Basement moisture
Basement Vapor Barrier: Should You Put One on Basement Walls and Floors?
Usually not a plastic sheet. Building Science Corporation and the US Department of Energy advise against polyethylene on the inside of below-grade basement walls because it traps moisture against the concrete. The common recommendation is rigid foam board or closed-cell spray foam against the wall, seams sealed, then framing. Floors are a different case.
Quick answer
Basement vapor barriers in one look
- Walls: skip the plastic sheet; use foam against the concrete.
- Floors: a sheet under a new slab or under floating flooring is normal.
- Code: the IRC exempts basement walls from its vapor retarder rule.
- Wet walls: a vapor barrier is not waterproofing. Fix the water first.
Local code and your climate zone govern. This is general guidance, not a design for your house.
Short answer
Why the plastic sheet is usually the wrong layer
A basement wall is not like an above-grade wall. The concrete is in contact with damp soil on one side, so it is always carrying some moisture, and it stays cool most of the year. The wall only stays healthy if that moisture has somewhere to go, and the only direction it can go is into the basement.
Six-mil polyethylene on the inside blocks that drying. Moisture collects between the plastic and the wall, and any humid room air that gets behind the finishes meets a cold surface and condenses. Building Science Corporation's basement guidance says plainly that no interior vapor barriers should be installed, and the Department of Energy's building science guidance says the same for basement walls with fibrous insulation.
What works better in most basements is rigid foam insulation directly against the concrete, with the seams sealed, and a stud wall in front of it. The foam keeps humid air off the cold concrete, slows vapor without stopping it entirely, and does not rot or feed mold if it gets damp.
Last reviewed: September 24, 2026
Terms
Vapor barrier vs vapor retarder: the three classes
"Vapor barrier" is the everyday term; codes and building scientists say vapor retarder, because almost every material lets some vapor through. They are graded by permeance, measured in perms: the lower the number, the less vapor gets through. When people say vapor barrier, they almost always mean a Class I material such as polyethylene sheeting.
| Class | Permeance | Common examples | In a basement |
|---|---|---|---|
| Class I (vapor barrier) | 0.1 perm or less | Polyethylene sheet (6-mil poly), unperforated aluminum foil, foil-faced foam boards, rubber membranes, sheet metal, glass | Right under a new slab. Not on the room side of a basement wall with fiberglass or wood framing behind it. |
| Class II | Over 0.1 up to 1.0 perm | Kraft-faced fiberglass batts, vapor retarder paint; thick unfaced XPS foam and closed-cell spray foam fall in or near this range | Foam layers in this range are the usual recommendation against basement concrete. |
| Class III | Over 1.0 up to 10 perms | Latex or enamel paint on drywall | Fine as the interior finish; it lets the wall dry toward the room. |
| Vapor permeable | Over 10 perms | Unpainted drywall, most housewraps | Lets vapor through freely; fibrous insulation needs foam between it and the concrete. |
Class definitions follow the 2021 IRC. Material examples are from the IRC definitions, DOE building science guidance and Building Science Corporation. Foam permeance depends on type and thickness; check the product data sheet.
The mechanism
Why interior plastic fails on a concrete basement wall
Poly on a basement wall fails in two ways, and both show up as the same musty smell and black spotting when the wall is opened:
| Failure | What happens | When it shows up |
|---|---|---|
| Moisture trapped on the concrete side | Concrete wicks moisture up from the footing and in from the soil. With plastic on the inside it can't dry, so the face of the wall, the plastic and anything touching them stay wet. | All year, worst after long wet spells or where the soil outside stays damp. |
| Room air condensing on cold plastic | Humid room air leaks into the stud cavity through outlets, gaps at the ceiling and bottom plate. The plastic is as cold as the concrete it touches, so the air reaches its dew point and water forms on the room side of the plastic, soaking batts and the bottom plate. | Humid summers, when the room's dew point is close to the wall temperature. |
| Both, with poly on the room side of the studs | Fiberglass against the concrete with poly stapled over the studs puts the cold, damp concrete and the batts in one sealed pocket that cannot dry either way. | Often found when a finished basement is opened after a few years. |
The second failure is the dew-point effect explained in our guide to what humidity a basement should be: a room reading that looks fine can mean near-saturated air at the coldest surface. Plastic on concrete creates exactly that cold surface, and hides it behind drywall.
Fiberglass and wood against bare concrete fail for a related reason. BSC's basement renovation report says moisture-sensitive materials such as wood, paper and fiberglass batts should not touch concrete or masonry walls. Foam against the concrete is what keeps them off it.
