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Basement Vapour Barrier Problems in Ontario: Wet Batts Behind the Poly, the Double Vapour Barrier, Where Poly Goes and Does Not Go Below Grade, and What the Fix Costs
Insulation & Air Sealing
The Basement Vapour Barrier: Poly Was Put There to Keep the Wall Dry, and It Is the Reason the Wall Is Wet
Pull the drywall off a finished basement from the 1990s or 2000s and you will see the same thing in most of them: 6 mil poly over fibreglass batts, a stud wall tight to the concrete, and behind the poly, at the bottom, a band of black. The poly did exactly what it was told to do, which was to stop water vapour from moving. The trouble is that below grade the vapour moves inward in summer and outward in winter, and a sheet that stops it on one side traps it on the other. This page is the post-mortem, the two rules that come out of it, and what it costs to open the wall and build it back properly.
How the wet batt happens, season by season
Winter. The living room is 21 C and dry. The concrete below grade is about 8 to 10 C. Vapour moves from warm to cold, so what little gets through the poly, and poly is never perfectly sealed at the plates and the boxes, goes outward through the batt and condenses on the concrete. It freezes against the top two feet of wall above the frost line. Not much, but it is water in the batt and it is not leaving.
Spring. The frost in the wall melts and runs down inside the batt to the bottom plate. The weeping tile is at its busiest and the concrete is as damp as it will be all year. The batt at the bottom is now wet from two directions.
Summer. The basement air is 22 C at 60% humidity, dew point about 14 C. The concrete is 12 C. Any air that reaches the concrete, and a stud wall tight to the wall is full of it, leaves its water on the concrete face of the batt. The poly is now on the cold side of the wall: the warm humid air is inside the room, the cold surface is the concrete, and the poly is between them, holding the water in. Nothing dries. By the third summer the bottom plate is soft and the drywall at the floor has a tide line.
Fibreglass does not stop any of this, because fibreglass is not an air barrier or a vapour barrier; it is a filter that holds water beautifully. The concrete wall page is what to build instead; this one is what went wrong.
The double vapour barrier
The rule every wall has to obey: it must be able to dry in at least one direction. A frame wall above grade dries outward through the house wrap, so the poly on the inside is fine. A basement wall cannot dry outward; the concrete is wet and the soil is wetter. It has to dry inward, into the room, which means the inside face of the assembly has to let vapour through. Poly does not. So a batt wall with poly below grade is a wall with no exit, before anything else goes wrong.
Now add foam. A contractor who learned that foam belongs against the concrete, and then puts poly over the batt anyway because the inspector always wanted it or the drywaller expects it, has built a double vapour barrier: 2 in. of XPS at about 1 perm on the concrete side and 6 mil poly at a fraction of a perm on the room side. Anything between them, the batt, the studs, the plate, is sealed in a bag. One plumbing leak, one wet spring, one humid summer with the window open, and the bag has water in it for good. The OBC in 9.25.4 asks for one vapour barrier on the warm side; 2 in. of foam against the concrete is it, and the wording does not say polyethylene anywhere. The three layers page has the permeance numbers for each material.
The two rules: below grade, foam against the concrete is the vapour barrier and nothing fibrous touches the wall; and there is never a second low-perm layer on the room side of foam. Poly over foam is wrong in every basement, every time.
Where poly goes below grade, and where it does not
| Location | Poly? | Why |
|---|---|---|
| Over foam on a concrete wall, Assembly A or B | No | The foam is the vapour barrier. Poly makes it a double. |
| Over a batt-only wall, Assembly C | Yes, or a smart membrane | Without foam, something has to stop winter vapour reaching the concrete. The smart membrane at $0.90 a square foot opens up in summer so the wall can dry inward; 6 mil poly at $0.50 does not. The smart membrane is the better $0.40. |
| Under the slab, before the pour | Yes, 6 mil, under or over the under-slab foam | Stops ground moisture and is the radon barrier the OBC asks for. The slab page. |
| The above-grade frame wall of a walkout | Yes, on the warm side | That wall is an ordinary frame wall and gets ordinary poly behind the drywall. |
| The ceiling of the basement, under the floor above | No | Both sides are inside the house. Poly between two heated floors does nothing but trap a leak. |
| An ICF wall, anywhere | No | The foam on both faces is the vapour retarder. The only poly in an ICF house I build is on the ceiling under the roof. |
Finding out whether your wall has the problem
- The smell. Musty in July, gone in January, back in July. That is a wall drying into the room every summer because it cannot dry anywhere else.
