Unvented flat roof, insulating from outside?
Boston, MA area here (climate zone 5A), with a flat roof, unvented. I’m replacing my roof and fascia boards on my 1950s era house, and am hoping to do what I can to insulate between the 2×10 ceiling joists while re-roofing (as well as adding 4″ of exterior rigid foam insulation ontop of the sheating/below the epdm membrane).
From my ceiling on up, it is a typical New England assembly with sheet rock, strapping, ceiling joists, sheating, epdm currently. I have a few recessed ceiling lights (low profile LED wafers).
What options do I have for exterior-installed insulation? Here’s what I’ve gone through thus far with options:
Dense Pack Cellulose
Pros: Cheap, easy to blow in (either blown through the fascia boards pre-replacement, or even from the inside ceiling and patched)
Cons: can lead to moisture problems? Reading articles here seem to offer contradictory advice where “it can sometimes work”
Closed cell spray foam from above
Pros: higher R-value
Cons: could create double air barrier sandwich (one from the closed cell against the ceiling sheetrock, the other in the exterior foam/epdm roofing layers)
Open cell spray foam from above
Pros: cheaper than closed cell, maybe would be permeable to allow drying to the inside?
Blown-in mineral wool
Pros: no concern about creating more plastics/global warming potential of blowing agents.
Cons: Literally cannot find a contractor in the area who does this.
I’m not even considering this due to the rot potential.
So my question to you, smart folks of this forum, is: what pros/cons for these materials am I missing? Am I missing any other insulating materials that might be a better compromise?
It really feels like the only way for me to insulate this roofing assembly is to rip down all the sheetrock from my ceilings and insulate from below, but for a variety of reasons that isn’t possible at the moment.
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