Hello-

I’m hoping to get some input on my plan for creating a hot roof. I’m doing a major renovation of an old 900 SF house in Seattle. For insulation I’m using pieces of polyiso foam board that I get for free from a local door manufacturer (these are the cutouts left over from installing full lite glass, etc- I peel the fiberglass skin off, cut to a loose fit, then foam the gaps). So far I’ve been using this approach in the walls only, and as everything outboard of the foam can dry to the outside, I don’t have any worries. I’m now looking at doing the roof, and am planning to build a straightforward hot roof assembly:

1) Framing would be existing 2×4 rafters (with lots of sisters to fix some really bad sags) with new 5/8″ plywood sheathing.

2) Roofing material: 30-lb felt covered with Certainteed Landmark TL shingles

3) insulation: 2×4 rafter bays filled with foam board, gaps sealed with foam; then 2x strapping perpendicular to the rafters with an additional 1.5″ of the reused foam pieces, gaps foamed; then a final layer of new 1″ polyiso board over everything, taped, to provide a thermal gap; then drywall, taped and painted. At R6 per inch that would give me an R-36 roof.

My question is this: I’m worried that because the foam board is not directly attached to the underside of the sheathing (as would be the case with spray foam), there exists the possibility for the underside of the sheathing to be a condensing surface, even if there is only a very very thin air space (fractions of an inch) between the board and the sheathing. If all goes well there shouldn’t be movement of vapor from inside the house, but I am assuming there will be some potentially moisture-laden outside air that makes it to the sheathing. I guess this is related to a general concern about sandwiching sheathing in between two vapor retarders.

Sorry for the long post- any advice would be much appreciated!