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Hey, I have a simple question about spray foam with XPS. Is there a "best" foam to use? Any kinds incompatible with the foam board? I'd prefer to use a single-can foam with a gun, 'cause otherwise I'm going to wind up with that stuff stuck to everything, including me.

Also, I'm aiming for a friction fit between rafters, but since the house was built in 1930 and literally *none* of the bays are regularly spaced (or regular polygons, even), there will be gaps. For small ones (say, up to 1/8" or so), should I try to squeeze foam in there, or just caulk the seams against the rafters and collar ties and call it good? What caulk would be best? And caulk, foam, or tape where the sheets meet?

Heh... So much for my one "simple question"! ;)

Thanks!
El Barbón
 

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Discussion Starter #3
Yeah, it's partially an attic, and the last roofers pushed a few nails through. Fortunately, most of them are old wooden-shingle nails, and they aren't long enough to hit the insulation. It's a half-story upstairs, and the rafters are old 2x4s, so I'm just putting up 1.5" board. Not ideal, but it leaves two inches of air space for the ridge vent, and it's better than the old insulation (3 layers of composite shingles over a layer of wooden ones and skip sheathing, plus some '50s-vintage 1/2" fiberboard).
 

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XPS boards also come in blue.:smile: If you can press fit, cheapest caulk such as painter's caulk is fine. If gap, hold the board with a few finish nails or shim or cardboard shims and fill with can foam.
 
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