Venting a Roof With Exterior Rigid Foam
I’m building a garage in central Maine (Climate Zone 6) with an apartment above the garage (I’m going to insulate the entire building so the garage can be kept at least decently warm in the winter), and I’m trying to build it with energy efficiency in mind while still not breaking the bank.
Following the concept of “the perfect wall” as explained in Joseph Lstiburek’s BSC article here, I’m planning to use 2″ of rigid foam (R10) on the exterior of the entire structure (outside the sheathing, inside the siding & roofing), with fiberglass insulation in the stud and rafter cavities.
My impression was that one of the purposes of the insulation outside the sheathing was to prevent moisture from condensing onto the sheathing, leading to rot.
When I went to my trusty local lumberyard with my plans, my guy there suggested venting the attic space with soffit and ridge vents and rafter baffles (diagram image included). That sounded wrong to me, because it would let moist air in underneath the foam insulation… which would seem to defeat the entire purpose of having the insulation there in the first place, both from a moisture and from a thermal perspective.
Instead, I had thought I would leave the attic space unvented and airsealed, with an air barrier between the finished, conditioned living space and the unfinished, unconditioned attic space (that air barrier is gypsum drywall, taped, mudded, and painted w/ latex paint). (my idea is included in the diagram attached)
I was searching through other previous questions here on GBA for the answer to this question, and I found another thread in which Martin referred to this article, all about insulating sloped ceilings:
In that article it explicitly says that vented attics are incompatible with my insulation plan of foam outside and fiberglass inside. So it looks like my lumberyard guy was wrong.
Additionally, that article also says that in my climate zone (6), the minimum exterior foam to keep sheathing above the dew point is R25…. the two inches of foam I was planning to use is only R10.
What should I do? How should I build this?
Thank you in advance for all your help. I am not a professional builder, just an amateur who knows just enough to be dangerous…
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