Insulating a low-slope roof in Zone 2b Phoenix?
I have a building with two low-slope roofs in zone 2B (Phoenix). The original building was built in 1930 out of block (CMU) exterior with no insulation anywhere. It has a traditional stick-framed addition. In my mind, that counts as two separate roofs as there is no air path between the two roofs other than in the living space (through the bedroom door).
On the high end, we have about 24 inches of space. The low end has about 1 inch between the rafters and ceiling joists.
The roof has 2+ inches of sprayed foam on the exterior with white coating. The interior still has zero insulation.
The space between the ceiling and the roof in the main building is currently vented with large holes through the CMU.
Right now there is no ceiling drywall anywhere in the house. We plan on attaching the ceiling drywall directly to the rafters in the addition. The rest of the house will have drywall attached to the ceiling joists like normal.
For the main building, I’m thinking that the best way to go is to use fluffy insulation directly against the underside of the roof decking and to seal all of the vent holes. We would then put no insulation on the ceiling.
The addition would have the entire joist space filled with fluffy insulation and all air leaks to the exterior sealed with caulk. Drywall would be installed directly to the bottom of the rafters (like in a vaulted ceiling?).
We are conditioning the space with ductless mini-splits, so there will be no air ducts in the ceiling, though we do plan on installing recessed can lights. Do we need to pay attention to sealing the ceiling and can lights?
Does this seem like a workable plan?
What are the “code” words I should use if my inspector questions this retrofit.
(EDIT) Changed “Roof Joists” to “Rafters”
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