Rigid insulation is exactly what it sounds like: insulation that comes as stiff boards instead of fluffy batts or loose fill. Made from plastic foams like polystyrene and polyisocyanurate, these boards give a lot of warmth per centimetre, shrug off moisture, and cut easily with a simple saw.
Canadian builders reach for them in basements, on exterior walls, under slabs, and around foundations. This guide compares the three main types, shows where each one shines, and walks through installation basics, costs, and mistakes to avoid.
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Table of Contents
- 1. What Makes It Different
- 2. EPS: The White Beadboard
- 3. XPS: The Coloured Boards
- 4. Polyiso: The Foil-Faced Boards
- 5. Cutting and Fitting Boards
- 6. Sealing the Joints
- 7. Thickness and Warmth per Centimetre
- 8. Foil Facings and What They Do
- 9. Layering Boards for Better Performance
- 10. Edge Sealer, Tape, and Foam
- 11. Recycling Offcuts and Old Boards
- 12. Buying and Storing Boards
- FAQs
1. What Makes It Different
Unlike batts that friction-fit between studs, rigid insulation is a structural board you can glue, screw, or tape in place. A 5 cm board of extruded polystyrene insulates roughly as well as 8 cm of fibreglass batt, so you get more warmth in less space.
Boards also block air movement far better than batts when the seams are taped. That air-sealing bonus is a big part of why they perform so well in real houses. Expect to pay $25 to $60 CAD per 1.2 m by 2.4 m sheet depending on type and thickness.
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2. EPS: The White Beadboard
Expanded polystyrene is the familiar white board made of fused beads, the same stuff as coffee cups but denser. It is the cheapest rigid foam, about $25 to $35 CAD per sheet, and it handles moisture reasonably well. You will see it on exterior walls and under concrete slabs.
Its warmth per centimetre is the lowest of the three, so you need thicker boards for the same result. EPS can also crumble at the edges if handled roughly. For budget jobs where thickness is not a problem, it is hard to beat on price.
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3. XPS: The Coloured Boards
Extruded polystyrene is the smooth, dense board sold in pink, blue, or grey depending on the brand. It insulates better per centimetre than EPS and absorbs very little water, which makes it the favourite for foundations, basement exteriors, and under slabs.
A 5 cm sheet runs $40 to $60 CAD. XPS cuts cleanly with a utility knife or fine saw, and the boards butt together tightly. One caution: it loses a little performance over many years as the blowing agents diffuse, but it remains a top performer for decades.
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4. Polyiso: The Foil-Faced Boards
Polyisocyanurate, usually called polyiso or PIR, is the warmest per centimetre of the three and almost always comes with a foil or coated facing. That facing reflects radiant heat and acts as a built-in vapour check, which is handy on walls and roofs.
Sheets cost $45 to $70 CAD. Polyiso's one weakness is cold: its rating dips a little in deep cold, so many builders use it on the warm side of the assembly or pair it with another foam outside. In heated basements and cathedral ceilings it is superb.
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5. Cutting and Fitting Boards
Measure twice, cut once, and leave a 3 mm gap you will fill later rather than forcing a tight board. Score XPS and polyiso with a sharp utility knife and snap them; cut EPS with a fine-tooth saw to avoid tearing the beads. A simple straightedge keeps cuts clean.
Wear a dust mask and gloves: foam dust is irritating, and fresh-cut edges are sharp. For warming up a garage, boards on the walls go up fast with construction adhesive plus a few concrete screws. Work from the bottom up so each board rests on the one below.
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6. Sealing the Joints
Boards only perform if the seams are sealed. Butt the edges tight, then tape every joint with foil or sheathing tape, pressing firmly for full contact. Gaps wider than 3 mm get a bead of low-expansion foam first, then tape over it once cured.
This taping step is what turns separate boards into a continuous air barrier. Skipping it can halve the real-world benefit, since air leaking through gaps carries heat straight past the foam. A $20 roll of tape is the cheapest performance upgrade in the whole job.
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7. Thickness and Warmth per Centimetre
As a rule of thumb, each 2.5 cm of XPS gives about R-5, polyiso about R-6, and EPS about R-4, in the imperial R-values Canadians still quote at the store. So a 5 cm XPS board lands near R-10, while the same thickness of polyiso nears R-12.
