Crawl space moisture and humidity

Crawl space humidity levels

Where crawl space moisture comes from

Advanced Energy's closed crawl space guide lists the 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). A damp crawl space can put as much water vapor into a house in a day as every other indoor source combined (EPA).

How to lower crawl space humidity

1. Keep liquid water out. Exterior water management is the first control on Advanced Energy's list above. See foundation drains.
2. Cover the ground. DOE's weatherization spec calls for a 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).
3. Close the vents in humid climates. DOE's Building America program notes that the outside air coming through vents is often wetter than the air already in the crawl space, which leads to structural, mold and pest problems (DOE Building America).
4. Dry what is left. In an unvented crawl space, the 2024 International Residential Code accepts a dehumidifier sized to the manufacturer's specifications (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).

Does a sealed crawl space fix the moisture?

Sealed, closed, unvented and conditioned crawl spaces are used interchangeably for designs with no ventilation to outside air (Advanced Energy). Whether sealing works depends on the build and the climate. Oak Ridge National Laboratory's modeling found a sealed crawl space needs an effective vapor retarder on all ground and wall surfaces, especially where the water table is high (Oak Ridge National Laboratory). In the dry West and the marine Pacific Northwest, DOE researchers found vented and closed crawl spaces both performed adequately when ducts were not in the crawl space (DOE Building America). More on crawl space encapsulation, and what it costs in the crawl space encapsulation cost calculator.

Where the ground and the climate make it worst

Two things in our county data push crawl space moisture up: a shallow water table (the share of mapped land where USDA rates it a limit for basements, from SSURGO) and heavy rain (NOAA, 1991-2020 average year). Of 3,122 counties with soil data, 1,061 have a shallow water table on most of their land and 741 average 50 inches of rain or more. 275 counties have both, and they hold 17.4 million homes (ACS 2020-2024). This ranks ground and climate; it does not measure any one crawl space.

States with the most of these counties

Counties with a shallow water table on most land and 50+ inches of average-year rain, by state. Each state page lists every county with its numbers.

Questions people ask

What humidity level is normal in a crawl space?

EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. That target is for living space, and a crawl space is usually damper. Crawl space researchers use 70 percent relative humidity as the level that supports mold growth, so a reading that sits above 60 percent for weeks is worth acting on and one above 70 percent is in the mold range.

What humidity should a crawl space be, and what is too high?

Aim below 60 percent relative humidity, EPA's target for indoor air. Above 70 percent is too high: it is the level crawl space researchers cite for mold growth. In Advanced Energy's study of 12 identical houses in the humid Southeast, wall-vented crawl spaces stayed above 70 percent for 98 percent of the 2003 summer, while closed (sealed) crawl spaces were above it 5 percent of the time.

How do I lower crawl space humidity?

Match the control to the source of the water. Advanced Energy lists exterior water management for liquid water, air-sealed walls for humid outside air, a vapor retarder over the ground for evaporation, and a mechanical drying system for the water vapor that remains. DOE's weatherization job aid calls for a ground vapor retarder at least 6 mil thick, covering as close to 100 percent of the ground as possible and running at least 6 inches up walls, columns and footings. The 2024 International Residential Code accepts a dehumidifier sized to the manufacturer's specifications as the drying method in an unvented crawl space.

Does a sealed crawl space stop moisture?

It depends on how it is built and on the climate. Oak Ridge National Laboratory's modeling found a sealed crawl space needs an effective vapor retarder on all ground and wall surfaces, especially where the water table is high, or moisture can accumulate. DOE researchers found that in the dry West and the marine Pacific Northwest, vented and closed crawl spaces both performed adequately when no ducts ran through the crawl space. In humid climates, DOE notes the air coming through vents is often wetter than the crawl space air.

What causes crawl space moisture?

Four sources, according to Advanced Energy's closed crawl space guide: liquid water from outside, humid outside air, evaporation from the ground and perimeter wall, and water from plumbing leaks or floods. EPA's building guidance adds that a damp crawl space can put as much water vapor into a house in a day as every other indoor source combined.

Which counties have the most crawl space moisture risk?

In our county data, 275 of 3,122 rated counties combine a shallow water table on most of their land (USDA SSURGO) with at least 50 inches of rain in an average year (NOAA, 1991-2020). They hold 17.4 million homes. The most common states are FL (53), LA (53), MS (51), AR (37). This ranks ground and climate conditions; it is not a measure of any one crawl space.