Condensation on framing in spray foam ceiling retrofit
I have an old stone farmhouse in Allentown, PA, and it is my only wish that I can figure out how to make he roof even slightly more energy efficient. Last week I bit the bullet and ripped down all the drywall in the ceiling of my 3rd story finished attic space. I discarded the stink-bug infested R13 batts that had been up there (unvented), and scheduled a closed-cell spray foam team to come turn my drafty attic bedroom into a sealed up hot roof.
The decision to use CCSF is the only way I could figure out to get a reasonable level of draft sealing and thermal insulation in my situation — the bays are slighly less than 5″ deep (The rafters are 3×5 at 32″ O.C.), and it is a simple gabled roof. (The foam is low-GWP “Gen 4” foam, so I don’t feel so guilty about filling the bays with it.)
I have read up on unvented roof assemblies on this site and elsewhere. The sticking point for me is that the 3x5s are nasty thermal bridges. Since they are hardwood, I assume ~R-1 per inch, and so I will have R-5 thermal bridges covering 10% of my otherwise toasty ceiling. My plan is therefore to come in after the spray team and fur out each rafter with strips of 3/4 or 1″ unfaced polyiso to bring them up to R-10 or so before the drywall goes up. I am using unfaced polyiso because it has a perm rating of about 4 (vapor retarder), so I can prevent a double vapor barrier while blocking moisture transit into the framing. Sort of like what they do in this article: https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge.
Here is my dilemma — won’t the framing get cold in the winter, after I insulate the face of each rafter? Then, won’t moisture condense underneath the polyiso strips? In my climate zone (4/5), the code specifies a max a 60/40 ratio between the insulation inside/outside of the air barrier in an unvented roof, and on paper my plan does meet that requirement, but it still seems iffy to me. Would gluing/foaming around each of my furring strips provide some insurance against moist air getting to the framing?
And a related question, which I think might shed some light on my problem: In a hypothetical “normal” flash-and-batt job on a modern 2×8 ceiling, with only a few inches of foam that is then buried in fiberglass, why wouldn’t the framing right up near the foam also get cold enough to cause condensation? That part of the framing is closer to the outside than it is to the living space, the wood provides little insulation, and it is buried in fiberglass that keeps indoor heat away from it… Seems like these jobs would get condensation problems too.
I realize over-roofing is something to consider, but I could not figure out how to make it add up $$ wise or how to resolve various other details it brings up, and it would not do anything for my draftiness problems.
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