Short answer: in South Carolina, almost always yes. Sealing your crawl space keeps most outside moisture out, but it doesn’t remove the humidity that’s already in the air and materials — and our climate is humid enough that a sealed space can still creep above the mold threshold without active drying. A properly sized dehumidifier is what holds an encapsulated crawl space in the safe zone year-round. Here’s the full reasoning.
Why Sealing Alone Isn’t Always Enough
Encapsulation stops new moisture from entering through the ground and vents. But moisture also comes from concrete curing, occasional plumbing sweating, the small amount of air exchange with the house, and humidity already present when the space was sealed. In a dry climate that residual moisture might dissipate on its own. In South Carolina’s humidity, it often doesn’t — and relative humidity can drift up past 60%, where mold becomes possible again. A dehumidifier removes that residual moisture and keeps you safely below the threshold (see our humidity chart).
When You Might Not Need One
Some encapsulated crawl spaces use a supply of conditioned air from the home’s HVAC instead of a standalone dehumidifier, and a small, well-sealed space in a drier microclimate may hold acceptable humidity on its own. But these need to be verified with real humidity readings — not assumed. If a contractor seals your crawl space and skips humidity control entirely, ask how humidity will be managed.
What Setting Should the Dehumidifier Be On?
Set a crawl space dehumidifier to about 50–55% relative humidity. That keeps you comfortably below the 60% mold line without the unit running non-stop. A quality crawl space dehumidifier drains automatically to your sump or the exterior, so there’s no reservoir to empty. Learn how it fits the full system in our encapsulation guide.
Not sure if your encapsulation needs a dehumidifier — or whether your current one is sized right? Attic and Crawl Space Solutions will measure and tell you. Book a free assessment.
Frequently Asked Questions
Does an encapsulated crawl space need a dehumidifier?
In humid climates like South Carolina, almost always yes. Sealing keeps new moisture out, but a dehumidifier removes residual humidity and holds the space below the 60% mold threshold year-round.
What should I set my crawl space dehumidifier at?
Around 50–55% relative humidity. That keeps you safely below the mold line without the unit running constantly.
How much does a crawl space dehumidifier cost to install?
A professionally installed crawl space dehumidifier typically runs $1,200–$2,500 depending on the unit’s capacity and drainage setup. It’s often included as part of a full encapsulation package.