I've been doing some air sealing in the attic with canned spray foam and I'm about to tackle my skylight tunnels. They are framed and covered with drywall. I have easy access to them in the attic and was wondering if it's okay to spray foam in the gap between the rafters/cross piece and roof sheathing?

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You can use spray foam as long it is a minimal expansion type of foam. Some brands call it door and window foam. This is usually only a 3 to 1 or 4 to one expansion factor. If you use the more aggressive expansion foam, it can cause deflection of the joists and actually cause leaks or cracked windows. Don't overfill the gaps, spray an even bead, let it cure, if you still have a lot of air space, you can put in a second bead of foam.

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