Compact re-roof with polyiso over spray foam?
I’m in the Lehigh Valley, Climate zone 5A (borderline 4A), and I have a roof that needs to be redone and insulated far more than it is now, it was originally a cold cellar back in the day, and is a relatively small area- 17′ x 17′.
Right now It’s true roughcut 2×8″ on 24″ centers, topped with 1″ planking that looks to be original, all in good shape, followed by a vapor barrier, a layer of 2×6″ run parallel to the 2×8″ under them with fiberglass in between, then at most 2″ of rigid on top, though i suspect less, or nothing but sheathing. Roof is something between a 1 and 1.5 pitch.
I plan on removing at least the roof sheathing and roof (old rubber roof) to install metal roofing and new insulation, unvented due to the low pitch, and I’m trying to figure out a cost effective way to improve on the insulation.
I’m height limited outside to roughly 8-9″ due to a roof intersection, and Ideally I want all of the old 2×8 exposed inside, but I’m willing to settle for less if needed. I want to keep them open as much as possible as the ceiling is fairly low as-is, having the open bays helps make the room feel much more open, plus its an 1801 Stone house, with exposed beams and stone throughout, so hiding them altogether would be criminal.
I looked into SIP’s but didn’t like the cost and crew required. I’m fine doing a roof with contractor friends, but renting a crane/pro crew sounds like more than I want to do. I enjoy working slow.
Currently my plan is to use 6-7″ of Polyiso on top of the planking, (either 3 or 3.5 inch-depending on what i can find recycled) then furring strips or plywood, and standing seam. I realize that will only be R30-35, if there is headroom I’d love to go up more unless its unadvisable to just roof with rigid over 6″ deep?
I also had the idea of doing a 1-2″ flash of spray foam on the inside of the planking between the 2×8″ to help boost the R-value and air-seal the living space, but I wasn’t sure if this (having the planking sandwiched between the foil-faced polyiso and the spray foam) was advisable- would it just trap moisture and rot, or would it be fine?
If it IS ok, another option would be flash and batt under the planking, and rigid on the outside, but again I’d hate to hide the joists.
If there are other options I’d love to hear them! Cellulose in the 2×6 bays and rigid on top? Some other combination?
Mike in LV
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