Crawl space is infested with mold, how do we go about properly fixing it?

Fix the moisture first, then the mold. A moldy crawl space means water and damp air are getting in, and until you stop that, any mold you scrub off comes right back. The durable fix in New England is crawlspace encapsulation: seal the dirt and walls off from ground moisture, then keep the air dry. Encapsulation runs $3,000 to $25,000 depending on the size of the space, how wet it is, and what shape the wood is in.

Why crawl spaces grow mold

Mold needs three things: moisture, something to feed on, and time. A vented dirt crawl space hands it all three. Ground moisture wicks up through bare soil, humid outside air drifts in through the vents and condenses on cool framing, and your floor joists and subfloor are wood, which is food. Leave that running a season or two and you get the fuzzy white and black growth most folks find when they finally crawl under there.

The water often comes from the same place it does in a basement. The soil against your foundation is disturbed backfill, the looser dirt put back when the house was built. It drains slowly and holds water, so the area around your house stays damp, and NH frost can push on the walls too. Bottom line, the crawl space is wet because the ground around it is wet and the air is humid, not because of one leak you can patch.

How we actually fix it

We do not just spray and leave. We seal the space so moisture stops coming in, then keep the remaining air dry. That is crawlspace encapsulation. Here is what goes in:

• A 20-mil floor vapor barrier across the dirt, over dimpled drainage matting so any water has a path to move.
• A 12-mil wall vapor barrier up the foundation walls.
• Spray foam sealing the seams so the barrier is one closed shell.
• A dehumidifier sized for the space to hold the air dry.
• A sump where the water needs somewhere to go.

Once the space is sealed and the air is dry, mold has nothing to live on. We address existing growth as part of the work so you are not sealing a problem in. If wood is already rotted or sagging, that is a separate structural fix, and we will tell you straight if you have it.

You should also know what you do NOT need. If the rest of your home is fine and the issue is contained to the crawl, you do not need a full basement system. The Forever Dry System is for basements. For a crawl, encapsulation is the right call, and we will not sell you the bigger job.

What drives the cost

The price of encapsulation, $3,000 to $25,000, moves with a few things. Square footage is the big one, and a taller crawl you can move around in costs more to seal than a low one. How wet it is matters too, because a soaked space may need a sump and more drainage matting where a damp one does not. And if there is rotted wood or a sagging beam, that repair is priced on its own after we see it.

If your crawl is dirt or fieldstone and you are worried about radon, that is a real concern in New Hampshire. We can install sub-membrane radon mitigation under the new barrier while we are down there. Radon mitigation runs $900 to $6,000, and most homes land around $1,950 to $2,250, and a radon test is $50, credited toward the job if you move ahead. The EPA action level is 4.0 pCi/L, and they suggest considering a fix between 2 and 4 (epa.gov/radon). NH DHHS has guidance for our state too.

A sealed crawl also helps your heating bills. The Department of Energy notes that a good chunk of a home’s heat loss, up to about a third, can come from an uninsulated foundation or crawl space (basc.pnnl.gov).

A quick word on the cheap route. Some contractors will dig outside and waterproof the exterior. In our field experience that fails in a couple of years, and you have paid for excavation twice. Sealing from the inside lasts.

What you get from us

We are 603 Basement Solutions out of East Kingston, NH, and we serve New Hampshire, Maine, and Massachusetts. We self-perform the work, so the crew sealing your crawl works for us. The inspection is free, the estimate is free, and we get your quote back within 24 hours. Crawlspace encapsulation carries a 25-year warranty, and the wall vapor barrier is warrantied 25 years too. Financing is available through Hearth, subject to approval.

You can learn more on our crawl space services page, see how radon mitigation fits a dirt crawl, and read the 603 guarantee.

Get your crawl space looked at

Call us at 603-610-1770 or book your free inspection and we will come crawl under there, find where the water is coming from, and price the fix. No charge to look.

Related questions

Can I just kill the mold myself with bleach or a spray? You can knock it back for a while, but it comes back unless you stop the moisture. Bleach on wood is surface deep. Until the crawl is sealed and the air is dry, the mold has water and food, so it returns. Fix the cause, not the symptom.

Do I need a dehumidifier in a sealed crawl space? Yes. Sealing keeps ground moisture and outside humid air out, and the dehumidifier holds the remaining air dry so mold cannot get a foothold. It is part of the encapsulation, not an add-on. Ours carries a 5-year warranty.

Is crawl space mold a radon problem too? They can travel together in a dirt or fieldstone crawl. If you are sealing the space anyway, that is the right time to add sub-membrane radon mitigation underneath. Radon mitigation runs $900 to $6,000, and most homes land around $1,950 to $2,250, and a test is $50, credited toward the job if you move ahead. EPA’s action level is 4.0 pCi/L (epa.gov/radon).

Will encapsulation lower my heating bill? It helps. A sealed, dry crawl stops the cold damp air your floors sit over. The DOE notes up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl (basc.pnnl.gov).