How does a vapor barrier function?

A vapor barrier is a sealed plastic liner that stops ground moisture and water vapor from passing into your crawl space. It works by covering the dirt floor and walls so soil moisture cannot evaporate up into the air your house breathes. In a 603 crawlspace encapsulation we use a 20-mil floor vapor barrier and a 12-mil wall vapor barrier, sealed at the seams, and that job runs $3,000 to $25,000.

Here is what is happening down there. A bare dirt crawl space is wet ground. Soil holds water and releases it slowly as vapor, all day, every day. That vapor rises, soaks into your floor joists, your insulation, your subfloor. You get the musty smell. You get mold on the wood. You get the cold floors upstairs in January. The dirt is not the enemy by itself. The moving moisture is.

A vapor barrier blocks that movement. Plastic does not let water vapor through it. So when we lay a thick liner across the whole floor and run it up the walls, the moisture from the soil hits the plastic and stops. It cannot reach the wood. It cannot feed mold. The air in the crawl gets drier and stays that way.

A sheet of plastic laid loose does not do much. The seams matter. Vapor finds any gap. That is why we overlap the panels and seal the seams, and we spray-foam-seal where the liner meets the walls and any penetrations. We also lay a dimpled drainage matting under the floor barrier, so if liquid water ever shows up it has a path to the sump instead of pooling on top of the liner. Then a dehumidifier holds the whole space dry, and a sump pump handles standing water if it ever comes.

Why does water show up at a crawl space wall in the first place? When the house was built, the crew dug out the ground and then put it back around the foundation. That backfill is loose. It drains slow and it holds water. So rain and snowmelt pool against the wall right where you do not want it. Up here in New Hampshire, frost makes it worse. The ground freezes and pushes on the wall through the winter. A vapor barrier alone does not fix wall movement, and we will tell you that to your face. If your crawl has structural cracking or a leaning wall, that is a foundation repair conversation, not an encapsulation one.

What drives the price? The square footage of the crawl, how much grading and prep the dirt needs, whether you need a sump and dehumidifier, and how rough the access is. A tight, low crawl takes longer to work in. We quote $3,000 to $25,000 after we crawl in and measure it. The free inspection is how we get you a real number, not a guess.

You may not need the full system. Some crawls just need a clean floor liner and a dehumidifier. Some need the whole encapsulation with drainage and a sump. We would rather tell you the smaller fix when the smaller fix is right.

One thing worth knowing on the energy side. 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 space. A sealed, dry crawl is easier to keep conditioned, so this job does more than handle moisture.

If your crawl has a dirt or fieldstone floor and you have a radon concern, the same membrane work pairs with a sub-membrane radon system. The EPA action level for radon is 4.0 pCi/L, and they say to consider fixing between 2 and 4. We install those systems, so you can handle both problems with one crew in the crawl.

Our crawlspace encapsulation carries a 25-year warranty, and the wall vapor barrier itself is warrantied 25 years. The transfer is conditional, so it stays good as long as another contractor does not touch the work.

Want a real answer for your crawl? Call us at 603-610-1770 or book your free inspection and we will come look, measure, and tell you what it actually needs.

See the full scope on our crawl space services page, and read what we stand behind on the 603 guarantee. If the problem turns out to be the wall and not the moisture, here is our structural and foundation repair work. For the radon side, here is radon mitigation.

For the building-science and health basics, the DOE Building America program covers crawl space moisture control, and the EPA and NH DHHS cover radon. [verify]

Related questions

Do I need a vapor barrier if my crawl space is dry now? Probably yes. A crawl that feels dry today still has soil giving off vapor underneath. A 12-mil wall and 20-mil floor liner keeps it that way through a wet spring. We will tell you if you can get by with less.

Is a vapor barrier the same as crawl space encapsulation? No. The vapor barrier is one part. Full encapsulation adds sealed seams, drainage matting, a sump, and a dehumidifier so the whole space stays dry, not just the floor.

How thick should a crawl space vapor barrier be? We run a 20-mil liner on the floor where you walk and crawl, and a 12-mil liner on the walls. Thicker on the floor because that is where it takes the wear.

Will a vapor barrier stop standing water? Not by itself. A liner stops vapor. If liquid water gets in, you need the drainage matting and a sump pump under the barrier to carry it out. That is why we install them together.