We are building a new home in Genelle with several cantilevered window bays that stick out beyond the main floor. There is a big debate going on about how to deal with the underside of these bays. Can you offer any opinions?

I have seen several different strategies. Here are some examples of assemblies that should be avoided. Do not install a plastic vapour barrier either under the sub-floor plywood or under the insulation before installing the soffit. Do not install a sealed, unvented soffit under the floor assembly. Here are the reasons for avoiding these designs. The sub-floor acts as a vapour barrier in this floor assembly. Therefore, you do not need any other vapour barrier anywhere else in this floor cavity. It will only provide a condensing surface for any stray moisture that gets through the sub-floor and traps that moisture in the assembly. You also don’t want to seal this floor assembly tight with a plywood soffit under the cantilever. Again, any stray moisture will get trapped on the upper surface of this “lid” and cause dampness and mildew in the floor cantilever cavity.

The most functional design, in my opinion, is as follows:

1.      Insulate between the floor joists, under the plywood sub-floor, with fiberglass batt insulation.

2.      Nail a vented plastic or metal soffit material to the underside of the floor joists (under the insulation) so any moisture that does leak through the plywood sub-floor can escape to the exterior without hitting a condensing surface inside the assembly.

Some folks install plywood as a soffit with breather holes drilled in the plywood. This can work, but I prefer the fully vented soffit so moisture has no opportunity to lay on a flat surface. If you have an existing fully sealed plywood soffit under your cantilevered bay window(s) or cabinet nook(s), drilling holes in the plywood is a good retro-fit because it will help these cavities breath. 

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This article was written by Steven Cannon, a Registered Building Official and partner in Lynch Building Inspection Services Ltd. of Nelson, British Columbia. It originally appeared in The Nelson Express.

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