Best Insulation for Concrete Basement Walls Ontario: Foam First, the Gap, the Poly Question

Insulation & Air Sealing

Concrete Basement Walls: Put the Foam Against the Concrete, Keep the Wood Off It, and Stop Arguing About the Poly

A concrete wall below grade is cold, it is damp on the outside face, and it stays that way in July when the basement air is warm and wet. Anything fibrous pressed against it ends up holding water on the concrete side, and a sheet of poly over the top of that just keeps the water in. So the rule I build to and the rule I tell people over the phone: foam goes against the concrete, continuous and taped; the wood stays off the wall on a foam sill with a gap; the batt, if there is one, sits behind the foam where it is warm; and there is no poly over any of it. That gets a poured wall to the code R-20, full height, without a mould line. An ICF basement skips the whole page because the foam is already there.

Poured and block walls in Ontario, finished basements, the OBC 2024 and SB-12 as in force in October 2026. Rates before HST where quoted.

Why a batt against concrete fails

Concrete below grade sits at about the soil temperature, 8 to 12 C most of the year. In January that is warmer than outside and the wall is the warm side of nothing, so vapour from the house moves outward through the batt, meets the cold concrete and condenses. In July the basement air is 22 C and 60% humidity, with a dew point around 14 C, and the same 12 C concrete is now colder than the dew point. The batt against it is wet on the concrete face again, from the other direction. Fibreglass does not stop air or vapour any more than a sweater does, so the water arrives either way.

Put 6 mil poly over the batt, the way the 1990s basement finishing packages did, and you have trapped that water between the plastic and the concrete with a fibreglass sponge to hold it. The bottom plate rots first, then the drywall at the floor goes soft, and the smell is what finally gets someone to open the wall. Every mouldy basement I have been asked to look at had batts, poly and a stud wall tight to the concrete. The vapour barrier problems page has the whole post-mortem.

The foam-first rule

Foam against the concrete fixes both directions at once. Two inches of XPS or three inches of closed cell is below the OBC line for a vapour barrier, under 60 ng/(Pa.s.m2) or about 1 perm, so vapour from the room cannot reach the concrete in winter. The inside face of the foam sits within a degree or two of room temperature, so there is no cold surface for summer humidity to find. The concrete is free to stay damp behind the foam, which it will, and nothing organic is touching it.

Two ways to do it. Rigid board, 2 in. XPS or Type II EPS at $3.50 a square foot, glued and joints taped, with the top edge foamed to the sill plate and the bottom foamed at the slab. Or closed cell spray foam, 2 in. at $3.00 or 3 in. at $4.25, which also seals the cold joint at the top of the wall, every form tie, every crack and the rim joist in the same visit. The spray foam is the better air seal; the board is the one a carpenter can do without a truck. Both count as the vapour retarder and both are continuous over the concrete, which is why a wall with R-10 of foam and an R-14 batt behind it is R-21 effective and a 2×6 wall with an R-22 batt alone is not. Spray foam basics covers the material; open cell does not go below grade.

Rule of thumb: at least R-10 of foam against the concrete, taped or sprayed continuous, before any wood or batt goes near the wall. Less than that and the inside face of the foam can still hit the summer dew point behind the batt.

The gap

The stud wall does not touch the concrete. It stands on a bottom plate set on a foam sill gasket, with the plate held off the slab edge and the studs 1/2 to 1 in. clear of the foam face. The gap is not for drying; the foam has done that job. It is so that a wet spring or a slab crack does not wick straight into the bottom plate, and so a bump in the concrete does not push the wall out of plumb. A pressure-treated bottom plate is cheap insurance on a slab that has ever been wet.

At 24 in. on centre a 2×4 wall has fewer studs and more room for the batt, and the drywall spans 24 in. fine at 1/2 in. thickness. The $4.50 a square foot stud-wall rate on the cost page includes the sill gasket, the gap and the blocking; a quote that frames tight to the wall is not cheaper, just worse.

