Search for the best vapor barrier for basement walls and you will find a lot of advice to staple a sheet of plastic to the wall and cover it with drywall. That is the one thing the U.S. Department of Energy tells you not to do. The reason is simple: a basement wall is concrete that can pass moisture from the ground, and a plastic sheet on the room side traps it in the wall instead of letting it dry out.
So the real question is not which plastic to buy. It is which wall assembly you are building, what has to touch the concrete, and where a vapor-control layer belongs at all. This guide sorts the common basement wall setups, shows what the Department of Energy’s Building America program says about each, and then covers the two products that fit a basement wall job: a variable-permeance retarder and a dimple drainage mat. If the problem is water coming through the wall rather than humid air, start with the foundation waterproofing sealant guide instead.
Vapor, liquid water and condensation are three different problems
The Department of Energy’s Building America profile of basement insulation systems (PNNL-SA-90546, January 2013) names the sources: interior air carries vapor that condenses on a basement wall that is colder than the dew point, rainwater or groundwater can wick through footings or walls, and freshly poured concrete is itself a moisture source in new construction. Each one needs a different fix. A sheet of plastic only addresses vapor from one direction, and it does nothing for liquid water. Martin Holladay of Green Building Advisor says in the comments of his article on basement and crawl space vapor retarders that bulk water is handled by a drainage system. That means gutters, grading and a working footing drain come before any wall treatment.
What the Department of Energy says to do and not do
The same profile gives a short list of rules for the inside of a basement wall:
- Do not install polyethylene sheeting, vinyl or foil wallpaper, or epoxy, oil or alkyd paint on the interior of basement walls. They act as vapor retarders and trap moisture in the wall.
- Do not put fiberglass batt or cellulose insulation in contact with a foundation wall. Against a damp wall they absorb a lot of moisture and stay damp.
- Insulation on the interior must be airtight against the wall and must allow moisture coming through the wall to dry to the interior (semi-permeable).
- Rigid foam (XPS, EPS or polyisocyanurate) must adhere completely to the wall with no air gaps behind it, and its edges and seams must be sealed with acrylic-latex caulk, foam sealant or mastic.
- Closed-cell spray foam gets the best moisture-control rating of all interior foundation insulations, and it can be a Class I, II or III vapor retarder depending on thickness.
The profile’s own wall example is 2 inches of XPS (R-10) against the concrete, a 2×6 wall in front of it with unfaced fiberglass (R-19), and gypsum board with latex paint as the vapor retarder. Notice what that drawing does not contain: a plastic sheet.
Which assembly are you building
| Your basement wall | What touches the concrete | Where vapor control comes from |
|---|---|---|
| Bare wall, damp or with minor seepage, not finishing yet | Nothing yet | Fix water entry first. A dimple drainage layer only makes sense if the water has somewhere to go, such as a floor drain or sump |
| Finishing with rigid foam board | Foam, fully adhered, seams sealed | The foam itself. No separate sheet |
| Stud wall with batts | Never the batts. Foam goes against the concrete first | The foam layer, plus a retarder on the room side only if your local code calls for one |
| Cold climate, code asks for a retarder on the warm side of the cavity | Foam against the concrete | A variable-permeance (smart) retarder instead of plain polyethylene |
| Dirt floor crawl space | A ground cover over the soil | See the crawl space vapor retarder guide. This is the one place plain polyethylene is the standard answer |
Climate and local code decide the last two rows, so check with your building department before you buy. Sources disagree in the details, and several of the pages that rank for this question are written by companies that sell a wall liner or a waterproofing service.
The two products that fit
CertainTeed MemBrain Smart Vapor Retarder, 8 ft 4 in by 100 ft (model 902008). A variable-permeance film: tight in dry conditions, more open when humidity rises, so a wall cavity can dry in summer. Distributor copies of CertainTeed’s spec sheet give it as 1 perm or less in the dry cup test and above 10 perms in the wet cup test (one trade source cites a higher wet figure, so check the current data sheet). CertainTeed positions it for heating and mixed climates, not for hot humid ones. The Amazon listing says it is for use with unfaced insulation where a vapor retarder is required, and it was rated 4.6 stars from 50 ratings, with a 100+ bought in the past month badge and in stock when I read it. The reviews are not uniform. One buyer used it on interior walls in the northeast and followed the CertainTeed install guide, and later reported condensation on the inside face of one wall in humid weather until the heat was installed. Another gave three stars and called it overpriced compared with taped plastic, which is a fair challenge: you only pay for a smart retarder where your code or climate actually asks for one. Whether it suits a particular below-grade wall is a question for CertainTeed’s install guide and your inspector, and the Department of Energy rules above still apply to what touches the concrete.
Check price on Amazon: CertainTeed MemBrain Smart Vapor Retarder, 8 ft 4 in by 100 ft. This is an affiliate link (see our disclosure for details).
Whitecap Dimple Drainage Matting, 2 ft by 50 ft. A high-density polyethylene sheet with raised dimples that holds the surface off the wall or floor and leaves a channel for water to run toward a drain or sump. This is a drainage layer, not a vapor-control layer, and it does not make a leaking wall dry. The listing names crawl space encapsulation, basement flooring underlayment, foundation walls and below-slab drainage as uses, and it was rated 4.8 stars from 129 ratings, an Amazon’s Choice, sold by CrawlSpace-DIY and in stock. Reviewers mostly describe crawl space floors, where it protects a ground cover from sharp gravel and foot traffic. Green Building Advisor’s Holladay mentions an exterior dimple mat together with a working footing drain as a way to handle bulk water, so on a wall the mat’s best role is outside during excavation work. On the inside it only helps if the channel ends at a drain.
Check price on Amazon: Whitecap Dimple Drainage Matting, 2 ft by 50 ft. This is an affiliate link (see our disclosure for details).
How much to buy: a 20 by 30 foot basement
The perimeter is 100 feet, and with 8 foot walls that is 800 square feet of wall. A MemBrain roll covers 8 ft 4 in by 100 ft, which is about 833 square feet. That leaves roughly 33 square feet for overlaps and cut waste, which is tight if you also need to lap seams and wrap corners. Subtract the windows and doors and you gain room, so one roll may be enough, but order two if the wall has many offsets, and plan to tape the seams and edges the way CertainTeed’s install guide describes.
The dimple mat is 2 by 50 feet, or 100 square feet per roll, so covering the same 800 square feet takes 8 rolls before overlaps. That number is the reason to ask whether you need it. On a damp wall that ends at a floor drain it can be the right tool. On a wall that is only cold and slightly humid, the Department of Energy’s foam-first assembly costs less in time and money and does the vapor job itself.
When a product is the wrong fix, and who to call
Skip every product above if the wall shows white mineral deposits, a visible wet line after rain, active seepage at the floor joint, or a crack that leaks. A plastic sheet or a dimple mat only hides these, and the water still ends up behind your finished wall. Grade the soil away from the house, clean the gutters and send downspouts away from the foundation. If water still comes in, you need drainage work, which a foundation or waterproofing professional can scope. Angi’s foundation pros are one way to find one. This is an affiliate link (see our disclosure for details).
For damp walls you plan to paint rather than insulate, see our basement waterproofing paint guide, and for a leaking crack, the concrete crack filler guide.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.