How to get rid of moisture in a crawl space

The lasting way to get rid of crawl space moisture is to seal the space off from the wet ground with encapsulation. That means a 12-mil wall vapor barrier, a 20-mil floor vapor barrier over dimpled drainage matting, spray-foam-sealed seams, a dehumidifier, and a sump to move water out. Full crawlspace encapsulation runs $3,000 to $25,000. That stops the moisture at the source instead of fighting it forever.

Here is what is happening down there. Your dirt or fieldstone crawl floor lets ground moisture rise straight into the air. Vents that were supposed to air it out pull humid outside air in during summer, and that warm air hits cool surfaces and condenses. So the crawl stays damp. Wood gets musty. You smell it upstairs.

A fan or a single dehumidifier alone will not win that fight. You can run a dehumidifier for years and still lose, because the ground keeps feeding new moisture in faster than the machine pulls it out. The fix is to stop the ground first.

That is what encapsulation does. We line the walls with a 12-mil vapor barrier and the floor with a tougher 20-mil barrier. Under that floor barrier we lay dimpled drainage matting so any water that does get in has a path to travel. We seal the seams with spray foam. Then a dehumidifier handles the small amount of humidity that is left, and a sump pump carries off real water. Sealed, dry, done.

What drives the cost

Crawlspace encapsulation lands at $3,000 to $25,000, and the spread comes from a few plain things. Square footage is the big one. A bigger crawl needs more barrier and more matting. A dirt floor with a lot of debris or old fieldstone takes more prep than a flat clean floor. If there is standing water now, the sump work grows. And if the crawl has structural sag, that is a separate repair, not part of the seal.

You do not always need the full system. If your crawl is bone dry and just a little humid, you may only need a dehumidifier and some sealing. We will tell you that at the free inspection. We are not going to sell you a sump you do not need.

Watch for radon while you are at it

A dirt or fieldstone crawl is also the most common spot for radon to come up through the soil. If yours is one of those, ask about a sub-membrane radon system at the same time. The vapor barrier and the radon membrane work together, so doing both at once saves a second trip under the house. The EPA action level for radon is 4.0 pCi/L, and they suggest considering a fix between 2 and 4 (EPA radon).

A little New Hampshire context

Sealing the crawl helps more than the air. The U.S. Department of Energy notes that up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl (DOE Building America). In our winters, a sealed crawl means warmer floors over it and less work for the furnace. Damp also drives wood rot and mold claims that homeowners insurance often will not cover (Insurance Information Institute), so dry pays off twice.

One thing we do not do is dig up the outside and slap on exterior coating. In our field experience that approach fails in a couple of years and you are back to square one. We seal from the inside, where it lasts. Our crawlspace encapsulation carries a 25-year warranty, and the dehumidifier is covered for 5 years.

Get it looked at

Want to know which fix your crawl actually needs? Call us at 603-610-1770 or book your free inspection. We come out, crawl under the house, and give you a real plan with a quote inside 24 hours. Free inspection, free estimate, every time!

See our full crawl space services, the radon mitigation page, and the 603 guarantee.

Related questions

Do crawl space vents help with moisture? Usually no. Open vents pull humid summer air into a cool crawl, and that air condenses on the wood. Most modern building science says to seal the crawl, not vent it. Encapsulation closes those vents and controls the air with a dehumidifier instead.

Will a dehumidifier alone dry out my crawl space? Only if the ground is already sealed. On a bare dirt or fieldstone floor the soil keeps adding moisture faster than a dehumidifier can remove it, so the machine runs forever and loses. Seal the floor and walls first, then a dehumidifier finishes the job easily.

How long does crawl space encapsulation take? Most crawls are done in a day or two, depending on size and how much prep the floor needs. We confirm the timeline at your free inspection. Encapsulation runs $3,000 to $25,000 and carries a 25-year warranty.

Is crawl space moisture covered by homeowners insurance? Often not. Slow damage from humidity and ground moisture is usually treated as a maintenance issue, not a covered event (Insurance Information Institute). Sealing the crawl is the cheaper path than waiting for rot.