Rigid Foam Insulation for Concrete: XPS vs EPS, Thickness & R-Value (GTA Guide)

Q: What's the difference between XPS and EPS rigid foam?

XPS (extruded polystyrene, the smooth board — SOPRA-XPS and similar) runs about R-5 per inch, has high compressive strength and very low water absorption, which is why it's the default under slabs and against foundation walls. EPS (expanded polystyrene, the white bead board) runs about R-3.6 to R-4.2 per inch, costs less, but soaks up more water unless it's treated. For concrete work below grade, most GTA crews use XPS.

Q: How thick does under-slab insulation need to be?

It's set by your energy-code compliance package and your permit drawings, not a rule of thumb. As a feel for the numbers, XPS gives about R-5 per inch, so 2 inches is roughly R-10 and 3 inches is roughly R-15. Confirm the required R-value against your drawings before you order.

Q: What compressive strength do I need under a slab?

For a typical residential slab on grade, 25 psi XPS (Type IV) is the common choice — it carries the slab and normal floor loads without crushing. Heavier loads, thickened slabs, or point loads can call for 40, 60 or 100 psi board. Match it to your engineer's spec.

Q: Does 2AZ stock rigid insulation in Mississauga and Brampton?

Yes. 2AZ Group stocks SOPRA rigid foam insulation board in common thicknesses at our Mississauga and Brampton yards, with same or next-day GTA delivery. Call 647-926-2597 for current sizes and pricing.

If you’re pouring a heated slab, a garage floor, or a foundation that has to meet Ontario’s energy code, rigid foam insulation is part of the job — not an afterthought. Get the board type, thickness, and compressive strength right and the slab stays warm, dry, and doesn’t crack the foam under load. Get it wrong and you’re either failing an inspection or crushing your insulation the day you pour.

This guide covers the two rigid foam types you’ll actually see on a GTA concrete job, how to think about thickness and R-value, and where the board goes so it does its job.

XPS vs EPS: which rigid foam for concrete?

Almost all rigid foam under and around concrete is one of two materials:

	XPS (extruded)	EPS (expanded)
Look	Smooth, coloured board (e.g. SOPRA-XPS)	White, made of fused beads
R-value	~R-5 per inch	~R-3.6 – R-4.2 per inch
Water absorption	Very low	Higher (unless treated)
Compressive strength	25 psi standard, up to 100 psi	Lower for the same price point
Typical use	Under slabs, against foundation walls, below grade	Above-grade walls, void fill, budget jobs

For concrete work that sits below grade or under a slab, XPS is the default across the GTA. It shrugs off ground moisture, carries the slab load, and holds its R-value in wet soil — the three things that matter most where foam meets concrete. EPS has its place on above-grade walls and where budget rules, but under a slab it’s the wrong tool unless it’s a moisture-rated grade.

Thickness and R-value: do the simple math

You don’t guess thickness — your energy-code compliance package and permit drawings set the required R-value, and you buy the thickness that gets you there. Because XPS is roughly R-5 per inch, the math is easy:

XPS thickness	Approximate R-value
1”	R-5
1.5”	R-7.5
2”	R-10
2.5”	R-12.5
3”	R-15

So if your drawings call for R-10 under the slab, that’s 2” of XPS. Need more than a single board gives you? Stack two layers and stagger the joints so there’s no straight thermal path through the seam. Always size to the R-value on your drawings — the numbers above are just there to help you picture it.

Compressive strength: don’t crush your insulation

Foam under a slab has to hold the concrete and everything that ever sits on that floor. That’s what compressive strength (measured in psi) is for:

25 psi — the standard choice under a normal residential slab on grade. Handles the slab plus regular floor and foot loads.
40 – 60 psi — thickened slabs, garage and shop floors that see vehicle or equipment loads.
100 psi — heavy point loads, racking legs, and engineered industrial floors.

If you spec 25 psi board and then park a loaded trailer on that spot, you’ll dish the foam and crack the slab above it. When in doubt, match the board grade to your engineer’s number, not the cheapest sheet on the pile.

Where the board goes

Two placements do most of the work on a GTA job:

Under-slab (horizontal): a continuous layer of XPS over compacted, level granular, with a 6-mil poly vapour barrier between the foam and the concrete. This is what keeps a heated slab from dumping its warmth straight into the ground.
Slab-edge and foundation wall (vertical): insulation at the perimeter and against the foundation wall stops the thermal bridge at the edge of the slab — the spot that leaks the most heat and is easiest to forget. Skipping the edge undoes a lot of what the under-slab layer bought you.

Keep the boards tight, stagger the seams, and don’t leave gaps at the perimeter. Foam that’s cut sloppy or left with open joints is just an expensive way to almost insulate.

A few field notes

Protect it until you pour. Rigid board is tough but not indestructible — stacked flat and kept off mud, it stays flat and clean. Boards left curled in the sun or walked on over gravel telegraph every high spot into your slab.
Tape or detail the seams where your drawings or vapour strategy call for it, especially under a heated slab.
Cut clean. A sharp utility knife and a straightedge beat a hacked edge every time — tight joints are what make the R-value real.
Order a little extra. Perimeter pieces, stacked layers, and cut waste add up. A few extra sheets beat stopping the crew to chase one more board.

Pair it with the rest of the pour

Insulation is one layer of a slab package. Most crews grab these on the same order:

6-mil poly vapour barrier to sit between the foam and the concrete.
Wire mesh or rebar to reinforce the slab — see wire mesh vs rebar for which to use, and the rebar sizing guide for sizes.
Chairs and expansion joint to hold steel at height and let the slab move.

Getting the base ready first? Our foundation prep checklist walks through grading and compaction before the foam ever goes down.

Buy rigid insulation in Mississauga & Brampton

2AZ Group stocks SOPRA rigid foam insulation board in common thicknesses at our Mississauga and Brampton yards, alongside vapour barrier, rebar, wire mesh and the rest of the slab package. Contractor volume pricing, same or next-day GTA delivery — Mississauga, Brampton, Toronto, Vaughan, Oakville and across the Greater Toronto Area.

Browse supplies → Request a quote → Call: 647-926-2597 for current sizes and pricing.

Frequently asked questions

What’s the difference between XPS and EPS rigid foam?

XPS (extruded polystyrene — the smooth board, like SOPRA-XPS) runs about R-5 per inch, with high compressive strength and very low water absorption, which is why it’s the default under slabs and against foundation walls. EPS (expanded polystyrene — the white bead board) runs about R-3.6 to R-4.2 per inch and costs less, but absorbs more water unless it’s a treated grade. For concrete below grade, most GTA crews reach for XPS.

How thick does under-slab insulation need to be?

Your energy-code compliance package and permit drawings set it, not a rule of thumb. As a feel for the numbers: XPS is about R-5 per inch, so 2” is roughly R-10 and 3” is roughly R-15. Confirm the required R-value on your drawings before you order.

What compressive strength do I need under a slab?

For a standard residential slab on grade, 25 psi XPS (Type IV) is the common choice. Thickened slabs, vehicle loads, or point loads can call for 40, 60 or 100 psi board. Match it to your engineer’s spec.

Does 2AZ stock rigid insulation in Mississauga and Brampton?

Yes. 2AZ Group stocks SOPRA rigid foam insulation board in common thicknesses at our Mississauga and Brampton yards, with same or next-day GTA delivery. Call 647-926-2597 for current sizes and pricing.