• Insulated assemblies must be constructed with a vapour barrier to prevent water vapour from entering wall spaces, floor spaces, attic or roof spaces

Illustration Source: Summerhill

9.25.4.1.(1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into the wall spaces, floor spaces or attic or roof spaces.