OK to use commercial roofing boards for wall insulation?
I am insulating an early 1950s Cape Cod style ventilated attic in Lexington, KY (IECC Zone 4) and have reached the following tentative conclusions:
1. I can’t replace the roof, so I can’t insulate above the deck.
2. There is too much thermal bridging through the rafters, too much thermal mass in the concrete board finished walls, and not enough room in the 2×6-framed rafter bays to effectively insulate between the rafters.
3. The best way to make the attic living space habitable is to take out the existing finished ceiling and knee wall, install 4″ of polyiso below the rafters and inside the wall studs, and refinish the living space with drywall.
1. Does this sound like a reasonable approach, or is there a better/easier way? Is this enough insulation?
2. I can get 2″ Firestone ISO 95+ boards for a much better price than XPS foam here ($22 vs ~$30 for 4×8) but the ISO 95+ boards are faced with a fiberglass reinforced craft paper on both sides. Manufacturer says boards are intended for commercial roofing, and offers no guidance on sealing seams. Has anyone used commercial roofing insulation for interior walls and ceilings? How did you seal the seams? Is one kind of foam or tape better than another?
A search on the GBA site showed one thread from 2011 questioning whether the ISO 95+ boards were impermeable. The technical data sheet on the manufacturer’s web site says ASTM E96 “Moisture Vapor Transmission” is <1 perm. Any reason not to use this product as wall insulation? If this question has already been answered, please point me to that thread. Thanks for you consideration!
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