What is a vapor barrier?
Updated August 2026
A vapor barrier is a sheet material, usually 6-mil or thicker polyethylene, that slows water vapor from diffusing through a wall, ceiling, or crawl-space floor and condensing somewhere it will cause rot. Codes rate these by permeance. Class I materials such as poly sheet and foil facing pass 0.1 perms or less, Class II kraft paper on batt insulation passes 0.1 to 1.0, and Class III latex paint passes 1 to 10. The tighter the class, the less forgiving the installation.
Where it is
Four locations. Over the dirt floor of a crawl space, where a 6-mil to 20-mil liner runs 6 to 12 inches up the foundation walls and laps 12 inches at every seam. Under a basement slab, rolled out before the pour. On the warm-in-winter face of insulated walls and ceilings in cold climates, either as loose poly under the drywall or as the kraft facing already stapled to the batts. And under laminate or engineered flooring as a thin underlayment film. Inside a finished wall you cannot see it without opening drywall, so the kraft-paper tell in an unfinished basement or an attic knee wall is the easiest one to read.
What it does
Warm air carries more moisture than cold air, so vapor drifts from the warm side of an assembly toward the cold side. Let it meet a cold surface on the way and it condenses. Stopping that trip is the entire assignment. The catch, and it is the whole reason this part has a bad reputation, is that the correct side depends on climate. In cold zones the barrier belongs on the interior. In hot, humid zones it must not go there: air conditioning makes the interior drywall the cold surface, moisture drives inward from outside, and interior poly (or vinyl wallpaper, the accidental version) traps it against the framing. Two barriers in one assembly is worse still, because whatever gets in cannot dry in either direction. Rot found in a 10-year-old wall is frequently this mistake rather than a leak. Bare crawl-space dirt is the other big source, commonly estimated to give up 10 to 15 gallons of water a day per 1,000 square feet.
Maintenance
The crawl-space liner is the only vapor barrier you can actually inspect, so put it on the calendar once a year, in spring. Look for tears left by plumbers and cable installers, rodent damage, seams peeled apart, and standing water sitting on top of the plastic, which is groundwater and needs a french drain or a sump rather than thicker plastic. Re-tape seams with contractor tape as you find them, and check the underside of the subfloor for damp spots or fuzzy growth. The rest of what to watch down there is in what a crawl space is.
What it costs to replace
A replacement vapor barrier runs $0.05–0.15 per square foot for 6-mil poly you lay yourself, $1,200–3,000 for a contractor to install a taped 12-mil to 20-mil crawl-space liner, and $5,000–15,000 for full encapsulation with sealed walls and a dehumidifier.
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Frequently asked questions
Which side of the insulation does a vapor barrier go on?
The side that stays warm in winter. In cold and mixed-cold climates, roughly zones 5 through 8, that means the interior face, which is why kraft-faced batts get stapled with the paper toward the room. In hot, humid climates skip interior poly entirely and let the wall dry inward; a coat of latex paint is all the vapor control needed. Mixed climates favor a smart membrane that tightens in winter and opens in summer. When unsure, ask a local building inspector, not the internet.
Is a vapor barrier the same as house wrap?
No, and confusing them causes rotten walls. House wrap is a water-resistive and air barrier: it sheds liquid water off the outside of the sheathing while staying vapor open, often near 50 perms, so the wall can still dry outward. A vapor barrier does the opposite, blocking vapor at 0.1 perms or less. Wrapping a house in poly instead of wrap seals moisture inside the assembly, which is exactly the failure everyone is trying to avoid.
Does my crawl space really need a vapor barrier?
Yes. Most codes require a minimum 6-mil ground cover, and bare soil in a crawl space feeds the floor above with humidity all summer, which shows up as cupped hardwood, musty air, and mold on joists. A taped 10-mil to 20-mil liner running up the walls costs $1,200–3,000 installed and does most of the work; full encapsulation with a dehumidifier at $5,000–15,000 is the version for chronically wet sites. Fix drainage outside first either way.