Rigid Foam vs Spray Foam — Ontario Insulation Guide

Rigid foam comes in three common products: XPS (extruded polystyrene, blue/pink), EPS (expanded polystyrene, white beadboard), and polyiso (yellow foil-faced). Spray foam comes in two: open-cell (0.5 lb) and closed-cell (2 lb).

Rigid foam is the better choice when:

1. continuous exterior insulation over framing — XPS or polyiso boards installed over wall sheathing reduce thermal bridging through studs and is increasingly required for high-performance Ontario builds (Net Zero, Passive House, R-2000)
2. below-grade foundation exterior — XPS bonded to the outside of the foundation wall is the gold-standard basement insulation method
3. flat roof above-deck — polyiso is the dominant commercial flat roof insulation
4. DIY budget retrofits — rigid foam can be screwed and taped by an experienced homeowner
5. under concrete slabs — XPS is the only insulation rated for direct concrete contact in below-grade applications.

For exterior insulation continuous over framing, rigid foam is essentially the only practical option.

Spray foam wins when:

1. cavity is irregular — basement walls with stone foundations, rim joists, attic transitions, cathedral ceilings
2. air sealing is the priority — closed-cell foam continuously seals every gap, fastener hole, and crack
3. tight cavities — 2x4 walls hitting R-22 require closed-cell foam at depth, where rigid board can't fit
4. retrofits with limited access — small basement crawl spaces and rim joists are easier to spray than to cut and install rigid board
5. one-pass insulation + air + vapour — closed-cell delivers all three, eliminating separate poly sheet and tape labour.

For irregular cavities and air sealing, spray foam beats rigid foam on labour even though material cost is higher.

Rigid foam is generally cheaper on flat, regular surfaces; spray foam catches up on irregular cavities where the labour to fit rigid board accurately exceeds the spray premium.

Many high-performance Ontario builds use both. A typical approach: closed-cell spray foam against the structural element (interior of basement wall, underside of roof deck) for air and vapour control, then rigid foam continuous over framing for thermal bridging. This combines spray foam's air sealing with rigid foam's continuous coverage.

For basement walls specifically, the standard approach in GTA new builds is: 2-3" closed-cell spray foam directly against the concrete (vapour control + air seal), then a 2x4 stud wall with batt fill for total cavity R-22+. This eliminates separate vapour barrier work and gives basement walls that perform like they're above grade.