Many builders have heard the phrase, “Walls should be able to dry in at least one direction.” The rule usually makes sense. If you want to install rigid foam on the exterior side of a wall — foam that will prevent outward drying — it’s usually a good idea if the wall can dry to the interior.
What about insulated roof assemblies? Should the same rule apply?
If a building has an insulated roof assembly — either a cathedral ceiling or an insulated low-slope roof — there are several ways that the roof sheathing may be able to dry.
1. In a conventional vented roof assembly — one with a ventilation channel between the underside of the roof sheathing and the top surface of the insulation — the roof sheathing can dry by giving up its moisture to ventilation air that enters at the soffit vents and leaves at the ridge vent.
2. In an unvented roof assembly with rigid foam above the roof sheathing, the roof sheathing can dry toward the interior by diffusion, as long as the insulation between the rafters is vapor-permeable, and as long as the ceiling materials are also vapor-permeable. Fiberglass batts, mineral wool batts, and cellulose are all vapor-permeable. Gypsum drywall is also vapor-permeable. This approach works well, as long as the builder remembers not to install any interior polyethylene. (Of course, in the illustration above, the second layer of roof sheathing — the outermost layer — can’t dry inward. But that’s a discussion for another article.)
[Image credit: ATAS International]3. Ordinarily, an unvented roof assembly with closed-cell spray foam installed on the underside of the roof sheathing (for example, a flash-and-batt assembly) does not allow the roof sheathing to dry inward — at least, not much. However, this type of roof assembly may be able to…