Insulating shed roof
(I asked a version of this a year ago but new parameters/information make it easier to ask again):
I will be re-roofing a 4:12 pitch shed roof (about 880 sq. ft.) on our mid-century modern home in Maryland/DC region. The room below is fully finished and so any insulation work has to happen from above. The roof is 2×10 rafters (the low side has a 2′ overhang and is vented at the soffit; the high side terminates on an upper clerestory wall and has venting through the fascia that overhangs the clerestory wall about 2″). It is currently insulated with 9.5″ of fiberglass insulation installed (in 1977) (that’s right, the 9.25″ “vented” cavity is filled with 9.5″ faced fiberglass; thermal images show lots of poorly placed insulation, gaps, etc.). The roof faces western exposure. Under the rafters is 1/2″ drywall with latex primer/paint (no poly). The building has 2×6 and 2×8 walls and has poly vapor barrier; everything has been carefully airsealed and the windows are fairly airtight for their age.
I will re-roof with Union Corrugating standing seam, and they spec plywood sheathing, hi-temp ice-water, a slip sheet of synthetic underlayment, and the standing seam panels attached with pancake heads into the plywood.
I want to redo the insulation when we re-roof. I see a couple of options and want to hear what others would argue is the proper approach.
Option 1) Plan to add 7.5″ of XPS in cut/cobble assembly (~R35) and leave a biut less than a 2″ vent space. This is the cheapest approach to achieve foam, allows me to (maybe) leave some pieces of plywood in place and while a pain, save a lot of money (when plywood is $50 a sheet), and allows me to work in stages on my schedule, not on foam installers schedule. Questions: Do all the edges need to be foamed all seven inches, given there will be a vent? Do the butt joints need to be sealed? Since vented, can I be fussy about the first layer of edge sealing and then just dry fit the top layers?
Option 2) The $6800 quote I just got from the most recommended foam insulator: I remove existing roofing, existing plywood decking, leave it all tarped and vulnerable until they spray foam between the rafters (closed cell to 5 1/2″ to achieve R-38, leaving 3-4″ of vent space)), then I lay new decking, and proceed. Company estimator said beyond 5 1/2″ is decreasingly pointless. (they also tried first to suggest fully filling the cavity with open cell was acceptable and I said, not according to everything I’ve read, and they quickly moved to closed cell with a vent)
or Option 3) I add 2″ SPF strips to the rafters and make the 2x10s effectively 2x12s and then place R-38 of Rockwool and maintain a 2″ vent channel. Seems to have the advantage of cost effectiveness, I’ve been told not to worry about windwashing, and the work can all be prepped. Getting the 2″ strips attached effectively so the sheathing has the appropriate strength will be key (I guess construction adhesive and attach the sheathing with 3 1/2″ framing nails so there is 1″ embeddment into the existing rafter?)
Any other options I’m missing? Thanks for any guidance. and thanks to those who responded a year or so ago.