I'm replacing a 30+ yr old 20'x'22 flat roof over a 1 story extension off the back of my 2 story colonial. It has leaky spots that will drip during a heavy rain, even after having it resurfaced a couple years back and numerous attempts to patch after that. After dissecting from the inside of the house I found considerable amounts of deck rot. The leaks seem to be coming in behind the flashing. It will be interesting to see what's going on where the extension meets the side of the main house.
I'm in upstate NY (Zone 5) so we can get both sides of the hot&humid/cold spectrum. The roof is shaded in the summer so heat isn't normally an issue. It has baseboards on 3 of the 4 sides but can still get chilly in the winter. I'd like to improve that if I can while re-roofing. I can't tell if the current roof is technically a warm roof or a cold roof.
It consists of:
-) 2 layers of sheet rock on the ceiling attached to 1x3 furring strips running perpendicular to the ceiling joists.
1) 2x6 joists (I think)
2) 2x? furring strips that sit on-top/parallel to the joists to give slope to the roof
3) plywood decking
4) 1/4"-1/2"...insulation board/stuff? it looks like drop ceiling tile but was too wet and degraded to really identify
5) 2+ layers of membranes
-) Fiberglass batts stuffed in the joist bays at the walls (some of which has been matted flat from water damage)
-) Aluminum soffits that don't seem to have much in the way of venting (even if the batts were're kind of blocking things).
...so little to no ventilation or insulation.
The contractors that I've had come give estimates have all suggested redecking, putting down 1"-6" of ISO, and EPDM rolls wide enough to get only 1 seam. Most of them said they rarely go above 3" even in their commercial projects. But 3" only gets me at R16-20 (which is probably still a step up compared to what I have now). Recommended amount for the area is R38. 6" gets me R33-40, even if the added thickness looks silly (it's on the back of the house so I don't really care).
I'm looking for advice on insulation. Should I go with the 6" and call it a day? Is that overkill? Could I supplement with fiberglass in the joists? If so can I fill the whole cavity or do I still need an air gap even with the ISO on top? Could I spray in foam when I (eventually) get to gutting the inside of the extension and will that impact whether or not I can have some batts up there?
Any feedback would be greatly appreciated.