It depends which foam. Closed-cell becomes a vapour retarder at about two inches (approaching a Class I barrier at 3"+); open-cell is vapour-permeable and never acts as a barrier. In Alberta's cold climate, that distinction decides how a wall is built.
Why it matters here
Alberta assemblies need vapour control on the warm side to stop interior moisture from condensing inside a cold wall. Where you use closed-cell at sufficient thickness, the foam is that control - you often don't add a separate poly sheet, which simplifies the wall. Where you use open-cell, the assembly still needs a vapour retarder from somewhere.
The practical rule
- Closed-cell at 2"+ : self-vapour-controlling; below grade and exterior friendly.
- Open-cell : needs a separate vapour retarder; interior only.
Getting this wrong causes hidden condensation - which is why a knowledgeable installer specs the assembly, not just the R-value. See closed-cell and Alberta code.
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