My plan to insulate a low slope roof in a moderately snowy climate (Zone 6)
I have posted 2 questions recently. However, having just signed up for a membership, I now am better read and a little more clarified. I’m in Burlington, VT, Zone 6. The more I learn, the more I realize I have a very poor candidate of a roof to insulate. However, I already stripped it down to studs and think I will move forward with the best practices I can implement. I am asking for one last review of the plan and any revisions or feedback folks may propose.
I have a low slope roof on an addition to my house (added on 2 owners ago). The roof slopes from 120″ to 110″ over 10 feet. So, I think this is a 1:12 pitched roof? I have 2×10 rafters, and it seems like one of the previous owners added soffit vents, thin foam baffles with about an inch of air space, ceiling and wall insulation, but no other venting or heating. In fact, there’s some minor evidence of mold against the ridge beam where the rafters run from. Probably because of the inflow from the soffits and then the bafles that terminate a few inches from this ridge board, releasing air with nowhere to go? On this forum and elsewhere I’ve been pointed to adding high point exhaust venting options like Cor-a-vent. However, it appears most of these have minimums like a 3:12 roof, which I do not have.
In Martin Holladay’s article, “Insulating Low Slope Residential Roofs,” he does describe a venting method that Joe Lstiburek recommends. However, it sounds like the execution needs to be flawless and I don’t know if this is less risky than where I’ve landed.
So, I’ve landed on going with an unvented attic, with 4″ of spray foam that has an aged r value of 30 followed by R-23 rockwool. This addition also has a gravely asphalt roof in 3 separate long sections – perhaps built up roofing or modified bitumen? I don’t believe there’s any insulation above the sheathing. I don’t see any evidence of a snow/ice membrane.
I was also warned about ice dams, including potentially on this set up. It seems from a study I read by Tobiasson, Buska and Greatorex, from either 1998 or 2001, that with my ground snow load and my plan for insulation, that I will be low risk for ice dams. That seems to conflict with some info I believe I’ve come across here on GBA.
I believe the primary recommendation would be to pull the roof apart and add insulation above the sheathing. Is that correct? That seems out of reach for my budget and timeline (I’m willing to compormise on the timeline, but can’t really on the budget).
So, essentially, I’m looking for one last round of feedback or advice before I authorize the spray foam contractor who is ready to spray this Thursday.
Thanks, as always.
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