What are the potential disadvantages of having a crawl space under your house?

A crawl space causes real problems when it sits open and unsealed. The big ones are damp air and mold, wood rot in your floor framing, higher heating bills, radon and musty smells pushed up into your living space, and pests that like the dark and the moisture. Most of that traces back to one thing. The crawl space breathes in ground moisture, and that moisture goes nowhere good. The fix is crawlspace encapsulation, which usually runs $3,000 to $25,000 depending on what we find down there.

Here in New Hampshire, Maine, and Massachusetts, we crawl under a lot of these. The trouble starts with the dirt floor and the open vents. Both let humidity in.

Why a crawl space causes problems

Water pools around your foundation because the soil down there is disturbed backfill. That’s the looser fill the builders put back when the house went up. It drains slow and holds water. Then NH frost heave pushes on the walls through the winter. So the ground stays wet, and a vented crawl space pulls that wet air right in.

Damp air does damage. Mold grows on the wood. The floor framing above your head can soften and rot over time. You feel it as bouncy floors or a musty smell in the rooms upstairs. That smell isn’t trapped down low. Air moves up through the house, so crawl space air becomes the air you breathe.

Energy is the quiet cost. The Department of Energy notes that up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl space (see basc.pnnl.gov). Cold air under the floor means cold floors and a furnace that runs harder.

Then there’s radon. It’s a soil gas, and it seeps up through a dirt or fieldstone crawl. The EPA action level is 4.0 pCi/L, and they say to consider fixing between 2 and 4 (epa.gov/radon; also NH DHHS radon). A crawl space is one of the most common entry points we see.

Pests round it out. Mice, ants, and other critters like the dark and the moisture. A wet crawl space is an open door.

How we fix it

We don’t waterproof a crawl space the way we do a basement. The Forever Dry System is basement only. A crawl space gets crawlspace encapsulation instead. That means a 12-mil wall vapor barrier and a 20-mil floor vapor barrier, dimpled drainage matting under the floor barrier, spray-foam-sealed seams, a dehumidifier, and a sump. We seal the dirt off so the ground moisture can’t get into your air. The dehumidifier holds the space dry, and the sump handles any water that does show up.

If you’ve got radon coming through a dirt or fieldstone crawl, the right tool is sub-membrane depressurization. We install those. It pulls the soil gas out from under the membrane before it reaches your floor.

You may not need every piece. If your crawl is bone dry and your radon test comes back low, you don’t need a radon system yet. We’ll tell you that on the free inspection. We won’t sell you a fan you don’t need.

What drives the cost

Size is the first thing. A bigger crawl is more barrier and more labor. After that it’s how wet it is, whether you need a sump, and whether there’s radon to deal with. Crawl space encapsulation usually lands around $3,000 to $25,000, and radon mitigation is its own line at $900 to $6,000, and most homes land around $1,950 to $2,250. If you want a radon test first, that’s $50, credited toward the job if you move ahead. We give you a real number after we see the space, not a guess over the phone.

The encapsulation carries a 25-year warranty, and the wall vapor barrier is 25-year too. The dehumidifier is 5-year and the sump pump system is 3-year, batteries excluded. Transfer is conditional. It stays good as long as nobody else touches the work. See the 603 guarantee.

One more New England note. We don’t do exterior excavation to fix moisture. Nik has seen those fail in about two to three years. The durable fix is sealing and drying from the inside.

Want it looked at? Get your free inspection booked, or call us at 603-610-1770. Free estimate, quote within 24 hours, and financing through Hearth if you want it (subject to approval).

Start at crawl space services. For related work see basement waterproofing, radon mitigation, and structural and foundation repair.

Related questions

Does a crawl space need to be encapsulated, or can I just leave it vented? Vented crawl spaces in our climate pull in damp air most of the year, which feeds mold and rot. Sealing it off with a vapor barrier and dehumidifier keeps the space dry and the air upstairs cleaner. We’ll tell you if yours is fine as is.

Can a crawl space cause high radon in the house? Yes. A dirt or fieldstone crawl is a common radon entry point because the gas seeps straight up from the soil. Test first. The EPA action level is 4.0 pCi/L (epa.gov/radon). If it’s high, sub-membrane depressurization is the fix, and we install it.

How much does crawl space encapsulation cost in NH? It usually runs $3,000 to $25,000, depending on the size of the crawl, how wet it is, and whether you need a sump or radon work. We give you a firm number after the free inspection.

Will encapsulating my crawl space lower my heating bills? It helps. The DOE notes a large share of a home’s heat loss can come from an uninsulated foundation or crawl space (basc.pnnl.gov). Sealing and drying the space takes that cold, damp air out of the equation, so your floors feel warmer and the furnace runs less.