Vapour barriers are one of the least glamorous and most important elements of a renovation. Get it wrong and you’ve built a moisture trap inside your walls.
The Basic Principle
Warm air holds more moisture than cold air. In a heating-dominated climate like Ontario, indoor air (warm and humid) wants to move through walls toward the outdoor (cold and dry). A vapour barrier on the warm side of the insulation prevents this moisture-laden air from reaching the cold side where it would condense.
The rule: warm side of insulation, not outside it.
Common Mistake 1: Vapour Barrier on the Wrong Side
Putting poly on the exterior side of insulation in a cold climate is a disaster. Moisture from the indoor air moves through insulation and hits the cold exterior barrier — it condenses and has nowhere to go. This creates mould and rot within years.
Correct for Ontario walls: poly (6-mil polyethylene) on the interior face of stud bays, facing the heated space.
Common Mistake 2: Using Vapour Barrier in a Humid Climate Zone
This is less common in Ontario (we’re decidedly cold climate), but worth knowing: in climate zones 1–4 (southern US, coastal BC), vapour barriers can cause similar problems in reverse — trapping summer moisture.
Common Mistake 3: Penetrations Without Air Sealing
A vapour barrier full of unsealed holes at outlet boxes, pipes, and light fixtures is largely ineffective. Air leakage through penetrations carries far more moisture than diffusion through the poly. Every penetration must be sealed with acoustical sealant or red tape.
Common Mistake 4: Compression of Insulation Behind Poly
Poly stapled tightly to studs compresses faced insulation in the stud bay. The batt should fill the cavity loosely (un-faced batts) with poly installed flat and smooth across the face. Compression creates cold bridges and reduced effective R-value.
The Ontario Building Code Requirement
OBC Part 9 requires a vapour barrier with a maximum permeance of 15 ng/(Pa·s·m²) for walls and ceiling assemblies in the thermal envelope. 6-mil poly meets this requirement easily.
Building science questions, inspector experiences, and renovation techniques at home.renovation.reviews — Canada’s renovation community.
LF Builders | home.renovation.reviews