Vaulted ceiling insulation
Hi all. Nate here.
I’ll describe my situation then post my question at the end.
I have an empty addition room that I am turning into our kitchen. It orginially had flat ceiling, but we don’t want that and are turning it into a vaulted ceiling. 2×6 roof rafters were used. I want to get near R30 insulation. I plan to combine (R6) 1″ Rmax polyiso & (R15) 3.5″ Roxul (R21) in the cavity, and give it a 1″ air gap between the sheathing and polyiso. On the inside of the rafters I plan to use additional whole sheets of (R6) 1″ Rmax polyiso to cover the entire interior of the ceiling. That should put me around R27. I am going to use ship lap to cover the interior of the ceiling.
My question is.
Do I need to have a slight air gap between the back side of the shiplap & polyiso boards or can they be in direct contact with each other?
I thought about using 1/4″ sill gasket if a small gap is needed.
Thanks for your time & any info given.
Have a good one.
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