- The tide line. Drywall discoloured or soft in the bottom 6 in., baseboard swollen, paint bubbling at the floor. Worst on the north and east walls and in corners.
- A moisture meter on the drywall at the floor, with a pin meter through a baseboard nail hole. Over 18% in the plate is a wet plate.
- One cut. A 6 by 6 in. hole behind a baseboard or in a closet, low on an outside wall. If the batt is grey or black at the bottom and the plate is dark, you have your answer in five minutes. Photograph it with a ruler.
- Do not assume a leak. Most wet basement walls with this assembly have no crack and no leak; the water came from the air. The leak or condensation page separates the two, and the mould page says what the black is and what to do about it.
What the fix costs
There is no fix from the room side that leaves the batt in place. The wall comes open, the wet insulation comes out and goes into bags, not under new insulation, and the framing is dried and checked. A soft plate or stud is replaced; black surface staining on sound wood is cleaned; a wall that is structurally rotted at the plate is rebuilt. Then it goes back as Assembly A or B from the cost page: 2 in. XPS or 3 in. of closed cell against the concrete, the stud wall reset on a sill gasket off the wall, an R-14 batt or none, no poly, drywall.
On a 12 ft section of one wall, 8 ft high, 96 sq ft: strip and bag, about two hours; reframe if needed; foam board 96 sq ft at $3.50, $336, or closed cell at the minimum charge if the crew is coming for this alone, which is why you do the whole wall or the whole basement at once; new batt $106; drywall and paint on top. A whole 984 sq ft basement redone in Assembly A is the same $9,446 as a new one plus the strip-out and whatever the framing needs, and the strip-out on a dry-ish wall is a day for two men. The thing that makes it expensive is waiting: the plate that is soft this year is the stud that is gone in three.
Questions people ask
Do you need a vapour barrier in a basement in Ontario?
You need a vapour retarder on the warm side of the insulation under OBC 9.25.4. Below grade, 2 in. of rigid foam or closed cell against the concrete is that layer. Poly is only for a batt-only wall, and a smart membrane does that job better.
Should I put poly over foam board in the basement?
No. Foam against the concrete is already the vapour barrier; poly over it makes a double vapour barrier that seals the studs and batt in a bag with no way to dry. Drywall goes straight over the stud wall.
Why is the insulation behind my basement poly wet?
Because the concrete is colder than the summer dew point and the batt against it collects condensation from the room air, while the poly keeps it from drying inward. In winter a little vapour goes the other way and freezes in the top of the wall. It is the assembly, not a leak, most of the time.
Can I just cut slits in the poly to let the wall dry?
No. A slit lets more warm air reach the cold concrete and makes more condensation; it does not make the batt stop holding water. The wall has to come open and the batt has to come out.
What is a smart vapour retarder?
A membrane that is tight to vapour when the air around it is dry, in winter, and opens up when it is humid, in summer, so the wall can dry inward. $0.90 a square foot against $0.50 for poly. It belongs over a batt-only basement wall if someone insists on batts; it is not needed over foam.
Price the rebuild in foam
The spray foam cost calculator prices closed cell against your wall area so you can see what the wall costs done right, and whether one wall or the whole basement clears the minimum charge. If you have a quote to “replace the vapour barrier” that lists poly and batts again, the $99 review will say what you are about to pay to rebuild the same problem.
Sources and further reading
- Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
- Building Science Corporation: the cold-climate basement, wall and roof assembly guidance that explains where the condensation goes and why foam goes against the concrete.
- Addressing moisture and mould in your home: Health Canada: what to do about wet insulation and mould before anything gets covered up.
