Crawl space dehumidifiers
A dehumidifier holds a sealed crawl space below the 60 percent humidity the EPA recommends indoors. In an open, vented crawl space it mostly dries the outdoors. Fixr lists crawl space dehumidifiers at $750 to $1,700; This Old House puts installation at $800 to $2,000.
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Why Charleston crawl spaces need help
Charleston's summer air carries a lot of water. The airport averages an outdoor dew point of about 73.2°F in July and 73.2°F in August. When that air reaches a cooler spot under the house, like a duct carrying air conditioning or the underside of the floor, it can condense. Outdoor relative humidity averages about 78 percent in August by our calculation from NOAA data.
Average outdoor relative humidity by month (%)
Sealed first, then dehumidify
A dehumidifier works on a fixed volume of air. Seal the vents and cover the ground, and it can hold the space dry. Leave the vents open, and it fights the whole neighborhood's humidity and wears out early. That's why most dehumidifier installs come as part of an encapsulation.
What to ask about the unit
- Capacity for the crawl space size, and whether it is built for low temperatures and dusty spaces, unlike a basement or room unit.
- Drain: a hose to daylight or a condensate pump. A unit with a bucket under a house is not a plan.
- Humidity sensor and target. Ask what level it will be set to, and whether you can see readings from upstairs.
- Filter and service. How often, and who does it.
- Warranty on the unit and the labor, in writing.
What it costs
| Item | Published range | Source |
|---|---|---|
| Crawl space dehumidifier | $750 to $1,700 | Fixr, Jan and Aug 2025 |
| Dehumidifier, installed | $800 to $2,000 | This Old House, Mar 2026 |
| Added to an encapsulation | $800 to $1,200 | Bob Vila, Jul 2022 |
Watch for rodents and standing water
Charleston owners report finding rodent droppings on the liner when a technician came to service the unit, and water pooling on top of a vapor barrier after storms. A dehumidifier won't fix either. Water on the liner needs drainage; gaps that let animals in need sealing.
Questions Charleston homeowners ask
Does a Charleston crawl space need a dehumidifier?
A sealed one usually does, or a small supply of conditioned air, to stay below 60 percent humidity in summer. An open, vented crawl space with a dehumidifier mostly dries the outdoor air coming in.
Can I use a regular dehumidifier under the house?
Room units aren't built for a cool, dusty crawl space and usually rely on a bucket. Crawl space units drain continuously and keep working in cooler air.
What humidity should a crawl space stay at?
The EPA advises keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Many crawl space systems are set in the 50s. Ask the installer what target they will hold and how you can check it.
Sources
- US EPA: A Brief Guide to Mold, Moisture and Your Home (updated February 18, 2026)
- Fixr: Crawl Space Repair Cost (updated January 31, 2025)
- Fixr: Crawl Space Insulation Cost (updated August 29, 2025)
- This Old House: How Much Does Crawl Space Encapsulation Cost? (updated March 13, 2026)
- Bob Vila: How Much Does Crawl Space Encapsulation Cost? (updated July 20, 2022)
- US DOE Building America Solution Center: Capillary Break at Crawlspace Floors (updated July 25, 2017)
- NOAA NCEI: Global Summary of the Day and daily summaries, Charleston International Airport