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Vaulted ceiling insulation with skip sheathing

Jamescrawford | Posted in General Questions on

This is how I plan on insulating my upstairs bedroom. I’m vaulting the ceiling and leaving the collar ties exposed. I put furring strips to maintain a 1 1/2” vent space. I’m putting two 2” layers of R-Max polyiso in the rafter bays and one 2” layer over the rafters to achieve R-39. I will spray foam the 1/4” spaces in between the foam board and rafters/collar ties and tape the seams on the final layer that’ll cover the rafters. Thoughts?

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  2. Expert Member
    Akos | | #2

    Same advice as above link. You get pretty much the same ASSEMBLY R value by going with 3.5" mineral wool batts instead of the 4" of polyiso and way less work. Center of cavity will be less but the extra R value of the plyiso between rafters is wasted.

    The inner layer of plyiso is a good thing, you do want that. If you want a higher R value assembly, spend your poyiso $ here by going thicker. The inner rigid should be detailed as your warm side air barrier.

    Collar ties are hard to air seal as the building tends to move. I've tried a number of ways, the only ones that seems to work is properly installed backer rod and stretch caulk and flex flashing tape. It might not look like much but a dozen 2x6 collar ties even with a small 1/16" gap works out to something like a 50 sq in hole.

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