Bare soil under a house gives off water vapor constantly. In a crawl space, that moisture raises the humidity, feeds mold on wood framing, and rises into the living space above. A moisture barrier stops it at the source.
A crawl space moisture barrier is a polyethylene sheet laid over the dirt floor so ground moisture can't evaporate into the crawl space. A basic 6-mil ground cover is the cheapest step, with inexpensive material for 1,000 sq ft; a full encapsulation, with a heavier liner up the walls, sealed vents and usually a dehumidifier, runs into the thousands.
Why a crawl space needs one
The EPA recommends keeping indoor humidity between 30% and 50%, and below 60%, to keep mold from growing. A crawl space open to damp soil easily sits above that. Covering the soil removes the biggest moisture source, and air sealing and a dehumidifier handle the rest.
Liner materials
Liner thickness is measured in mils, thousandths of an inch.
- 6-mil polyethylene: the basic ground cover. It blocks vapor well but tears easily if people crawl on it.
- 10- to 12-mil reinforced polyethylene: a woven reinforcement makes it much harder to puncture, a sensible minimum if anyone will work in the space.
- 20-mil liners: used for full encapsulation, tough enough for regular foot traffic and for running up the walls.
All of these are Class I vapor retarders. For unvented crawl spaces, the International Residential Code (section R408.3) requires a continuous Class I vapor retarder over the exposed earth, with seams overlapped and sealed and the edges run up the walls.
Ground cover vs. full encapsulation
- Ground cover: a liner over the dirt floor, seams taped. It's the cheapest step and blocks most ground moisture, while the vents stay as they are.
- Full encapsulation: a heavier liner over the floor and up the walls, sealed seams, sealed vents and usually a dehumidifier. It turns the crawl space into a dry, semi-conditioned space.
Fix any water coming in first. A liner over standing water or an active leak just hides the problem; drainage, gutters and grading come before the barrier.
What it costs
Cost depends mostly on the liner you choose and on whether you install it yourself or hire a contractor. For a 1,000 sq ft ground cover, 6-mil polyethylene is the cheapest liner, 12-mil reinforced costs more, and 20-mil costs the most. Seam tape adds a little on top of the liner.
A DIY ground cover is inexpensive, mostly the price of the plastic and tape. Hiring a professional adds labor, which makes the same job cost more.
Full encapsulation costs more because it adds materials and work beyond a simple ground cover. It uses a heavier liner over the floor and up the walls, seam tape and mastic, wall fasteners, and sealed vents, plus a dehumidifier with its own drain and electrical outlet. Where water gets in you add an optional drainage mat or a sump pump, some jobs add wall insulation, and professional labor is the largest part, so a full encapsulation together runs into the low thousands.
Humidity control after encapsulation
An encapsulated crawl space still needs its humidity controlled, usually with a dehumidifier set to about 50%. A dehumidifier adds a modest amount to the monthly power bill, depending on its size and how many hours a day it runs; see dehumidifier running cost and the dehumidifier guide.
Common mistakes
- Leaving seams untaped or the edges loose, which lets vapor around the barrier.
- Sealing vents without adding humidity control, which can trap moisture.
- Covering standing water or ignoring leaks instead of fixing drainage first.
- Using thin plastic where people will crawl, which tears within a few visits.
Frequently asked questions
What thickness vapor barrier should I use in a crawl space?
At least 6-mil polyethylene to block vapor; 10- to 12-mil reinforced if anyone will work in the space; 20-mil for full encapsulation.
Is crawl space encapsulation worth it?
In a humid climate, or where the crawl space has mold, musty odors or damp insulation, usually yes. In a dry climate with a dry crawl space, a ground cover may be all it needs.
Should I close my crawl space vents?
Only as part of a full encapsulation with humidity control. Closing vents without a sealed liner and a dehumidifier can make moisture worse.
Does a crawl space moisture barrier stop mold?
It removes the main moisture source, which stops most mold growth. Keeping humidity below 60%, as the EPA recommends, keeps it that way.