Insulating cathedral ceiling from inside out?
Renovating a home in zone 6. The home has a cathedral ceiling with old 1x bead board on interior. we are removing the existing shingles and the old 1x material sheathing. The exposed rafters are 2×12 no sheetrock. The bead board is nailed directly to the rafters. We were thinking about closed cell foam at a thickness of either 3 or 4″ on the outer side of the bead board between the rafters stopping the foam from going through by putting tar paper down first. Then installing fiberglass or rock wool bats to bring it up to the necessary R value. Finishing with zip system sheathing and asphalt shingles. We were also going to allow it to be vented. Doing it from the inside is not an option as much of the demo has taken place. Any potential issues with this?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Your suggested method will work. A friend of mine with a leaky cathedral ceiling did exactly the work that you describe, and it made a huge difference in comfort.