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Basement Floor and Slab Insulation in Ontario: Under-Slab Foam Before the Pour, Over-Slab Options for an Existing Basement, R-10 Under a Heated Floor, Radon and What It Costs
Insulation & Air Sealing
Under the Slab: Two Dollars a Square Foot the Day Before the Pour, or a Floor That Is Cold for the Life of the House
A basement slab sits on ground that is 8 to 12 C all year, and concrete conducts heat about as well as a cast iron pan. Without foam under it, the slab is the ground temperature, the floor is cold in slippers in August, and in a house with a heated floor you are paying to warm the gravel under the house. Two inches of Type II EPS over compacted stone, with 6 mil poly, costs $1.75 to $2.25 a square foot the day before the pour, call it $2,000 on a 1,000 sq ft slab. The same R-value after the slab is in costs the headroom of the basement and three times the money. This is the one insulation decision in a house that cannot be fixed later at any reasonable price, so it is worth getting right on the drawings.
The numbers
$2.002 in. Type II EPS, R-8, with 6 mil poly, per sq ft, before the pour
R-10under a heated slab, the SB-12 number
$2.751.5 in. XPS over an existing slab, per sq ft, before the subfloor
2.25 in.headroom lost to foam and plywood over an existing slab
| Option | Per sq ft | R | Notes |
|---|---|---|---|
| Under-slab, 2 in. Type II EPS + 6 mil poly, new pour | $1.75 to $2.25, default $2.00 | R-8 | Over 6 in. of compacted clear stone. Type II because Type I crushes under a slab. The default for every basement I pour. |
| Under-slab, R-10 under a heated slab | about $2.25 | R-10 | 2.5 in. of EPS or 2 in. of XPS. Required by SB-12 under a heated slab, and the right number anyway. |
| Slab edge, 2 in. vertical at the wall | in the wall foam | R-10 | Where the slab meets the foundation is where it loses the most. The wall foam runs to the footing or the under-slab foam turns up. |
| Over existing slab, 1.5 in. XPS taped + 5/8 in. plywood | $2.75 + the subfloor | R-7.5 | Loses 2.25 in. of headroom. Stairs, doors and the furnace plenum all have to be looked at. |
| Over existing slab, dimpled subfloor panel | see the flooring pages | R-1 to R-2 | A moisture break with a little air space. Warm feet, not insulation. |
| Radon rough-in under the slab, new build | in the slab price | Required under OBC 9.13.4: a sealed poly, a capped pipe through the slab into the stone, sealed penetrations. |
The subfloor and finished flooring over any of this is on best basement flooring; the slab rate here stops at the top of the foam.
Before the pour, which is the only cheap day
The order under a new slab, from the bottom: compacted clear stone, 6 in. or whatever the drawing says, which is also the radon collection layer; the radon pipe stubbed up through where the slab will be, capped; 2 in. of Type II EPS laid tight with the joints staggered; 6 mil poly over the foam, lapped and taped, run up the wall and sealed to it; then the slab. Some crews put the poly under the foam. Either is accepted; over the foam keeps the concrete water out of the joints and makes the poly the radon barrier in one layer, which is how I do it.
The slab edge is the detail that gets missed. A slab touching a cold foundation wall loses heat sideways into the wall and down into the footing, and the first 2 ft in from the wall is where a floor feels cold. The wall foam on an ICF basement already runs to the footing, so the slab is isolated by the form. On a poured wall, the 2 in. of foam against the concrete goes all the way to the footing before the slab is poured, or a strip of foam goes vertically at the slab edge; the wall page covers the wall foam that makes this work.
1,000 sq ft slab, unheated: 1,000 at $2.00, $2,000 before HST. The same slab with a radiant floor: R-10 at about $2.25, $2,250, plus the tubing and the mechanical, which is the radiant floor page. The extra $250 for R-10 over R-8 on a heated slab is the cheapest money on the whole job; without the foam, something like a third of the heat in the slab goes down.
R-10 with a heated floor, and why the code insists
SB-12 requires insulation under a heated slab, and R-10 is the number the common packages carry. The reason is arithmetic. An unheated slab sits at maybe 14 C and the ground at 10 C, four degrees of difference, not much heat moving. Heat the slab to 27 C for a radiant floor and the difference is 17 degrees, four times the flow, all of it into the gravel if there is no foam. I have been putting radiant floors in since the early 1990s and the first thing I check on someone else’s cold radiant basement is whether there is foam under it; often there is not, and there is nothing to be done except pay the gas bill.
