insulation for vaulted ceiling (on one side against roof deck)
We have an addition going on. Atlanta GA. Climate zone 3.
The addition has a pitched roofline and there is one portion we are attempting to finish as a vaulted ceiling (for a sunroom). There are no mechanicals in the attic so the rest of the addition attic will be vented with blown cellulose or fiberglass on the flat ceiling portion.
Stressing on what the best type of insulation for our vaulted portion of the addition. It’s about 90 square feet on both sides of the vault. One side of the vault will be open to the rest of the vented attic (sort of like a slanted attic knewall really) but my concern is the other side which will be right against the roof decking. Also, it’s a 40’s early ranch home so original home doesn’t have soffits/overhang and addition was designed to match (so no soffit vents). picture attached for clarity.
I know fiberglass is asking for moisture/condensation gain on the portion of the vault against the roofline. I was going to go open cell spray foam but reading a lot about potential issues if not sealing/conditioning the attic space and the moisture gain that could be created by the open cell foam which is vapor permeable. I have had 3 spray foam installers- 2 suggested open and another closed.
Latest idea is to install 3 layers of XPS R-10 board to get my R-30. This would allow me to leave a .5″ to 1″ air gap between the XPS board and the roof decking- at least allowing some ventilation behind the insulation.
Any thoughts on this approach with the foam board or just going with an open cell foam considering the one side of the vault wouldn’t have the benefit of any ventilation?
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