Weighing insulation options for attic, roof, and gable ends
I’m seeking insulation advice to turn our 1950s cape cod attic into living space. Here is a description of the situation:
(1) Location is near Harrisonburg, VA (zone 4)–code level roof insulation is R38.
(2) There is zero insulation in the walls and roof (there is cellulose in the attic floor).
(3) The walls from the inside out appear to be 2×4 frame sheathed in 1x lumber wrapped in tar paper covered in rigid siding (transite, maybe?) covered by a plasticy, spongy WRB (I assume) clad in vinyl siding.
(4) There are no (intentional) soffit vents, just a fan at each gable end. However there is daylight at the eaves.
(5) Insulating the exterior roof deck is not feasible.
(6) I’d like to minimize encroachment of the roof into the room. The rafters are 5.5″ deep, 16″ o.c.
(7) Gable ends are north and south. There are two west-facing dormers.
I think the air-sealing qualities of spray foam would be greatly beneficial here, and the local contractors use Demilec closed cell which claims R7.4/inch. That would allow me to reach code-level insulation without deepening the rafter bays. Another option is for spray foam up to the collar ties (where a ceiling will be installed) and then blow in cellulose on that ceiling. That is significantly cheaper, but does it make sense to spray foam against the lower part of the roof and have no insulation against the upper part? What would I have to do to ensure proper venting of that space (preferably without the fans)? Also, how can I best address the daylight at the eaves? Backing rod & caulk? I assume it’s a bad idea to leave it up to the foam to take care of it since some would could end up on the exterior.
As another option, if I can source Kingspan Kooltherm, does it make sense to cut and cobble the first 2 or 4 inches of the roof insulation and encase it in closed cell spray foam (an idea derived from https://www.greenbuildingadvisor.com/article/high-humidity-in-unvented-conditioned-attics)? I am assuming this would be cheaper than a 100% spray foam fill (using myself for labor), but do not yet know the cost of Kooltherm.
Then again, I haven’t even addressed thermal bridging. Would 1″ polyiso be sufficient? This article seems to say “yes”: https://www.greenbuildingadvisor.com/article/walls-with-interior-rigid-foam.
As for the walls, there are gaps in the sheathing large enough to stick my fingers in, so they need a lot of air-sealing help. Does it make sense to use closed cell spray foam on them when the drying potential to the exterior is suspect (see point 3 above)? Would I be better off making the gable-end air-barrier at the drywall and using cellulose to provide a hygric buffer? The contractor dissuades me from using open cell on the walls because because it is a different installation crew from the closed cell (ergo high fixed cost).
Thanks for any and all help!
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