Insulating Unvented Cathedral Ceiling
My wife and I are designing and building a home in Western Washington (I’m a professional structural engineer). One of the spaces is a great room with a cathedral ceiling featuring exposed trusses. Due to the configuration of the roof (specifically, an adjacent roof with a perpendicular ridge that eliminates the possibility of eave vents for approximately 3/4 of one side of the roof), I’m leaning toward an unvented assembly. I would have preferred venting it, but I don’t think the configuration of the roof will allow it.
I found a presentation Martin Holladay did in 2016 regarding cathedral ceilings online – great resource! (This is the presentation I’m referencing: http://conference2016.eeba.org/Data/Sites/2/media/presentations/Holladay-Getting-Cathedral-Ceilings-Right.pdf) Specifically, I’m currently planning to do what the presentation calls Option 1 – full closed-cell spray foam. Here in WA, the energy code allows R-38 for cathedral ceilings, so that’s my goal. I’ll have 2×8 roof framing members at 24″ OC that’ll interrupt the insulation periodically and some air space between the insulation and the gyp ceiling.
Do I need to be concerned that the 2x’s interrupting the spray foam will cause discontinuities in my thermal envelope and vapor barrier? I’m somewhat nervous about moisture collecting and causing rot of the roof sheathing. Also, do I need to be concerned that the dew point of the assembly is somewhere in the spray foam, so condensation can form in the sheathing? Or will the sheathing be able to dry to the exterior in that case, so no concern?
Are my concerns valid, or am I being overly cautious?
Any other ideas?
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