Adding polyiso to existing roof – GreenBuildingAdvisor

I’m in the planning stage at this point.

CZ6 (Catskills region of NY state).

I am an old homes contractor working on my own house.

Existing roof rafters are 2×6 true dimension. I furred out the rafters an additional 1.5”. Cavity R-value is close to R-25.

The air barrier is established at the 1/2” layer of polyiso that is set an inch below the roof deck, and sealed with spray foam.

The plan is to eventually re-roof the entire house, and at that point add polyiso to considerably raise the total R-value of the roof assembly.

I know that by “the rules” I should have at least 4” of polyiso on top of the sheathing to avoid condensation issues.

4” on this historic house (original structure dates to 1795) is pushing the aesthetic boundaries. Aesthetically, I feel like I could get away with 2” of polyiso added to the roof deck, but am I basically ensuring moisture issues to come without the minimum 4” of polyiso?

The other big question is whether I need to tear down to the rafters to remove the original board sheathing from the stack-up. Essentially, can I have an air space between the sheathing and the exterior rigid insulation?

Thank you in advance!