The better assembly
What to put on basement walls instead
This is the finished-wall assembly Building Science Corporation and DOE guidance describe. It is general guidance for a wall that is damp at most, not leaking. Your local code, fire rules and climate zone set the details.
- Deal with liquid water first. If the wall is visibly wet at times or water runs onto the floor, BSC's guidance is to install drainage before insulating. See the section on waterproofing below.
- Rigid foam directly against the concrete. BSC's renovation report suggests about 1 to 2 inches of extruded polystyrene (XPS), depending on climate and whether you add insulation in a stud wall. Unfaced EPS and closed-cell spray foam are the other common choices.
- Seal every seam and edge. Tape the joints or use mesh and mastic, and seal the top and bottom edges so room air cannot get behind the foam to the concrete.
- Deal with the rim joist. It is a cold spot in both summer and winter. BSC recommends air-impermeable insulation there, such as foam board sealed in place or spray foam.
- Frame in front of the foam with a capillary break (a sill gasket or foam strip) under the bottom plate so the wood doesn't wick moisture from the slab. The cavity can take unfaced fiberglass or mineral wool if you want more R-value.
- No plastic sheet anywhere in the wall. Use unfaced batts, not kraft- or foil-faced ones.
- Cover the foam with an approved thermal barrier, typically half-inch gypsum board; foam insulation is flammable. BSC suggests paperless gypsum below grade. Finish with latex paint so the wall can dry toward the room.
| Foam option against concrete | BSC's guidance on permeance | Notes |
|---|---|---|
| Unfaced XPS board | Up to about 2 in. (R-10) stays semi-permeable enough to allow inward drying | The most common DIY choice; tolerates damp; must be covered with a thermal barrier. |
| Unfaced EPS board | Up to about 4 in. (R-15) | More vapor-open than XPS at the same thickness; lower R per inch. |
| Closed-cell spray foam | About 3 in. (R-18) medium density | Fills irregular walls and seals air leaks in one step; professional install; BSC calls interior spray foam the least risky approach. |
| Foil-faced polyiso or foil-faced XPS | Class I at the foil | BSC says foil-faced boards can be used in certain situations but need careful attention to other moisture control; get the design checked. |
From BSC Information Sheet 511 and BSD-103. Thicknesses are BSC's examples of foam that remains semi-permeable, not code minimums; your climate zone sets required R-values.
Decide
Do you need a vapor barrier? Decision table by situation
| Your situation | Vapor barrier? | What usually makes sense | Watch out for |
|---|---|---|---|
| Finishing a basement wall that is dry or only slightly damp | No plastic sheet | Rigid foam or closed-cell spray foam against the concrete, seams sealed, then framing and drywall. | Poly or faced batts in the stud wall; wood or fiberglass touching concrete. |
| Bare, unfinished wall you just want drier | No | Control humidity and fix outside drainage. Plastic taped to the wall only hides moisture. | Condensation on bare walls in summer; see the humidity guide. |
| Wall is wet after rain, has a tide line, efflorescence or active seepage | No, fix the water first | Find the source, then drainage or waterproofing. Start with water coming through the wall. | Any finish or sheet over a leaking wall traps the water and hides the leak. |
| Interior waterproofing with a wall membrane | Not a vapor barrier; a drainage liner | A dimpled or plastic liner that sends seepage down into an interior drain. See interior waterproofing. | It must connect to a working drain and sump, and it doesn't replace insulation if you finish the wall. |
| New slab being poured | Yes, under the slab | Code normally calls for a vapor retarder under the slab; BSC puts it over any sub-slab foam, in direct contact with the concrete. | A sand layer between the poly and the concrete can hold water. |
| Existing slab, installing flooring | Often yes, under the flooring | Test the slab, then follow the flooring maker's underlayment instructions, or build a sealed foam-and-subfloor system. | Impermeable flooring glued to a damp slab; carpet pad on a cold slab in summer. |
| Above-grade part of a walk-out or partial wall | It depends on climate zone | Framed walls above grade follow the IRC's climate-zone rules for vapor retarders. | Mixing below-grade and above-grade details on the same wall; get local advice. |
General guidance. Local code, your climate zone and an on-site look at the wall decide what is right for your house.
Not sure whether the wall is wet from inside or outside? The plastic-sheet test takes 24 to 48 hours and shows which side the moisture is on.
