The mechanics of vapour diffusion
Water vapour moves through building materials from areas of higher vapour pressure to areas of lower vapour pressure — a process called diffusion. In a Canadian winter, warm, moist interior air carries significantly more vapour pressure than the cold, dry exterior air. This pressure differential drives vapour outward through any permeable material in the wall assembly.
If that vapour reaches a surface within the wall that is at or below the dew point temperature, it condenses into liquid water. In a typical 2×6 wall in central Canada, the dew point is often located somewhere within the insulation layer or at the exterior sheathing during cold weather. Liquid water accumulating inside a wall cavity encourages mould growth, can degrade insulation performance, and over time causes structural wood to rot.
A vapour retarder — commonly but imprecisely called a vapour barrier — slows the rate of diffusion, reducing the amount of vapour that can reach and condense at the cold sheathing.
Vapour barrier versus vapour retarder
The terminology matters. A true vapour barrier, with a permeance of 0.1 perms or less, stops nearly all vapour diffusion. A vapour retarder allows some diffusion but at a substantially reduced rate. Most polyethylene sheeting used in Canadian residential construction falls in the Class II vapour retarder range at 0.1 to 1.0 perms, and is commonly referred to colloquially as a "vapour barrier" even though it does not stop vapour entirely.
The distinction becomes significant when considering the drying potential of an assembly. A wall that has a highly impermeable layer on both the interior and exterior sides cannot dry in either direction if moisture enters. This is a known problem with walls that have polyethylene on the interior and closed-cell spray foam on the exterior — any moisture that gets into the cavity has nowhere to go.
Where the vapour retarder belongs in a cold-climate wall
In a heating-dominated climate — which describes most of Canada for the majority of the year — the vapour retarder belongs on the warm side of the insulation, closest to the interior. The standard Canadian residential assembly positions 6-mil polyethylene sheeting directly behind the interior drywall, on the warm side of the fibreglass or mineral wool insulation filling the stud cavities.
This placement keeps the retarder warm. Because it is on the warm side of the insulation, the temperature at the polyethylene surface is close to interior air temperature, well above the dew point. Vapour that diffuses through the drywall reaches the polyethylene before it gets cold enough to condense.
The exterior sheathing — typically OSB or plywood — is at a much colder temperature and is permeable enough to allow any vapour that does get through the assembly to eventually diffuse outward and escape during warmer periods. This one-way drying to the exterior is an important component of the assembly's moisture management strategy.
The vapour retarder belongs on the warm side of the insulation. Placing it anywhere else in a cold-climate wall increases condensation risk.
Common installation errors and their consequences
The most frequent installation error is puncturing or incompletely lapping the polyethylene, leaving gaps at electrical boxes, plumbing penetrations, and joints between sheets. Even small gaps allow significant vapour flow — vapour is driven by pressure difference, not just by the area of the opening. At those gaps, warm humid air contacts the cold sheathing directly and condenses.
A second common problem is installing insulation between the vapour retarder and the exterior, rather than between the interior finish and the retarder. This pushes the retarder into the cold zone of the wall, where its temperature can fall below the dew point, causing condensation on the retarder surface itself.
A third scenario involves adding exterior insulation without adjusting the vapour control strategy. When rigid foam insulation is added to the exterior of an existing wall, it raises the temperature of the entire original wall assembly. In some cases, this moves the dew point outside the original wall entirely, eliminating the condensation risk at the sheathing. However, if the existing interior polyethylene is left in place and the total assembly becomes very air-tight, drying potential can be reduced. The Building Science Corporation has detailed guidance on vapour control in assemblies with exterior insulation.
Variable permeance retarders
Membrane products with variable permeance — sometimes called "smart" vapour retarders — adjust their permeance based on relative humidity. At low humidity levels, they resist diffusion, performing like a Class II retarder. At high humidity, they become more permeable, allowing the assembly to dry inward when exterior conditions are warm and humid (a concern for mixed-climate locations that experience significant cooling loads in summer).
Products like Intello and MemBrain are used in high-performance building envelopes where the designer wants both winter moisture control and the ability for the assembly to dry inward during summer. For standard Canadian residential construction in purely cold climate zones, standard 6-mil polyethylene remains the most common and code-compliant approach.
Air barriers and vapour retarders: a critical distinction
The vapour retarder deals with diffusion — the slow movement of vapour molecules through solid materials. Air sealing deals with convective transport — the bulk movement of warm, moisture-laden air through gaps and cracks. Convective moisture transport moves far more water vapour than diffusion does, and it does so much faster.
In practice, addressing air leakage is more important than achieving a perfect vapour barrier installation. A wall assembly with a slightly imperfect vapour retarder but excellent air sealing will perform considerably better than one with a flawless vapour barrier but gaps around electrical outlets, plumbing penetrations, and top-plate connections.
The polyethylene sheeting used as a vapour retarder in Canadian homes also serves as the primary air barrier when it is properly lapped, taped at seams, and sealed around penetrations. The airtight drywall approach — using gaskets, caulk, and acoustic sealant to seal the drywall itself as the air barrier — is an alternative that eliminates the need for a separate polyethylene sheet.
Practical checklist for vapour retarder installation
- Ensure the polyethylene sheet is at least 6 mil (0.15 mm) thickness
- Lap sheets by at least 300 mm (12 inches) at all seams
- Tape all laps with a compatible acoustic sealant or approved tape
- Seal around all electrical boxes with gaskets or caulk before drywalling
- Seal around all plumbing and mechanical penetrations with acoustical sealant
- Ensure the sheet is continuous at the top plate and carried down to the sill plate
- Inspect for tears before drywalling — small tears can be patched with compatible tape
Provincial building codes in Canada have specific requirements for vapour retarder installation. The National Building Code of Canada 2020 sets the baseline; provincial amendments may require stricter standards in certain climate zones.