Poly and Canadian building codes
This comes up quite frequently and I thought it might be useful to lay out what is actually required by the code. My description relies on the BC Building Code, but I am fairly confident it applies to all provinces that have based their codes on the NBC.
The code requires all building assemblies to include an air barrier and a vapour barrier.
The air barrier can consist of a variety of sheet or panels materials. It can be located anywhere in the assembly. That is the inner, intermediate or outside surfaces of the assembly (220.127.116.11).
The vapour barrier can be any material less than 1 perm. It has to be located “sufficiently close to the warm side of the assembly to prevent condensation at design conditions (18.104.22.168). The requirements further explain it must be “at a location where the ratio between the total thermal resistance of all materials outboard of its innermost impermeable surface and the total thermal resistance of all materials inboard of that surface is not less than that required by table 22.214.171.124.” This table has columns for degree days, minimum RSI ratio, minimum outboard thermal resistance, and sheathing thermal resistance.
The code anticipates the use of exterior foam as a vapour barrier and requires that the inner-surface of the foam meet the requirements of maintaining a condensation free temperature or the assembly is required to include another vapour barrier closer to the interior. ((126.96.36.199 Appendix A (thermal Insulation).
So in summary: Poly is only one of a variety of materials described in the code accepted as both air and vapour barriers. Other sheet goods, panel materials such as plywood and OSB, paints and coating, and foam are all explicitly mentioned. Exterior foam may be used without an interior vapour barrier as long as it is thick enough.