Canadian building code question

What are the building code requirements for vapour barriers in Canada?

The Canadian building code requires a vapour barrier with permeance generally not exceeding 60 ng/Pa·s·m² installed on the warm side of insulation in conditioned residential assemblies, subject to provincial overlays and exemptions for some assemblies.

Vapour barriers are routinely confused with air barriers, but the code treats them as separate building-science functions. The vapour barrier controls diffusion; the air barrier controls bulk air movement. Both are required, but they can be combined or separated depending on the assembly.

What to check first

  • Confirm the required vapour barrier permeance and the location in the assembly for the applicable code edition.
  • Distinguish the vapour barrier role from the air barrier role; a single membrane can serve both, but the test criteria differ.
  • Check whether the climate zone or specific assembly type allows or requires a different permeance class.

Jurisdiction notes

National baseline

Part 9 of the NBC sets a default vapour barrier permeance threshold for residential conditioned assemblies.

Cold-climate provinces are strict

Northern and prairie provinces apply the vapour barrier rules consistently because diffusion failures are more visible there.

Air barrier is a separate requirement

The air barrier has its own continuity, durability, and air-leakage criteria that the building code addresses independently.

Work through it in this order

  1. Identify the conditioned assemblies that need a vapour control layer.
  2. Select a vapour barrier material with permeance below the code threshold and compatible with the air barrier strategy.
  3. Place the vapour barrier on the warm side of insulation per the code direction.
  4. Detail penetrations, laps, and terminations consistently with both vapour and air control roles.
  5. Document the assembly so inspectors and trades follow the intended layer order.

Common questions

Is polyethylene still required as a vapour barrier in Canada?

Not specifically. The code requires a material below the permeance threshold; 6-mil poly is one common option, but other materials qualify.

Can the air barrier and vapour barrier be the same layer?

Yes, if the chosen membrane meets both the permeance criteria and the air-leakage requirements for its role.

Where in the assembly should the vapour barrier go?

Generally on the warm side of insulation in conditioned spaces; specific assemblies and exterior insulation strategies can shift this.