What the code actually requires
The National Building Code (adopted in Manitoba and Saskatchewan as the NBC/NECB 2020 edition, and referenced through Ontarios SB-12 supplement in Northwestern Ontario) requires a vapor barrier on the warm-in-winter side of most above-grade wall assemblies, with a maximum permeance rating tied to the climate zone. That requirement exists to stop interior humidity from migrating into the wall cavity and condensing against a cold surface, the same mechanism discussed on our air barriers vs. vapor barriers page. Historically this layer was a sheet of 6-mil poly installed on the interior face of the studs before drywall. Spray foam changes the picture because some formulations, at sufficient thickness, are dense enough to qualify as the vapor barrier layer themselves.
By foam type
Closed-cell foam
- Vapor barrier at
- Typically qualifies at around 1.5 inches, per the specific products CCMC evaluation report. Below that thickness, it is insulation and an air barrier, but not yet a vapor barrier.
- Practical result
- At full cavity depth in most Prairie wall assemblies, closed-cell foam commonly eliminates the need for a separate interior poly sheet.
- What to verify
- Ask for the CCMC report on the exact product being sprayed. Evaluation numbers are product-specific and not interchangeable between brands.
Open-cell foam
- Vapor barrier at
- Never, at any thickness. Open-cell foam remains vapor-open by design.
- Practical result
- A separate vapor control layer is required on the warm side of the assembly, commonly vapor-retarder paint or a smart (variable-permeance) membrane.
- What to verify
- Confirm which vapor control product is specified and that it is installed as part of the same job, not left as a "by others" gap between trades.
Why "the foam handles it" is an incomplete answer
Some contractors tell every customer that spray foam eliminates the need for a vapor barrier, full stop. That is true for closed-cell foam applied at the correct thickness, and false for open-cell foam or closed-cell foam sprayed thinner than the evaluation report requires. The only way to know which situation applies to your project is to ask which product is being used and at what thickness, then check that products CCMC listing.
Mixed and hybrid assemblies
Some wall assemblies use a modest layer of closed-cell foam (often 1 to 2 inches) against the sheathing for air sealing and a thermal break, then fill the remaining cavity depth with open-cell foam or batt insulation for cost efficiency. In these hybrid assemblies, whether the closed-cell layer alone reaches vapor-barrier thickness depends entirely on how many inches of closed-cell are actually applied, not the total cavity depth. This is a detail worth confirming in writing before the crew shows up, not after.
Frequently asked questions
Does spray foam replace poly vapor barrier in Manitoba or Saskatchewan?
Closed-cell spray foam can replace a separate poly vapor barrier, but only at the thickness specified in the products CCMC evaluation listing, generally around 1.5 inches or more, where it qualifies as a Class II vapor retarder. At lesser thicknesses, or with open-cell foam at any thickness, a separate vapor control layer is still required to meet the National Building Code.
Does open-cell spray foam need a vapor barrier?
Yes. Open-cell foam stays vapor-open even at full cavity depth, so it does not satisfy the vapor barrier requirement in the National Building Code on its own. A separate vapor control layer, such as vapor-retarder paint or a smart membrane, is needed on the warm side of the assembly in a cold climate like Manitoba, Saskatchewan, or Northwestern Ontario.
How do I know if my spray foam contractor is meeting code for vapor control?
Ask which foam product is being used and ask to see its CCMC evaluation report, which states the exact thickness at which that specific product is recognized as a vapor retarder. A contractor who cannot produce this report, or who assures you foam alone handles vapor control regardless of thickness or type, is not giving you a code-accurate answer.
Want the CCMC report for what we spray?
We will show you the evaluation listing before the crew arrives, not after.
