Insulate rafter bays under a flat roof, or just seal them?
The roof of my guest wing now has exterior foam with a minimum value of R-33. I am ready to start on the next phase of retrofitting the envelope.nIn this phase I have to decide how (or even whether) to get insulation in the 5 and a half inch bays above the ceiling, between the rafters/joists in this miniature, non-accessible “attic”. This attic is currently vented, and I must convert it to an unvented attic to get any value out of the external roof insulation.
The most important objective is to stop the movement of outside air into the space to that the cold roof becomes a warm roof. It would be great to add additional insulation of course but that’s secondary to stopping the air movement. I’m planning to wrap the fascia and the soffit vent with rigid foam and seal the whole assembly. So one option would be to leave the bays empty, but that means relying solely on the integrity of my sealing job.
Since I can’t visually inspect this “attic”, I have some concern about whether the wiring is sound enough. Given what I’ve seen in attics around town, I have no trust that there isn’t an open junction box or worse. I also don’t know if this concern would be allayed by blowing in fiberglass rather than cellulose. I did buy an inspection camera/flashlight tool (which I’m struggling to get to work) just to help satisfy myself that there’s no knob and tube wiring up there.
I have access to the bays for a hose from one side only, by removing the fascia, and the length of the bays I would be trying to fill is 12 feet. Another option is to “drill and fill” through the ceiling. Although I’m not eager to do it, I’m willing to demo and replace the ceiling if that’s the only way to do a good and safe job.
As usual, my thinking proliferates to fill the void of my lack of experience, so any and all suggestions are welcome.
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