The poly question, answered once

OBC 9.25.4 asks for a vapour barrier on the warm side of the insulation. On a frame wall above grade that is 6 mil poly behind the drywall and nobody argues. Below grade, the foam against the concrete already is the vapour barrier at 2 in. of XPS or closed cell, so a second sheet of poly on the room side makes a wall with a vapour barrier on both faces and no way out for any water that gets in. That is the double vapour barrier, and the inspector who insists on poly over foam has not read 9.25.4 all the way through: a material with the right permeance in the right place is the vapour barrier, whatever it is made of.

So: foam assembly, no poly. Batt-only assembly, poly on the warm side, and a dry wall, a real gap and a prayer. If a finisher will only work with poly, a smart variable-perm membrane at $0.90 a square foot opens up in summer and lets the wall dry inward, which is the one case where a sheet on the room side below grade makes sense.

What to write on the quote

The wall is dry before anything goes on it. A taped poly square on the concrete for 48 hours, no efflorescence line, no active crack. Written as a line, not assumed.
2 in. XPS or Type II EPS, glued, joints taped, top and bottom edges foamed, full height from the underside of the subfloor to the slab; or 3 in. closed cell CAN/ULC S705.1 foam by a CUFCA-certified installer with the daily work sheet showing thickness and density.
Rim joist and top of wall done in the same assembly, 2 in. closed cell or cut foam board sealed at the edges, $5.50 a linear foot. The rim joist page.
2×4 wall at 24 in. o.c. on a sill gasket, bottom plate pressure-treated, 1/2 to 1 in. off the foam, blocked for the drywall.
R-14 batt in the stud bays, no poly, or no batt and the thicker foam. Either way the words “no polyethylene over foam” on the quote.
1/2 in. drywall over everything as the thermal barrier over the foam, OBC 9.10.17, before occupancy. Spray foam left bare in a finished basement is not finished.
The number: R-20 effective, full height, which is what the SB-12 form and the permit drawing will say. Not “R-20 batts”.

Block walls get the same list with one change: the top course and every open core get foamed first, because a block wall breathes through its cores like a chimney and no amount of batt stops that.

And if the basement is ICF

None of this. The ICF wall is 2.5 in. of EPS on each face of the concrete, R-22 and up effective, continuous, with the foam already doing the vapour job on both sides. No stud wall, no batt, no poly, no gap; the drywall screws to the plastic webs at 6 or 8 in. on centre. I have built basements this way since 1995; the finisher always asks where the poly goes, and the answer is the ceiling under the roof, not the wall. The ICF R-value page has the numbers and the ICF vs conventional basement page prices the two side by side.

Questions people ask

What is the best insulation for concrete basement walls?

Foam against the concrete: 2 in. of XPS or EPS board taped, or 2 to 3 in. of closed cell spray foam, continuous and full height. A stud wall off the foam with a gap, an R-14 batt behind the foam if you want the extra R, drywall over everything, no poly. That reaches the R-20 effective the code wants and keeps every organic material away from the concrete.

Can I use fibreglass batts in a basement?

Behind foam, yes; the foam keeps them warm and dry. Alone against concrete with poly over them, they get wet from both directions and grow mould. If a batt has to do the whole job, a 2×6 wall with a real gap and mineral wool is the least bad version, and it is still not what I would build.

Do I need a vapour barrier on a basement wall in Ontario?

You need a vapour retarder on the warm side, and 2 in. of foam against the concrete is one. Adding poly over foam makes a double vapour barrier that traps water. Poly is only for the batt-only assembly, and a smart membrane is the better choice there.

How thick should the foam be?

R-10 minimum against the concrete, which is 2 in. of XPS or about 1.75 in. of closed cell; 3 in. of closed cell at R-18 lets you skip the batt. In the code packages that ask for R-12 plus R-10 continuous, the 2 in. foam is the R-10 continuous.

Price the foam layer on your walls

The spray foam cost calculator prices 2 or 3 in. of closed cell on your wall area and the rim together, with the masking and the minimum charge shown. If a quote you already have says “insulated to code” and lists batts and poly, the $99 review will tell you what it is actually describing.

Price foam on my basement walls Review my quotes, $99

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.
Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.

Best Insulation for Concrete Basement Walls in Ontario: Why Batts Against Concrete Fail, the Foam-First Rule, the Gap, the Poly Question and What to Write on the Quote