The same job, two ways
On a crawl space or basement wall these compete directly. Both give you insulation and vapour control in one layer against the concrete, which is what that assembly needs. The decision is rarely about performance — it is about whether the surface is flat and whether you are doing the work yourself.
Cost
Board is $0.70–$0.95 per square foot per inch in material, at R-5 per inch for XPS. Installed on a wall by a contractor, reckon on $2.50–$5.00 per square foot.
Closed-cell spray foam is $1.50–$4.50 installed, at R-6.5 per inch. It is never a realistic DIY material — it needs two-component equipment, a supplied-air respirator and a clear-out while it cures.
So the real cost comparison is board you fit yourself against foamsomebody sprays, and on that basis board usually wins on money and loses on time.
Geometry decides it
Board only works on a surface it can sit flat against. A poured concrete wall is ideal. A rubble foundation, a stone wall, block with proud mortar joints, or anything with pipes and ducts crossing it is where board stops making sense — every irregularity becomes an air gap behind the sheet, and a gap behind rigid foam is a convection loop, not insulation.
Spray foam has no such constraint. It takes the shape of whatever it hits, which is why it is the standard answer for rim joists, irregular masonry and anywhere pipework makes a continuous sheet impossible.
Whichever you use: seal every edge and seam. Unsealed board is a convection path. Taped and sealed board is a continuous insulation layer, and the difference between the two is most of the performance.
Which to choose
- Flat poured concrete wall, DIY: board. It is the cheapest route to a correct assembly.
- Rubble, stone or irregular masonry: spray foam. Board cannot sit flat and the gaps defeat it.
- Rim joists: spray foam, or cut-and-cobble board sealed with canned foam at every edge.
- Continuous exterior sheathing: board. This is its other main job — the unbroken layer outside the studs that defeats thermal bridging.
- Under a slab: board, rated for the load. Foam is not a slab product.
One code note for both: foam left exposed needs a covering. A crawl space gets the lighter ignition-barrier requirement; a garage does not, and exposed foam there is a straightforward violation.