Floors
Vapor barrier on a basement floor
Floors are the one place where a sheet vapor barrier is standard. A slab sits on the ground and can only dry upward, so a low-perm layer between the ground and the concrete keeps soil moisture from rising into the slab and the finishes on top of it.
| Case | Typical approach | Why |
|---|---|---|
| New slab | Poly (commonly 6 to 10 mil) over gravel, and over any rigid foam, directly under the concrete. | DOE guidance calls for a capillary break and vapor retarder under the slab; BSC says the poly should touch the slab, with no sand layer between. |
| Existing slab, adding insulation | Seal cracks, lay taped 6-mil poly or a dimpled plastic mat, then rigid XPS or EPS with taped seams, then a floating subfloor not fastened through the foam. | DOE's steps for insulating an existing slab; the foam warms the floor surface so summer condensation is less likely. |
| Existing slab, floating floor (laminate, engineered wood, some vinyl plank) | A poly sheet or vapor-retarding underlayment, per the flooring maker. | Protects the flooring from slab moisture; the maker's warranty usually specifies it. |
| Existing slab, carpet or glued flooring | Test the slab first; many products set moisture limits for the concrete. | BSC warns against impermeable floor finishes such as vinyl sheet on slabs that must dry upward. |
| Slab wet from below, water on the floor | No sheet yet. Find and fix the water. | See water coming through the floor and hydrostatic pressure. |
Cracks in the slab are a separate question; a hairline shrinkage crack is common and is not a reason by itself to skip flooring. Our basement floor cracks guide covers which ones matter.
Important
A vapor barrier is not waterproofing
Vapor barriers deal with water vapor, the invisible moisture in air and in damp concrete. They do nothing about liquid water pushed through cracks, the wall-floor joint or porous block by saturated soil. Covering a leaking wall with plastic, foam or drywall does not stop the water; it stops you seeing it until the finishes are ruined.
| What you see | What it usually means | Where to go |
|---|---|---|
| Water or damp patches after rain | Surface water or groundwater reaching the wall | Water in basement after rain |
| White powder on the wall | Water moving through the masonry and leaving salts | Basement efflorescence |
| Water along the base of the wall | Groundwater at the cove joint | Water at the wall-floor joint |
| Leaking crack | A path for water through the wall | Basement wall cracks leaking |
| Musty smell with no visible water | Humidity, condensation, or hidden wet materials | Musty basement smell |
Waterproof paints and sealers sit between the two: they can slow some vapor and damp, but they are not a fix for water under pressure. The comparison of waterproof paint vs a drainage system explains the limits, and interior vs exterior waterproofing covers the real fixes.
Code
What the building code says, and why climate zone matters
The International Residential Code handles vapor retarders in section R702.7. In the 2021 edition it calls for a Class I or II vapor retarder on the interior side of frame walls in climate zones 5, 6, 7, 8 and Marine 4, and restricts the lower-perm classes in warmer zones. It then lists exceptions, including basement walls and the below-grade portion of any wall.
So the model code does not require a vapor barrier on a basement wall, and the building science guidance above recommends against a plastic one. Three things can still change the answer for your house:
- Local amendments. States and cities adopt different IRC editions and amend them. Your building department decides, and a finishing permit is where this gets checked.
- Above-grade wall sections. On a walk-out or a partly exposed wall, the part above ground can fall under the climate-zone rule.
- Energy code insulation levels. Required basement wall R-values vary by climate zone and are set by the energy code, which can affect how thick the foam needs to be.
If a contractor or inspector asks for poly on a basement wall, ask which code section they are applying. It may be a local amendment or a misreading of the above-grade rule, and it is worth settling before the drywall goes up.
Avoid these
Common basement vapor barrier mistakes
- Stapling 6-mil poly over a stud wall filled with fiberglass, the most common failed assembly in finished basements.
- Using kraft- or foil-faced batts in the stud wall, which adds a vapor retarder on the wrong side.
- Leaving foam seams, top edges or outlet boxes unsealed, so room air reaches the cold concrete anyway.
- Skipping the rim joist, which is often the coldest and dampest spot in the basement.
- Finishing over a wall that is wet after rain, trusting the plastic or foam to keep water out.
- Leaving exposed foam without an approved thermal barrier.
- Treating a vapor barrier as the fix for high humidity. A ventilation or dehumidification plan deals with the air; the wall assembly deals with surfaces.
Related pages
Where to go next
What Humidity Should a Basement Be?
Targets, dew point and the plastic-sheet test.