The second reason is response. A heated slab on foam warms up in hours; one on bare ground spends the first day heating the earth under the house and never quite catches up. On an ICF house with a heated slab, the wall foam, the under-slab foam and the slab-edge isolation together are why the flooring over a heated ICF slab page can talk about a floor that holds 24 C on a 25 cent a day gas budget.
An existing slab: what you can do and what it costs you
- Measure the headroom first. The OBC wants 1.95 m (6 ft 5 in.) under beams and ducts in a finished basement, more under the joists. A 7 ft basement with 2.25 in. of foam and plywood on the floor is 6 ft 9.75 in. to the joists and less under the beam. The stairs lose a riser and the bottom door gets cut.
- Rigid foam and plywood. 1.5 in. XPS at $2.75, joints taped, 5/8 in. tongue-and-groove plywood screwed through to the slab with tapcons or floated. R-7.5, a warm floor, a moisture break. The best over-slab assembly, and the one that costs the most headroom.
- Dimpled subfloor panels. About an inch, an air gap and a thin foam or OSB layer. R-1 to R-2. It makes the floor feel warm by keeping the finish off the concrete; it does not insulate the slab. Fine under a rec room carpet, not an insulation line.
- Nothing under the floor, foam at the walls. For a low basement, the honest answer is to spend on the walls and the rim, where the heat loss is larger, and put a moisture-tolerant floor straight on the slab. The slab loses heat slowly to 10 C ground; the wall above grade loses it fast to minus 20 air.
- Do not put poly on top of an existing slab under a floor. Concrete needs to dry up through the floor or it does not dry at all; a vapour-open underlay or a dimpled panel, not plastic. The basement flooring page has what goes on the slab and why.
Radon, sealing and the slab edge
The under-slab poly is the radon barrier, and the OBC rough-in under 9.13.4 is a capped pipe into the stone layer plus sealed penetrations, so that a fan can be added if the house tests high. Sealing the slab edge and every pipe and sump penetration is part of the air sealing anyway; a 1/4 in. gap around the whole slab perimeter is a bigger hole than every window in the basement put together. On an existing slab the same sealing, with polyurethane caulk at the slab-to-wall joint and around the sump lid, is the cheapest radon and air-leakage work in the house. The OBC radon requirements page covers the rules and the test; do not take radon advice from an insulation page beyond this paragraph.
Questions people ask
Does a basement slab need insulation in Ontario?
Under a heated slab, yes, R-10, under SB-12. Under an unheated slab the common packages do not require it, and I put 2 in. of Type II EPS under every one anyway at $2.00 a square foot because it cannot be added later and the floor is never cold.
How much does under-slab insulation cost?
$1.75 to $2.25 a square foot installed with the 6 mil poly, before the pour; about $2,000 on a 1,000 sq ft basement, $2,250 for R-10 under a heated floor. Over an existing slab, 1.5 in. of XPS is $2.75 plus the plywood and 2.25 in. of headroom.
Can I insulate an existing basement floor?
Yes, with rigid foam and a plywood subfloor on top of the slab, if the headroom allows the 2.25 in. Dimpled subfloor panels give a warm feel but almost no R. For a low basement, spend the money on the walls instead.
Does the poly go above or below the under-slab foam?
Either is accepted in Ontario. Over the foam, lapped, taped and sealed to the wall, keeps the pour water out of the foam joints and makes the poly the radon barrier in one layer, which is how I do it.
Why does a heated slab need more foam than an unheated one?
Because the temperature difference between the slab and the ground is four times larger, so the heat loss downward is four times larger. R-10 under a 27 C slab is where the loss stops being most of the bill. SB-12 makes it a requirement for that reason.
Put the slab in the whole basement number
The spray foam cost calculator prices the walls and the rim, and the basement cost page adds the slab line to the four wall assemblies so the whole basement is one number. If a builder quote shows a slab with no foam line under it, the $99 review will ask the question before the truck does.
Sources and further reading
- Ontario’s Building Code, Supplementary Standard SB-12: the energy-efficiency packages that set the R-values for attics, walls, basements and slabs in a new or renovated Ontario house.
- Radon: Health Canada: why a tighter basement should be tested, how, and what level calls for mitigation.
- 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.
