What is a vapour barrier?

A vapour barrier is a continuous sheet of polyethylene plastic installed between the drywall and insulation in an interior wall. Its primary role is to block water vapour from passing through the wall assembly and condensing inside the insulation or sheathing where it causes structural damage.

In Ontario, vapour barriers are mandatory in heated and finished garages under the Ontario Building Code. When a garage is conditioned (heated or cooled), the temperature and humidity differences between indoors and outdoors create pressure that drives moisture into wall cavities. Without a vapour barrier, that moisture accumulates in the insulation and wood framing, leading to rot, mold, and material failure over time.

The barrier must be installed on the warm side of the insulation, typically between the insulation batts and the drywall that faces the garage interior. Proper installation includes sealing all seams, penetrations, and edges with tape or sealant to maintain a complete envelope. Common materials are 6-mil or 10-mil polyethylene sheets, chosen for their low permeability rating.

Unfinished or unheated garages do not require a vapour barrier under code, since moisture accumulation is less of a concern without significant indoor-outdoor temperature differences. However, many builders in Ontario now install them anyway as a best practice when adding insulation. Builders working on custom finishes and heated garages routinely specify vapour barriers as part of wall assembly details.