Can I use plastic sheeting as a vapor barrier?

You can, but the thin painter’s plastic from the hardware store will not last in a crawl space. A real crawl space vapor barrier is a 12-mil liner on the walls and a thicker 20-mil liner on the floor, sealed at every seam. Hardware-store sheeting is usually 4-mil or 6-mil. It tears, it slides, and it stops doing its job in a couple of years.

So the answer is yes and no. Yes, a vapor barrier is sheeting. No, the cheap roll is not the sheeting you want under your house.

Why the thin stuff fails

Your crawl space floor is rough. Rocks, old debris, a sharp corner here and there. A 4-mil sheet drapes over all of that and the weight of anyone crawling through punches holes in it fast. Once it tears, ground moisture comes right back up through the gap. Now you have a barrier that looks installed but is doing nothing.

Thin plastic also will not stay put on the walls. It curls. It pulls loose at the top. Tape lets go in the damp. Within a season or two you find half of it on the floor.

The whole point of a vapor barrier is to seal the dirt and the block off from your living space. Bare crawl space soil gives off water vapor all day long. That vapor feeds mold, rusts your ductwork, and rots wood framing. A floor liner that is torn or a wall liner that has fallen down does not seal anything.

What a real barrier looks like

When we encapsulate a crawl space, we do not just lay down plastic. The full job is a 12-mil wall vapor barrier and a 20-mil floor vapor barrier, with dimpled drainage matting under the floor liner so any water that gets in has a path to a sump. We spray-foam the seams so air and moisture cannot sneak through, and we add a dehumidifier to hold the space dry.

That is a system. The barrier is one piece of it. Plastic alone, even good plastic, will not control a wet crawl space on its own. If you have standing water under there, sheeting over the top of it just traps the water and hides the problem.

We will tell you when you do not need us yet. If your crawl space is already dry, stays dry through mud season, and has a barrier that is intact and taped down, leave it be. Go look first. Crawl on a tarp, bring a light, and check the corners and the low spots after a few days of rain.

What it costs

A full crawl space encapsulation runs $3,000 to $25,000. That covers the wall liner, the floor liner, the drainage matting, the sealed seams, and the dehumidifier. The price moves with the size of the crawl, how rough the floor is, how much old debris we haul out, and whether you need a sump and a radon piece too. We give you the number after a free inspection, and you get the quote within 24 hours.

If the crawl is on dirt or fieldstone and your radon reads high, the same membrane can double as part of a sub-membrane radon system. We install those. Ask us at the inspection and we will price both together.

A little New Hampshire context

Our soil holds water. The ground around most NH foundations is disturbed backfill, the loose fill that got pushed back in when the house was built, and it drains slow and stays wet. Add a freeze-thaw winter and a crawl space takes on moisture from every direction. Thin plastic was never built for that. A 12-mil wall and 20-mil floor liner, sealed and backed by a dehumidifier, was.

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, so sealing the space helps your heating bill too, not only the framing. See DOE Building America (basc.pnnl.gov).

Our crawl space encapsulation carries a 25-year warranty, and the wall vapor barrier carries its own 25-year warranty. The warranty transfers if you sell, with conditions, as long as nobody else cuts into our work.

Want a real answer for your crawl? Call us at 603-610-1770 or book your free inspection at /contact/. Free inspection, free estimate, quote in 24 hours.

Related questions

Is 6-mil plastic enough for a crawl space floor? No. Six-mil tears under foot traffic and debris. We run a 20-mil liner on the crawl floor because it stands up to crawling, tools, and rough ground for the long haul.

Do I need a vapor barrier if my crawl space seems dry? Often yes. A crawl can feel dry to the touch and still give off vapor from bare soil all day. If you want to be sure before you spend, check the low spots after heavy rain, or book a free inspection and we will tell you what we actually see.

Will a vapor barrier fix standing water? No. Sheeting over water traps it. Standing water needs drainage and a sump first, which is why our crawl space encapsulation puts dimpled matting and a sump under the liner before the barrier goes down.

Is this the same as the Forever Dry System for basements? No. The Forever Dry System is for finished basements with full-perimeter interior drainage. A crawl space gets encapsulation instead, which is built for a dirt or block crawl, not a poured basement floor.

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