Roof Membrane/Underlayment Recommendations
Hi, we’re going to turn our uninsulated attic into conditioned space. It’s a 1900 era farmhouse in northwest Massachusetts with board sheathing over real 2×6 rafters. 12:12 pitch. Current plan is to remove existing slate (it’s in pretty rough shape) and put down peel and stick membrane as air barrier, then 1.5″ polyiso insulation between horizontal 2x4s, another layer of 2.5″ polyiso, then another layer of 3″ foil-faced insulation. (Total 7″ of rigid insulation outside sheathing.) This will be covered by a layer of plywood sheathing screwed through to the 2x4s, then underlayment and finally standing seam metal roof. We’re also planning to put batt insulation between rafters and cover with shiplap or bead board.
I’m looking for recommendations for 1) the peel and stick membrane to go over the board sheathing and 2) for the underlayment to go under the metal roof. Do we need a high heat underlayment under the metal roof (planning to use plain galvalume, which should be pretty reflective)? Should the underlayment be permeable? Is stick down better for the underlayment?
I was planning to run 3′ of ice and water around bottom of roof over plywood sheathing and in dormer valleys, then run underlayment over that. Is that a good idea?
Any advice much appreciated.
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