A few times in the last year, our inspectors have run into a critical oversight that’s caused problems: incorrect placement of vapour barrier in floors over an unheated space.

First off, there’s a general rule on where vapour barrier goes: the warm side of the building. For things like walls and ceilings, that’s a really obvious thing. However, when building a stud floor over, say, an unheated crawl space – which is usually insulated later in the build than other sections – it’s easy to overlook the vapour barrier placement.

The image provided here illustrates how it should be done: vapour barrier is drawn over the joists before the sheathing goes down.

What some folks might overlook is the vapour barrier when building a suite over a heated garage. Garages are considered unconditioned (unheated) spaces, and for good reason: they’re often exposed to serious drops in temperatures, even if insulated and heated like the rest of the house. Further, garages must have a layer of vapour barrier encapsulating them to keep vehicle fumes away from the rest of the house. In this case, the old rule – vapour barrier on the warm side – holds true.

There is one critical exception: if closed-cell foam is used as the insulation. In that case, the foam serves as the vapour barrier, and can be applied on the underside of a sheathed floor without vapour barrier being in place.