What is a vapour barrier?

A vapour barrier is a layer of material with very low permeability to water vapour, installed in building assemblies to control moisture movement. The barrier stops water vapour from traveling through walls, crawlspaces, and foundations, which protects insulation from absorbing moisture and prevents condensation that can lead to rot, mold, and structural damage.

Vapour barriers are typically made from polyethylene sheeting, kraft paper facings on fiberglass batts, closed-cell spray foam, or specialized membranes. In the Greater Toronto Area, they are most commonly placed on the warm side of insulation in exterior walls and in below-grade basement assemblies. The location matters: in heating climates like the GTA, the barrier sits between the heated interior and the insulation, blocking indoor humidity from migrating outward and accumulating in wall cavities during cold months.

Building code and best practice placement depends on climate zone and assembly type. Basement walls often include polyethylene sheeting against the concrete before insulation and framing. Exterior walls in new construction or renovation projects typically use kraft-faced batts or rigid foam that serves a dual function. Improper placement or missing barriers can trap moisture within the wall cavity, shortening the life of framing lumber and insulation. When planning basement renovation or exterior wall work, understanding vapour barrier location and material selection is important for long-term durability.