Ontario's Plain-English Guide to Custom Home Buildingπ 705-533-1633 | βοΈ 1-866-868-6606 | βοΈ info@icfhome.ca
Rigid Foam Insulation: EPS vs XPS vs Polyiso for Ontario Houses, R Per Inch, What Each Does With Water and Cold, Where Each One Belongs (Slab, Basement Wall, Exterior, Roof) and What It Costs Installed
Insulation & Air Sealing
EPS, XPS, Polyiso: Three Foam Boards, Three Jobs, and the One That Loses R in January Is the One With the Highest Number on the Label
Rigid foam is the insulation that goes where batts cannot: under a slab, against a concrete wall, outside the sheathing, on a flat roof. There are three kinds and they are not interchangeable. White bead board, EPS, is about R-4 an inch, the cheapest, takes a little water and gives it back, and it is what an ICF form is made of. Pink or blue XPS is R-5, stiffer, nearly waterproof, and the board for below grade. Polyiso with the foil face is R-5.6 to R-6 on the label and the only one of the three that gets worse as it gets colder, which is a poor habit for insulation in Simcoe County.
The three boards
| EPS (expanded polystyrene) | XPS (extruded polystyrene) | Polyiso (polyisocyanurate) | |
|---|---|---|---|
| Looks like | White bead board, the coffee cup foam | Pink, blue or green smooth board | Yellow-tan foam with foil or glass-fibre facers |
| R per inch | about R-4 (Type I 3.8, Type II 4.0 to 4.2) | R-5 | R-5.6 to R-6 nominal, long-term rated lower |
| Cold weather | Gains a little R as it gets colder | Gains a little | Loses R below about 10 C mean; closer to R-4.5 to R-5 in a Simcoe January |
| Water | Takes 2 to 4% by volume if sitting wet, dries out again; drains between beads | Lowest absorption of the three; the below-grade board | The foam itself is fine; the facers wick, and a soaked board stays soaked. Keep it above grade. |
| Vapour, 1 in. | 2 to 5 perms, semi-permeable | about 1 perm, a vapour retarder | foil faced near 0, a vapour barrier |
| Compressive strength | Type I 10 psi, Type II 15 psi | 25 to 40 psi, 60 and 100 psi grades | about 20 psi |
| Belongs | Under slabs (Type II), ICF forms, exterior walls, over-slab in a basement | Basement walls, foundation exterior, under slabs, exterior walls | Flat and low-slope roofs, above-grade wall sheathing in milder zones |
| Does not belong | Where it will be stepped on or leaned on without cover | Where cost rules and water does not matter | Below grade, anywhere wet, or where the cold-weather R has not been counted |
All three are foam plastics and all three burn. Exposed inside a living space, every one of them needs the thermal barrier the code asks for in 9.10.17, usually 1/2 in. drywall.
What it costs installed
| Where | Board | Per sq ft | R added |
|---|---|---|---|
| Basement wall, against the concrete, seams taped | 1.5 in. XPS or EPS | $2.75 | R-7.5 (XPS) or R-6 (EPS) |
| Basement wall | 2 in. | $3.50 | R-10 |
| Basement wall | 3 in. | $4.75 | R-15 |
| Under the slab, before the pour, with 6 mil poly | 2 in. Type II EPS | $1.75 to $2.25, default $2.00 | R-8; R-10 under a heated slab |
| Exterior wall at re-siding | 1.5 in. EPS or XPS | $4.50 | R-6 to R-7.5, continuous |
| Exterior wall at re-siding | 2 in. mineral wool board, for comparison | $7.00 | about R-8 |
The basement lines are what Assembly A on the basement pages starts with: 2 in. XPS at $3.50, then a 2×4 wall, an R-14 batt and poly, $9.60 a square foot for about R-21 effective. The exterior lines, with the $150 per opening for jamb extensions, are priced and argued on the exterior insulation cost page.
EPS: the cheap board that is in every ICF wall
Expanded polystyrene is beads of foam steamed together in a mould. It is the lowest R per inch of the three and the lowest price per R of any insulation on this site, and the only one of the three I would put in contact with the ground without worrying. Water gets between the beads, sits a while and leaves; a board dug up after twenty years under a slab is damp on the surface and dry inside. Type II at 15 psi is the slab grade; Type I at 10 psi is for walls, and the difference is printed on the board, so look.
An ICF form is 2.5 in. of EPS on each face of the concrete, which is why the ICF walls I build come out at R-22 to R-24 effective with nothing added, continuous, no studs bridging it, no poly. That number is on the ICF R-value page. EPS also dents when you lean a ladder on it and chews into pellets when you cut it, which is why nobody loves working with it and everybody specifies it.
