Does spray foam insulation act as a vapour barrier in NB homes? | Insulation IQ?

Whether spray foam can serve as your vapour barrier in a New Brunswick home is one of the most important — and most frequently misunderstood — questions in residential insulation. The answer depends almost entirely on which type of spray foam you're using and how thick it's applied, because the two main products behave very differently when it comes to vapour permeance.

Closed-cell spray foam is a highly effective vapour retarder and can function as the primary vapour barrier in most New Brunswick wall and roof assemblies when applied at sufficient thickness. Closed-cell foam has a vapour permeance of approximately 35 to 60 ng/(Pa·s·m²) at 1 inch, and as thickness increases, permeance drops further — at 2 inches, closed-cell foam typically falls below 30 ng/(Pa·s·m²). The NB Building Code (following the National Building Code of Canada) requires vapour barriers on the warm side of insulation in Climate Zone 6 to have a maximum permeance of 60 ng/(Pa·s·m²). This means closed-cell spray foam at 1.5 to 2 inches typically meets this requirement, and at 2 inches or more, it exceeds it comfortably. This is a key reason why closed-cell foam applied to the cold side of wall assemblies — against the exterior sheathing or on the interior face of a poured concrete foundation wall — is so effective in New Brunswick: it handles both the thermal resistance and the vapour control function in one product.

Open-cell spray foam is a completely different story. Open-cell foam is vapour-permeable, with a permeance typically in the range of 1,700 to 2,800 ng/(Pa·s·m²) — many times higher than the Code limit. This means open-cell foam provides essentially no vapour barrier function whatsoever. In New Brunswick's cold climate, using open-cell foam in a wall or attic assembly without a separate vapour barrier on the warm (interior) side would risk significant moisture accumulation in the wall assembly during winter, potentially leading to mould, rot, and structural damage. If you're using open-cell foam in NB, a 6-mil polyethylene vapour barrier or vapour barrier paint applied to the interior face of the framing is mandatory under the Building Code.

The placement of the vapour barrier also matters in NB assemblies. The Code requires the vapour barrier to be on the warm-in-winter side of the insulation — meaning the interior side in a wall assembly. When closed-cell foam is sprayed against the exterior sheathing inside a stud cavity (a common retrofit approach in Moncton and Fredericton renovation projects), it sits on the cold side of the assembly. This is actually advantageous from a building science perspective: it keeps the sheathing warm and dry by preventing interior vapour-laden air from reaching the cold sheathing surface. The closed-cell foam's low permeance then functions as a class II vapour retarder at the cold side, which is acceptable under the NBC's prescriptive compliance path when certain conditions are met regarding the ratio of exterior-to-interior insulation.

For attic applications, when closed-cell foam is sprayed to the underside of the roof sheathing to create a sealed (unvented) attic, it acts as both the air barrier and vapour retarder for the roof assembly. The NB Building Code's requirements for vapour control in sealed roof assemblies are satisfied by closed-cell foam at the specified thickness, typically 3 to 4 inches minimum for a sealed attic assembly in Climate Zone 6, achieving R-18 to R-28 while providing complete vapour and air control.

For foundation walls and rim joists, closed-cell spray foam is particularly well suited because it simultaneously provides moisture resistance (resisting bulk water and ground moisture), air sealing, thermal insulation, and vapour control — eliminating the need for the complex layering of separate membrane, batt, and poly assemblies that older NB basement renovation guides recommend.

A common practical hybrid used by many New Brunswick builders and contractors: spray 2 inches of closed-cell foam against the exterior sheathing or cold surface to satisfy vapour control requirements and seal air leakage, then fill the remaining cavity with blown-in cellulose or fibreglass batts to achieve the target R-value economically. This approach is code-compliant, building-science sound, and more cost-effective than filling the entire cavity with closed-cell foam.

If you're unsure whether your planned assembly requires a separate vapour barrier alongside spray foam, or whether your specific product and thickness meet the NB Building Code threshold, a qualified insulation contractor can review your assembly in detail. Professionals listed through New Brunswick Insulation and the New Brunswick Construction Network are familiar with provincial code requirements and can confirm your assembly is both compliant and durable in our Climate Zone 6 conditions.

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