Read nextBasement Ventilation
When fans help, when they add moisture, and the backdraft risk.
Read nextWater Coming Through the Basement Wall
What to do before you finish over a damp wall.
Read nextInterior Basement Waterproofing
Drainage liners, perimeter drains and sump pumps.
Read nextMusty Basement Smell
Often the first sign of a trapped-moisture wall.
Read nextBasement inspection checklist
What to record before finishing or getting quotes.
Read nextFAQs
Basement vapor barrier FAQs
Do I need a vapor barrier on my basement walls?
In most cases, no, and the model building code agrees: the IRC's vapor retarder rule (R702.7) lists basement walls and the below-grade portion of any wall as exceptions. What a finished basement wall does need is insulation that keeps humid room air off the cold concrete, such as rigid foam board with sealed seams, and no plastic sheet on the room side. Your local code and climate zone govern, so check with your building department before you finish.
When should you not use a vapor barrier?
Don't put a Class I vapor barrier, such as 6-mil polyethylene or foil, on the inside of fiberglass or other air-permeable insulation in a basement or crawl space wall. The Department of Energy's building science guidance says it can trap water in the wall with no way to dry. Also don't use one to fix a leak: if liquid water is getting through the wall, a sheet only hides it.
What is the code requirement for a vapor barrier in a basement?
Under the 2021 International Residential Code, section R702.7 requires a Class I or II vapor retarder on the inside of frame walls in climate zones 5, 6, 7, 8 and Marine 4, but it lists basement walls and the below-grade portion of any wall as exceptions. Floors are covered separately: a vapor retarder is normally required under a new slab. Local amendments vary, so your building department has the final word.
Is a vapor barrier recommended for a basement concrete wall?
Not as a plastic sheet on the inside. Building Science Corporation says no interior vapor barrier should be installed, so the wall can dry inward. Rigid foam insulation against the concrete, with sealed seams, gives the wall a warm, moisture-tolerant inner surface and still lets it dry slowly. Unfaced XPS up to about 2 inches or EPS up to about 4 inches are among the options BSC lists.
What is the best type of vapor barrier for a basement wall?
For a finished basement wall, the approach building scientists recommend is not a thin sheet but a foam insulation layer against the concrete: unfaced XPS or EPS board, or closed-cell spray foam, with seams taped or sealed. These are semi-permeable, so the wall can still dry. Foil-faced foam can work in some cases but needs extra care. We don't rank brands; choose by type and follow the manufacturer's instructions and local code.
What is the difference between a vapor barrier and a vapor retarder?
A vapor retarder is any material that slows water vapor; codes sort them into Class I (0.1 perm or less, such as polyethylene or foil), Class II (over 0.1 up to 1.0 perm, such as kraft-faced batts or vapor retarder paint) and Class III (over 1.0 up to 10 perms, such as latex paint). People usually mean a Class I material when they say vapor barrier.
Should I put plastic behind drywall in a basement?
Generally no. Plastic between the drywall and a stud wall full of fiberglass in a basement stops the wall from drying toward the room, and moisture from the concrete or from room air that leaks in can collect in the cavity, leading to mold and odor. Keep the plastic out and use foam against the concrete instead.
Do I need a vapor barrier under flooring in a basement?
Often yes for floating floors: many flooring makers call for a polyethylene underlayment or a vapor-retarding pad over concrete, so follow their instructions. That is a different situation from walls, because the slab dries mainly upward and the sheet protects the flooring. Test the slab first; a slab that is wet from below needs the water problem fixed, not just a sheet.
Sources
Sources
- Building Science Corporation — Information Sheet 511: Basement Insulation for All Climates (foam types and thicknesses; no interior vapor barriers; slab poly)
- Building Science Corporation — BSD-103: Understanding Basements
- Building Science Corporation — BA-0309: Renovating Your Basement (XPS thickness, sealed seams, thermal barrier, drainage first)
- US DOE Building Science Education — No vapor retarder on the interior side of air-permeable foundation insulation
- US DOE Building Science Education — Understanding vapor retarders for basements
- US DOE Building Science Education — Insulating existing slabs
- Continuous Insulation — Summary of 2021 IRC R702.7 vapor retarder requirements (class definitions, basement wall exception)
- Green Building Advisor — Vapor retarder for basement walls (Q&A)
This page is educational guidance for homeowners. It is not a wall design, a code interpretation for your jurisdiction, or a substitute for an on-site inspection. Local code and your climate zone govern.