XPS: the below-grade board
Extruded polystyrene is the same plastic pushed through a die as a continuous foam, which closes the cells. It is R-5 an inch, stiff enough to walk on, and takes almost no water, which is why it goes on the outside of foundations, under slabs where the water table is high, and against the inside of basement walls where the concrete will be damp every July. At 1 in. it is about 1 perm, a vapour retarder. Against concrete that is what you want: the wall can no longer push summer moisture into the stud wall in front of it, and with no poly on the warm side the assembly dries inward.
The blowing agents in XPS changed recently; the new low-GWP boards are the same colour and a hair lower in rated R. It does not change where the board goes. Tape the seams with a sheathing tape, foam the edges at the slab and the sill, and it is both the insulation and the air barrier on that wall.
Polyiso: the highest number and the catch
Polyisocyanurate is a different chemistry, closer to spray foam than to the polystyrenes, with a foil or glass-fibre facer on each side. It is the board of choice on flat commercial roofs because it has the most R per inch in a tapered system. The catch is in the fine print of every data sheet: the R per inch is rated at a mean temperature of 24 C, and as the board gets colder the gas in the cells starts to condense and the R drops. At a mean of minus 5 C, a cold January on a north wall, a board labelled R-6 is performing nearer R-4.5 to R-5. The polystyrenes go the other way, gaining a little as they cool.
So polyiso on an exterior wall in Simcoe County does its best work in October. On a roof under a membrane, or as the inner layer of a two-board roof with XPS or EPS outside it, it is a good product. Below grade it is not: the facers wick water, a soaked board does not dry, and the foil face against damp concrete is a vapour barrier on the wrong side. Keep it out of the basement.
Rule of thumb: ground and concrete get EPS or XPS; roofs get polyiso; an exterior frame wall gets whichever polystyrene the vapour ratio rule allows at the thickness you can afford, and 1.5 in. is the least worth the jambs. Which layer stops water, which stops air and which stops vapour, and why a foam board can be two of them at once, is on the three-layers page.
Where it goes wrong
- Foil-faced polyiso in the basement. It wicks, it traps, and the foil is a vapour barrier against a wet wall. XPS or EPS.
- Seams not taped. Untaped board on a basement wall is an air barrier with 400 lf of slot in it. Warm air goes through the slots, meets the concrete, condenses. Tape every joint and foam the top and bottom edges.
- Poly over the board in the basement. Foam board at 2 in. is already the vapour retarder. Poly in front of it makes a wall that cannot dry in either direction.
- Type I EPS under a slab. 10 psi under a garage floor or a loaded slab crushes. Type II, 15 psi, or XPS.
- Exposed foam in a finished room. Every foam board burns. Drywall over it.
- Counting polyiso at R-6 in the heat-loss calculation. The design R for a cold wall is the cold-weather R. Use R-5 and you will not be surprised in February.
Questions people ask
Which is better, EPS or XPS?
XPS has more R per inch, R-5 against R-4, and takes less water, so it wins below grade and where the thickness is limited. EPS costs less per R, dries out, and is what ICF forms are made of. Under a slab, either in Type II grade; against a basement wall, XPS if the wall is damp, EPS if it is dry and the budget is tight.
Does polyiso lose R-value in cold weather?
Yes. Rated at a mean of 24 C it is R-5.6 to R-6 an inch; at Ontario winter wall temperatures it drops toward R-4.5 to R-5. EPS and XPS gain slightly in the cold. Use the cold-weather number when a wall or roof is being designed for January.
Can you put rigid foam under a basement slab?
Yes, and in a new house you should: 2 in. of Type II EPS or XPS with 6 mil poly, about $2.00 a square foot before the pour, R-8. Under a heated slab the code wants R-10. Over an existing slab, 1 in. or more of XPS under a subfloor is the retrofit.
Does rigid foam need to be covered?
Inside a living space, yes, with a thermal barrier, usually 1/2 in. drywall, under OBC 9.10.17. All three boards are combustible. Under a slab or behind cladding no cover is needed beyond what the assembly already has.
How much does rigid foam cost installed?
On a basement wall, $2.75 a square foot for 1.5 in., $3.50 for 2 in., $4.75 for 3 in., taped. Under a slab, $2.00 for 2 in. with poly. On an exterior wall at re-siding, $4.50 for 1.5 in. plus the jambs and soffits. All before HST.
Board or foam, priced the same way
Where a quote has rigid board on one line and spray foam on the next for the same wall, the spray foam cost calculator prices the sprayed version surface by surface so the board can be held beside it at the rates above. If the quote says “foam board” and not which one or how thick, the $99 review will ask.
Sources and further reading
- 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.
- 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.
