In most Canadian heating climates, moisture moves from indoors toward the cold exterior during winter. If that moisture reaches a surface cold enough to cause condensation inside the wall assembly, the result is wet insulation, potential mould, and structural degradation that can take years to become visible. A vapour barrier placed on the warm side of the insulation slows that outward moisture drive — but only when it is installed correctly and in the right location for the wall system.
What a Vapour Barrier Actually Does
The term is sometimes used loosely. A vapour barrier (or vapour retarder, depending on its permeance rating) is a material with low water vapour transmission. In Canadian residential construction, the most common choice is 6 mil (0.15 mm) polyethylene sheeting, which has a permeance well below 1 ng/Pa·s·m² — the threshold the National Building Code of Canada uses to define a Type II vapour barrier.
It does not stop all moisture movement. It slows the rate enough that the wall assembly can dry toward the interior during summer months without accumulating moisture during winter. This drying potential is why the placement matters: a vapour barrier installed on the wrong side of a wall can trap moisture rather than manage it.
Where It Goes and Why
In a heating-dominated climate like most of Canada, the vapour barrier belongs on the warm-in-winter side — typically the interior face of the stud cavity, directly behind the drywall. This positioning keeps the barrier above the dew point during the heating season, meaning moisture in the air does not condense on the polyethylene itself.
The dew point location shifts through the wall depending on interior temperature, relative humidity, and exterior temperature. In Ottawa at –20°C with 35% indoor relative humidity, the dew point falls well inside the stud cavity. The vapour barrier needs to be warm enough that the dew point is always on the exterior (cold) side of the barrier — not behind it.
Permeance Ratings and What They Mean
Permeance measures how readily water vapour passes through a material, expressed in ng/Pa·s·m² (metric perms). The NBC categories used in Part 9 residential construction are:
- Type I (Class A) vapour barrier: Less than 15 ng/Pa·s·m². Polyethylene sheeting and foil-faced rigid foam boards typically fall here.
- Type II (Class B) vapour retarder: 15 to 60 ng/Pa·s·m². Kraft-faced batts and certain vapour-retarder paints are in this range.
- Type III (Class C): 60 to 170 ng/Pa·s·m². Latex paint on drywall falls here and is generally insufficient as a primary vapour barrier in cold climates.
For above-grade walls in Climate Zones 6 and colder (most of Ontario, Quebec, the Prairies, and northern British Columbia), a Type I barrier is the standard. Some builders working with advanced framing or double-stud walls use vapour-retarder paints as a secondary control layer, but these are not substitutes for polyethylene in a conventional stud wall.
Common Installation Errors
Penetrations Left Unaddressed
Electrical boxes, plumbing stacks, and recessed lighting are the most common sources of air leakage through a vapour barrier. Even a well-sealed polyethylene membrane can be undermined by a single unaddressed penetration. The NBC requires that all joints, seams, and penetrations be sealed with acoustical sealant, tape, or another airtight method. In practice, this step is often skipped or done hastily before drywall.
Compressed or Improperly Fitted Batts
Fibreglass batts that are stuffed into cavities where they compress or fold lose a significant portion of their thermal resistance. A batt rated at RSI 3.52 (R-20) installed with gaps or at reduced thickness may perform at RSI 2.5 or less. This affects not just thermal performance but also where the dew point falls within the assembly.
Wrong-Side Placement in Mixed Climates
Buildings in southern BC and parts of southern Ontario that have significant cooling loads in summer present a more complex situation. Running air conditioning at high humidity can reverse the vapour drive, pushing moisture from the humid exterior toward the cooler interior. A conventional interior polyethylene barrier can trap this moisture. In mixed climates, builders sometimes use a smart vapour retarder (such as MemBrain or similar) that changes permeance with relative humidity — low in winter, higher in summer — allowing the wall to dry in both directions seasonally.
Garage and Basement Walls
Attached garage walls and basement assemblies follow different rules. Basement walls contact the ground and are often at near-constant temperatures; the vapour drive can be bidirectional depending on groundwater and interior conditions. The NBC and the Canadian Home Builders' Association both note that below-grade walls present unique moisture challenges that call for site-specific assessment rather than a blanket rule.
For above-grade garage walls adjacent to conditioned space, the vapour barrier requirements are the same as for exterior walls. The garage side is treated as the exterior.
What the Code Requires vs. What Works Well
Meeting the minimum NBC requirement for vapour barriers is not the same as building a high-performance wall. Code compliance establishes a floor — the minimum that prevents health and structural problems in typical conditions. Builders aiming for EnerGuide 80 or higher, or for Passive House certification, typically use continuous air barriers in addition to vapour barriers, taped rigid foam on the exterior, and careful attention to sealing every penetration.
Natural Resources Canada's housing efficiency resources include climate-specific guidance on wall assemblies as part of the EnerGuide program, which provides a useful benchmark for understanding how installation quality affects measured performance.
A Note on Air Barriers
Vapour barriers and air barriers are often confused because polyethylene serves as both in a conventional assembly. Technically, an air barrier stops bulk air movement while a vapour barrier limits vapour diffusion. Polyethylene is an effective air barrier only when it is continuous and sealed at all penetrations; otherwise it controls vapour diffusion but allows air movement through gaps, which carries far more moisture than diffusion alone.
In many cold-climate failures, the culprit is not vapour diffusion but air exfiltration — warm, humid interior air leaking through electrical boxes, top plate gaps, and around window rough openings. Sealing these points first gives a better return than upgrading the polyethylene to a thicker grade.
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Last updated: April 15, 2026. Information reflects NBC 2020 requirements and current Canadian building practice.