepts a dehumidifier sized to the manufacturer's specifications as the drying method (2024 IRC). The 2018 code set a number instead: 70 pints of moisture removal a day per 1,000 square feet of crawl space floor (2018 IRC).

Where the ground is wettest

Counties with 20,000 or more homes and at least 50 inches of rain in an average year (NOAA, 1991-2020), ranked by the share of land where USDA rates a shallow water table a limit. Sealing a crawl space.