Grace Ice and water shield HT on roof sheathing below rigid insulation
Hello GBA community. I will soon be building our home in NC, on the boarder of climate Zone 3 & 4. I have been doing alot of home work on this and other sites about the different building techniques for green and energy efficiency. Today however i have a question about the G i&WS. i will be building out the roo like such. plywood sheathing, I&WS, 4 inch rigid foam insulation, plywood, standing seam metal roofing. my question is weather or not to use i&ws on the structural roof decking or something such as vycor env-s. if i were to use the vycor env-s on the sturctural seathing should i use i&ws on the top seathing below the metal roofing? thank you for your opinions.
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Caleb, the only reason you would need a vapor-permeable membrane over the top layer of sheathing would be to allow the sheathing to dry to the exterior. The sheathing should not get wet, and almost no drying happens through the roofing, so I don't see the need for a vapor-permeable membrane unless it's less expensive than other options.
ok thank you for the input.