Flat roof mid-century house with dropped ceiling
Hello – Thank you for the extensive resources your provide across the various insulation spectrum. I have read these both extensively:
I am in climate Zone4 (westchester NY) and have a 1955 build, flat roofed mid century house which has had a new roof installed (existing decking, +new felt pad+2″ polyiso+EPDM membrane). Yes, I should have gone with 4″.
The walls are concrete block, up to 10ft heigh, and 2×10 joists run across the block creating a 4ft overhang all around the exterior. The soffit this creates has a 1″ vent running around the entire perimeter. The ceiling is framed for a 9″ height (drop ceiling) using 2x6s, leaving about 14″ between the ceiling framing and the bottom of the roof joists. It also had a whole house fan (60″!) in a doghouse which has been removed and roofed over.
So it was historically a cold roof assembly. Which leaked water miserably due to neglect prior to purchase. upon taking down wire+plaster ceiling and stripping the tar and gravel, we found no rot amazingly, so seems the venting was working properly for last 70″ years, though the 4″ batt was disintegrated at this point.
considering we only have 2″ iso on top, and considering the 150+ openings created by the joists-on-bock to the vented soffit, what is best way forward?
1. adding 2″ closed cell right on the underside tongue-groove, 2″ all around in the 150+ openings and down the block wall to ceiling height creating a conditioned interior, then add another 8″ of open cell? Im concerned that we wont get a proper seal between the joists and block wall at overhangs. Also would leave about 19-20″ of conditioned space before sheetrock ceiling.
2. add 12″ + of open cell insulation to really get into all the nooks+crannies of the openings/vents, which would provide some vapor permeation around the ends. this would also enable the foam to continue over the bottom of the joists to assist even more in reducing any thermal bridging (again, only 2″ iso on top). would leave about 15″ of conditioned (if i can call it that with open cell) space between sheetrock ceiling and foam.
3. Just never think about the 2″ Iso on top, consider it lost, and re-create the cold roof assembly but with modern insulation between/around/over the 2×6 ceiling rafters and rely on the seemingly functional venting designed 70 years ago. if so, whats best here?
Additional notes: No Ducting. All mini-split Mitsubishi units and cast iron baseboard heat. There are to be about 30 IC rated can light housings throughout the 1700 sq ft home.
Thanks, apologies for the novel.
Its just such a unique setup im driving myself crazy. but less crazy than a moldy, rotten roof would make me.
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