Do You Need a Vapor Barrier in a Basement Wall?

The short rule

A vapor barrier on the inside of a basement wall is the opposite of what building science recommends. Building Science Corporation's information sheet states: “No interior vapor barriers should be installed in order to permit inward drying” (Building Science Corporation, Basement Insulation for All Climates (Information Sheet 511)). The reason is that moisture moves through concrete in small amounts all the time. If the wall can dry toward the room, that moisture leaves. If a sheet of plastic blocks it, the moisture collects behind the plastic, in the wood and drywall.

What counts as a vapor barrier

The Department of Energy lists the Class I vapor retarders, the least permeable type: polyethylene plastic sheeting, rubber membranes, glass, aluminum foil, sheet metal and foil-faced insulating sheathings. It says they should not be used interior to fibrous insulation in basements and crawl spaces because they can trap moisture inside wall assemblies (U.S. DOE, Understanding vapor retarders for basements). Its Building America lessons add: do not install polyethylene sheeting, vinyl or foil wallpaper, or epoxy, oil or alkyd paint on the interior of basement walls (U.S. DOE, Basement insulation systems (Building America Top Innovation)). Two of those are surprises. Wallpaper and certain paints act like a vapor barrier.

What to use instead

• Semi-permeable foam on the concrete. Unfaced XPS or EPS, or closed-cell spray foam, with sealed seams. Foam facings should not be foil or plastic (Building Science Corporation, Basement Insulation for All Climates (Information Sheet 511)).
• Class II or III retarders in a cavity. The DOE page says to use Class II or III vapor retarders such as kraft-faced fiberglass batts or latex paint on the drywall for below-grade walls. Building Science Corporation's sheet prefers unfaced fiberglass or damp spray cellulose behind a foam layer. The sources differ on facing; the common point is no polyethylene, and foam rather than fiberglass against the concrete.
• Latex paint on the drywall. One of the DOE-named options, and it lets the wall dry inward.

Where a barrier does belong

Under a new concrete slab. Building Science Corporation says that when slab insulation is provided, a sheet polyethylene vapor barrier should sit over the rigid insulation and in direct contact with the concrete slab, and a sand layer should never go between the barrier and the slab. If rigid XPS is laid under a new slab, the XPS itself is a vapor retarder, so no polyethylene is needed under it (Building Science Corporation, Renovating Your Basement for Moisture Control (Building America report 0309)). That is a slab-replacement detail, rare in a finish, and not a reason to cover the walls with plastic.

There is also a floor rule that follows from the same logic: if the slab is uninsulated, DOE says not to install carpet or vinyl flooring because they trap moisture. See the subfloor guide.

What a contractor may propose

If a bid includes a plastic wall liner, or poly behind the drywall, ask why and ask for the manufacturer's permeance data. Ask also what keeps water from collecting behind the sheet. A system with an interior drain that carries water to a sump is a different thing from a sheet that tries to hold the wall dry. Those are discussed in the waterproofing guide, and the full wall assembly is in the insulation guide.

If your basement already has plastic on the walls

Older finishes sometimes include polyethylene behind paneling or drywall. If you open a wall and find it, look at the concrete and the wood behind it. A damp or discolored surface behind plastic is the kind of moisture trap DOE describes. Remove the plastic, let the wall dry, find and fix the water source, and rebuild the assembly with foam against the concrete. If the wall behind it is dry, that is still not a reason to put plastic back. The sources are consistent that the safer assembly lets the wall dry toward the room.

Perm ratings in plain terms

Building Science Corporation expresses vapor permeability in perms. It calls a foam layer vapor semi-impermeable if it is greater than 0.1 perm, semi-permeable if greater than 1.0 perm, and permeable if greater than 10 perm; the greater the permeance, the greater the inward drying (Building Science Corporation, Basement Insulation for All Climates (Information Sheet 511)). When a product data sheet gives a perm rating, those are the numbers to compare. A rating below 0.1 perm is in the territory the DOE warns about.

Where to go next

This guide sits under Basement walls and moisture. Every price on this site, with its source and date, is on the basement finishing cost page. If this topic is on your list, these guides cover the neighbouring questions: Basement Wall Insulation in Northern Illinois, Preventing Mold in a Finished Basement, Subfloor Options for a Basement Slab, and Moisture-Resistant Drywall for a Basement.