Air gap in unvented cathedral ceiling
I’m in the midst of framing a small cabin in climate zone 6. The cabin will have an unvented cathedral ceiling that I am trying to keep to a minimum thickness while still hitting R-49. Because of snow loads I need to use 2×8 rafters or larger. I’m planning on using enough rigid exterior foam to meet at least 51% of the roofs R-value (ideally not much more than R25). Because I am trying to keep the roof as thin as is responsible, I was wondering if it is acceptable to not fully fill the rafter bays with insulation. I had planned on using rock wool and was wondering if I could use 5.5inch rock wool with a small air gap either above the sheetrock or below the roof sheathing. It seems as though installation would be a pain but would this air gap be problematic? If there are more elegant solutions to this I would love to hear them as well.
Thanks in advance,
GBA Detail Library
A collection of one thousand construction details organized by climate and house part