Building codes in most provinces want far more than one layer for walls and roofs, which is why you will see boards doubled up or combined with batts. Two staggered layers beat one thick layer: the offset seams block air paths. Always check your local code before buying.
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8. Foil Facings and What They Do
The shiny face on polyiso is not just for looks. It reflects radiant heat back into the room and slows vapour moving through the wall. On a basement wall, the foil faces inward toward the warm side, doing double duty as insulation and vapour control.
Do not puncture the facing more than necessary, and tape over screw heads on critical walls. If you need a non-foil board for a spot that must dry outward, EPS without facing is the safer pick. Match the facing to the wall's drying direction.
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9. Layering Boards for Better Performance
Two 5 cm layers with staggered joints outperform a single 10 cm layer, because the second layer covers the seams of the first. Offset both the vertical and horizontal joints by at least 30 cm. The result is closer to a continuous blanket with no straight-through air paths.
Glue the layers together with foam-compatible adhesive, or screw through both into the framing with long fasteners and wide washers. Staggering costs a little more labour but measurably improves airtightness. Blower-door tests consistently show the difference.
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10. Edge Sealer, Tape, and Foam
The accessories matter as much as the boards. Edge sealer in a spray can protects cut foam edges from moisture and UV during construction. Foil tape seals the seams, and low-expansion spray foam fills gaps around pipes, wires, and window frames.
Buy foam labelled for windows and doors: it stays flexible and will not bow frames. One can covers a surprising number of gaps, and at $12 to $18 CAD it is cheap insurance. Keep a rag and mineral spirits handy, since cured foam is miserable to remove.
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11. Recycling Offcuts and Old Boards
Foam offcuts pile up fast, and they do not belong in the regular bin. Many cities accept clean EPS at depots or through take-back programs, and some recyclers densify it into new products. Check your municipality's waste guide before tossing a truckload.
On site, keep offcuts dry and sorted by type: clean pieces can fill small gaps, and some ReStores accept usable full boards. Winterizing a cottage is a great use for leftover boards, since small buildings need less material and every offcut finds a job.
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12. Buying and Storing Boards
Buy from a building supply store rather than a general hardware shop for better prices on full lifts. A lift of 5 cm XPS (about 30 sheets) often costs 10 to 15 per cent less per sheet. Check each board for dents and broken corners before it goes on the truck.
Store boards flat, off the ground, and out of the sun: UV breaks down foam surfaces in weeks. A tarp over a flat stack works fine. Plan the delivery so boards go straight into a dry garage or shed, not onto a wet driveway.
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FAQs
Which rigid foam is best for basement walls?
XPS is the most popular for exterior foundation walls because it resists moisture. Inside a finished basement, foil-faced polyiso works well on the warm side. EPS is fine where budget matters and thickness is not limited.
Can rigid foam touch the ground?
XPS and EPS rated for below-grade use can, and often do, sit against foundations and under slabs. Keep any foam at least 15 cm above exterior grade unless it is rated for ground contact, and protect above-grade foam from sunlight.
Do I need a vapour barrier with rigid foam?
Often the foil facing or the foam itself is enough, but it depends on the assembly and your climate zone. In cold Canadian climates the vapour control goes on the warm side. When in doubt, ask your local building inspector before closing up the wall.
How long does rigid foam insulation last?
Decades. The foam does not settle or rot, and XPS and polyiso hold their rating well over time. What fails is usually the tape or the installation, not the board. Protect it from UV and physical damage and it will outlast the wall finish.
Is rigid foam a fire risk?
All plastic foams must be covered by a thermal barrier such as drywall when used inside living spaces: that is code everywhere in Canada. Never leave foam exposed in a basement or garage. The covering is about fire safety, not looks.
Final Thoughts
Rigid boards are the closest thing to a sure bet in insulation: predictable performance, real moisture tolerance, and a straightforward install. Pick the right foam for the spot, tape every seam, and cover it properly. Done well, it is a fifty-year upgrade you will never think about again.
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Photo: Wikimedia Commons
Photo: Wikimedia Commons
Photo: Wikimedia Commons
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