Last Updated on August 24, 2026 by Umar Farooq
Searching this question gets you a lot of instructions for stapling polythene to basement walls. Building science says that assembly traps moisture and rots the framing behind it.
Both things are true at once: vapour barriers are correct and code-required in some basement locations, and damaging in others. It comes down to which direction the assembly can dry.
Code references here are the US International Residential Code. UK practice differs — damp-proof membranes and BS 8102 tanking — so do not read these as UK requirements.
The short version
| Location | Vapour barrier? | Why |
|---|---|---|
| Under a new concrete slab | Yes. Required. | The wet side is the soil. The slab dries upward into the room. The barrier sits on the outside of the assembly. |
| Over crawlspace soil | Yes. Required if unvented. | Same logic. It sits between wet ground and the space, with nothing behind it that can rot. |
| On the interior face of a basement wall | No. | A buried wall cannot dry outward into wet soil. Inward is the only drying path it has, and this closes it. |
The fact that settles it
The building code does require an interior vapour retarder on framed walls in colder climates. That requirement is where the “vapour barrier goes on the warm side” rule comes from, and it is correct for an above-grade wall that dries outward.
The same code section then lists its exceptions. Two of them are:
So the code that people cite when they staple polythene to a basement wall specifically exempts basement walls from that requirement. This is checkable in a few minutes, and it appears on none of the how-to pages currently ranking for this.
Note also that the exemption is not climate-dependent. The climate-zone rules apply to above-grade frame walls. Basement walls are exempt in every zone.
Why the wall assembly fails
An above-grade wall has two possible drying directions. A basement wall has one. It is buried against soil that sits at essentially 100% relative humidity all year, so it cannot dry outward. Everything it takes on has to leave through the inside face.
Put a sheet of polythene there and the moisture arrives, condenses on the plastic, and has nowhere to go. Department of Energy guidance describes the outcome directly: water vapour condenses on the sheeting, cannot dry to the interior, and accumulates in the wall cavity, giving ruined insulation, mould, odours and structural rotting of framing members.
Building Science Corporation is equally plain about it, and their sentence is the one to remember: a plastic vapour barrier prevents inward drying, and the common outcome is odour, mould, decay and corrosion problems.
What “vapour barrier” actually means
The word gets used for anything plastic. Code classifies by permeance, and the class is what matters.
| Class | Perm rating | Typical material |
|---|---|---|
| Class I | 0.1 or less | Sheet polythene, unperforated aluminium foil. This is the one that causes the problem on a basement wall. |
| Class II | Over 0.1, up to 1.0 | Kraft-faced fibreglass batts, vapour retarder paint |
| Class III | Over 1.0, up to 10.0 | Latex or enamel paint |
Under a slab: the specification
This is the case where a Class I barrier is exactly right, and where the code is specific.
- 10 mil minimum, meeting ASTM E1745 Class A. The 2021 code raised this from 6 mil, so older guidance you find will be out of date.
- Joints lapped at least 6 inches.
- Placed between the slab and the base course, or the prepared subgrade where there is no base course.
- In direct contact with the slab. Building science guidance is explicit that a sand layer should never be installed between the polythene and the concrete, which contradicts older industry advice about blotter layers.
- Layer order from the bottom: crushed stone, rigid foam, polythene, concrete.
One thing worth knowing about the ASTM classes, because product marketing implies otherwise: Class A, B and C all share the same 0.1 perm ceiling. They differ in tensile strength and puncture resistance. Class A is specified because it survives trades walking on it before the pour, not because it is less permeable.
Crawlspace ground cover: the specification
Also correct, also code-required for an unvented crawlspace.
- Continuous Class I vapour retarder over all exposed earth.
- Joints overlapped 6 inches, sealed or taped.
- Edges extending at least 6 inches up the stem wall, attached and sealed to the wall or insulation.
- Plus one moisture-control method: continuous mechanical exhaust, a conditioned air supply, a plenum, or dehumidification.
That last requirement gets left out of a lot of encapsulation descriptions. A sealed crawlspace without a conditioning method is a half-built system.
On thickness, code specifies a class rather than a mil count. Ten or twenty mil reinforced sheet is common in practice, for puncture resistance rather than permeance, because people crawl on it.
What to do on a basement wall instead
You still want insulation and you still want to stop humid interior air reaching cold concrete. You do it with rigid foam rather than plastic.
- Rigid foam directly against the concrete, covering it completely. The point is to keep interior air off the cold condensing surface.
- Seal the seams and the joints to other materials. The foam is your air barrier. It is not meant to be a vapour barrier.
- Use unfaced foam, or strip the facing. Facings push the assembly toward Class I, which is what you are avoiding.
- Respect the thickness limits that still allow inward drying: up to 2 inches unfaced XPS, 4 inches EPS, 3 inches closed-cell spray foam, 10 inches open-cell.
- Frame in front if you want a finished wall, with unfaced batts or damp-spray cellulose in the cavities. No polythene anywhere.
- Hold the drywall about half an inch off the slab as a moisture break.
Two things not to do. Do not put fibreglass batts against concrete. And do not leave an air gap between a framed wall and the concrete so it can “breathe” — that ventilates humid interior air onto a cold surface, which is the opposite of the goal.
If you already installed poly on the walls
This is the situation with no good answer anywhere on the internet, so here is an honest one.
The failure is slow and it announces itself before it becomes structural. Look for a musty smell with no visible source, drywall that feels soft near the bottom, rust on fastener heads, or staining tracking the framing.
If any of that is present, open a section at the bottom of the wall and look. Wet insulation, black staining on the back of the drywall, or damp framing means the assembly is doing what the guidance predicts.
If it is dry, you have time, and you can plan the rebuild rather than rush it. If it is wet, take the poly out. Leaving it in place while adding ventilation does not solve it, because the trapped side is still trapped.
A second reason for the slab barrier
Under-slab polythene also acts as a radon retarder. In some jurisdictions sub-slab membrane is mandated partly on that basis. If you are pouring a slab anyway, that is a free second benefit, and worth knowing because radon is the one basement air risk with a quantified death toll.
FAQ
Everyone tells me to put plastic on my basement walls. Are they all wrong?
The advice is a correct rule applied to the wrong wall. Interior vapour barriers make sense on above-grade walls that dry outward. A buried wall cannot do that, which is why the code exempts basement walls from the requirement.
What about a dimple membrane against the wall?
Different product, different job. A dimpled drainage membrane creates a drainage and air gap and is used with a drainage system, usually as part of a designed assembly. It is not the same as stapling poly to studs.
Can I use faced rigid foam?
Prefer unfaced, or remove the facing. A foil facing turns the foam into a Class I layer and closes the drying path you are trying to keep open.
Do I need a vapour barrier if my basement stays dry?
Not on the walls. A dry basement is evidence the current assembly is working, and adding a Class I layer to the inside is how dry basements become damp ones.
My slab has no vapour barrier under it. What can I do?
You cannot retrofit one under an existing slab without removing it. Manage it from above instead: keep humidity controlled, avoid impermeable floor coverings that trap moisture in the slab, and expect the slab to keep releasing moisture indefinitely.
