Do I still need a vapour barrier if spray foam insulation is being installed?
There is some varied opinion on this point. Spray foam insulation at one and a half inches thick is a recognized vapour barrier. If it is installed between metal studs, there is no need for a vapour barrier. If the studs are wood, some argue that a vapour barrier is needed for the studs because wood is porous. Others would say that the penetration through a 2×6 stud is so minimal that it does not warrant the additional cost of a vapour barrier. In commercial applications, at any material transition point, a sealing membrane is required.
