I have a low pitch (1/12) single slope shed roof on my house. (Marine Zone 4c near the coast with very little snow and ice) The framing is 2×8 filled with insulation. Drywall on bottom and skip sheathing on top- no ventilation. The existing roofing is built-up asphalt.
I plan to remove the built-up roofing and install 2″ of polyiso, then 1 and 1/2 inches of EPS, 5/8″ of plywood, then a standing seam metal roof with mechanically seamed joints.
Should I Install 2×4 strapping between the foam and the plywood to leave a ventilation gap? Ice dams are not a problem here, so I’m not sure I need it. Might it help with cooling in the summer? I’m not sure this is a problem either, as the house never heated up beyond my comfort level last summer without any AC.
If it is not necessary to ventilate this assembly, could I install 2×4 strapping over the polyiso and then put the 1 1/2″ of eps between the 2×4’s? I realize this will decrease the thermal performance somewhat, but I think it would allow easier installation of the plywood and provide some room for adjustment in order to keep the roof deck in plane.
Finally, should I install ice and water shield over the old roof sheathing? I’m hesitant about trapping moisture, but it seems to me the foam will be fairly vapor impermeable anyway so I might as well seal it up good.
Any insights are appreciated! Thanks.
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