Polyiso on both sides of roof sheathing
I have 4in of polyiso on the exterior of my roof sheathing (zone 3) Using an r30 batt on the inside.
My question is in regards to areas where we have recessed lighting. We are using 4in cans (dont want to use the thin wafer leds).
I am considering cutting some squares of polyiso to use above the cans, full width of the rafter bay and 6in or so beyond the can on either side length wise. I can probably get 5in of polyiso (2in + 2in +1in) above the can, that will give me r27ish.
This way i can just butt the batt into the flat edge of the polyiso stack and not worry about compressing or cutting the batt around the light.
My only “concern” is that in those spots the roof sheathing will be sandwiched between polyiso on both sides with 0 drying potential.
I am inclined to think that this is ok because its just in some spots, most of the underside of the roof will be using the batts.
Just wondering if anyone has any thoughts about that?
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We did exactly that on a current project of mine. Because the areas are small, any moisture accumulation will be able to dry to the edges of the assembly.
Thanks Michael,
Just curious how you attached the polyiso on the underside? Im thinking of just taking some 2x blocks and screwing them into the rafters to hold the polyiso tight to the underside of the roof deck.
Also did you cut the polyiso short of each rafter and fill the gap w sprayfoam or just cut it as tight as you could to the rafter?
Thanks
I didn't specify a detail but that's what the construction crew did. My typical detail for that situation is shown here: https://www.greenbuildingadvisor.com/article/design-for-low-carbon-building, though not very clearly--2X framing with plywood behind, but that's because I usually do R-60 roofs.
You have 4” polyiso above the sheathing in an area where you need 1” for condensation control. Why do you think you will ever have condensation issues? The only way moisture should find its way in your rafter bays is if you have a roof leak.