Crawl Space Dehumidifiers

Not always, and not because the code says so. Washington's unvented crawl space rule does not mention a dehumidifier. Once a crawl space is sealed, its moisture still has to be managed, and a dehumidifier is one way to do that. Whether yours needs one depends on how much moisture the space still picks up once the ground is sealed and any water problem is fixed, which is what we look at before recommending one.

Under Washington's current residential code, in effect since March 2024, a vented crawl space needs at least 1 square foot of net vent area for every 300 square feet of under-floor area, arranged for cross ventilation, with openings covered by mesh or grilles no larger than 1/4 inch. Vent area can drop to 1 per 1,500 square feet with an approved Class I vapor retarder on the ground, but below 1 per 300, or with operable louvers, a radon vent is required. In Washington, a crawl space can be built without foundation vents only when the ground is covered with a continuous, sealed Class I vapor barrier that runs up and is sealed to the foundation wall, a radon system is installed, and an exhaust fan runs continuously to the outdoors at 1 cubic foot per minute for every 50 square feet of crawl space floor. Your local building department has the final say on how these rules apply to your home, and we confirm the requirements before any vents are closed.

There is no official crawl space humidity limit to measure against. The EPA's advice to keep humidity below 60 percent, ideally 30 to 50 percent, is for indoor living space, not crawl spaces. The WSU crawl space study treated 80 percent relative humidity as its critical point, because humidity sustained above that is needed to hold wood at 20 percent moisture or more, and USDA Forest Products Laboratory scientists say decay fungi need wood above about 20 to 25 percent moisture. So one reading matters less than the trend and the condition of the wood. We check both.

We would not pick a month. A dehumidifier in a sealed crawl space should respond to the moisture in that space rather than to the calendar, and its controls are how it does that. If you are thinking of switching yours off because it never seems to stop running, call us first. A unit that runs constantly can be a sign that water is getting in somewhere.

The most useful thing to put in a crawl space is not an absorber, it is a ground cover. DOE says a crawl space must always have a ground cover that prevents ground moisture evaporating into the space, and Washington's code requires one: 6-mil black polyethylene or an approved equal, overlapped at the joints and run to the foundation wall. After that, deal with water coming from outside: gutters, downspouts and the slope of the ground.

Yes, if moisture gets in. The EPA says the key to mold control is moisture control, and that it is impossible to get rid of all mold and mold spores indoors. A plumbing leak or water running in under the liner can still cause problems in a sealed crawl space, and nobody can honestly promise a mold-free one.