Vapour Diffusion vs. Air Infiltration
Moisture enters a wall assembly in two distinct ways. The first is vapour diffusion: water molecules migrate through solid building materials from an area of higher vapour pressure to lower vapour pressure. In a Canadian winter, the interior of a house is warm and humid relative to the cold, dry exterior, so moisture tends to diffuse slowly through wall materials toward the outside.
The second mechanism — air infiltration — is far more significant in practice. When warm, humid interior air finds a gap in the building envelope (around an electrical box, through an unsealed top plate, past a pipe penetration), it carries far more moisture into the wall cavity in a few hours than vapour diffusion would move in an entire heating season. Research published by the Building Science Corporation found that air movement can carry 30 to 100 times more moisture into a wall assembly than diffusion alone.
This distinction matters because the solution to each problem is different. A 6-mil polyethylene sheet — the classic "vapour barrier" — addresses diffusion but is not a reliable air barrier unless every seam, edge, and penetration is carefully taped. Treating the poly as both a vapour retarder and an air barrier requires meticulous installation. Many older Canadian homes have poly installed but not properly detailed, which means the vapour diffusion side of the problem is addressed but the air infiltration side is not.
Where the 1985 National Building Code Changed Things
Before 1985, Canadian building practice focused almost exclusively on vapour diffusion control. The 1985 National Building Code of Canada was one of the first codes in North America to formally recognise the air barrier as a separate building-science requirement from the vapour retarder. The code introduced the concept of a continuous air barrier system — not just poly between studs, but a continuous, connected assembly that includes drywall, rim boards, and all penetrations sealed.
The distinction is preserved in modern codes. The vapour barrier (a Class I or II vapour retarder with low permeance) controls diffusion. The air barrier controls convective moisture transport. They can be the same material if installed with full continuity and sealed laps, or they can be separate layers in the assembly.
Placement in a Standard Cold-Climate Wall Assembly
In a conventional wood-frame wall insulated with fibreglass or mineral wool batts, the vapour retarder goes on the warm-in-winter side of the insulation — that is, the interior face of the stud cavity, just behind the drywall. This is typically the 6-mil polyethylene sheet, though in some modern assemblies it is replaced by a variable-permeance membrane (sometimes called a "smart vapour retarder") that can let moisture move outward during summer drying while limiting inward diffusion in winter.
The warm-side placement rule holds for most of Canada, but it is worth noting that mixed-humid climates (parts of southern BC and southern Ontario near the Great Lakes) can experience summer cooling loads where the moisture drive reverses — exterior humidity now wants to move inward against the air conditioning. In those zones, a variable-permeance retarder is a safer choice than a fixed low-permeance poly, which can trap summer moisture in the assembly.
Common Installation Mistakes
Unsealed Laps and Penetrations
A polyethylene sheet stapled to studs but not taped at laps or sealed around electrical boxes, pipes, and fixtures does almost nothing as an air barrier. The poly may still slow vapour diffusion somewhat, but every unsealed penetration is a highway for moist air. The correct approach is to use acoustical sealant (not silicone) at all penetrations and at the poly's perimeter, and to tape seams with a compatible construction tape rated for poly-to-poly and poly-to-framing adhesion.
Poly Behind Vapour-Open Exterior Cladding
Some older renovation work placed poly on both the interior and exterior faces of a wall assembly, trapping any moisture that entered from either direction with no drying pathway. A wall needs to be able to dry to at least one side. In a cold climate, this is typically the exterior: the exterior sheathing and cladding should be vapour-open (permeable) to allow drying, while the interior poly retards inward vapour drive.
Continuous Poly Over Exterior Insulation
When continuous rigid insulation is installed on the exterior of the stud wall (a common approach to meeting higher R-value targets without increasing stud-bay depth), the position of the dew point in the assembly shifts. Enough exterior insulation moves the dew point outside the stud cavity entirely, which means the poly on the interior side may no longer be necessary — and in some assemblies can actively trap moisture. The general rule: if you are adding exterior continuous insulation, consult an assembly calculator (such as the one offered by Building Science Corporation) to confirm whether a vapour retarder on the interior is still appropriate for your climate zone.
The 2:1 Rule for Exterior Insulation. A commonly cited guideline for cold climates: the exterior insulation layer should provide at least one-third of the total assembly R-value to keep the sheathing warm enough to avoid condensation. In Climate Zone 6 (R-22 total wall), that means at least R-7.3 on the exterior. Exact numbers vary by climate zone and assembly type — verify with current code tables.
Smart Vapour Retarders
Products marketed as variable-permeance or "smart" vapour retarders (MemBrain by CertainTeed is one Canadian-market example) change their permeance depending on relative humidity. In winter, when interior air is relatively dry and the retarder needs to slow diffusion, permeance stays low. In summer or during wetting events, as the relative humidity near the retarder rises, permeance increases and allows the assembly to dry inward. This makes them a useful option in assemblies where both winter vapour retardation and year-round drying capacity are goals — particularly in mixed-climate zones and in renovations where drying to the exterior is restricted.
The Role of the Interior Finish
Drywall itself is a vapour retarder in the Class III range (1–10 perms), which in some applications is sufficient on its own without an additional poly layer — particularly when combined with a continuous air barrier on the exterior sheathing side. Modern high-performance walls sometimes eliminate the interior poly entirely in favour of a continuous taped exterior air barrier (Zip System, for example) paired with the drywall providing modest vapour retardation. Whether this approach meets the National Building Code requirements in a given province depends on the specific wall assembly and climate zone. The code provides prescriptive requirements; an engineer or certified building scientist can confirm compliance for non-standard assemblies.