Is this a Cathedral Ceiling that Works? – GreenBuildingAdvisor

Zone 4/5, slate roof over nickel-gap sheathing. Currently, there is R-30 fiberglass in the attic floor and R-19 fiberglass batts in the roof. But the attic is a conditioned space, and I would like to add more insulation and air sealing.

I have reviewed “Five Cathedral Ceilings that Work” as well as the more detailed “How to Build an Insulated Cathedral Ceiling” 

I am intending to follow Assembly #2 like so:

–            1” air gap

–            1” fiberglass-faced polyiso baffle, all edges sealed with foam

–            R-19 fiberglass batt

–            1” foil-faced polyiso attached to the underside of the rafters, all seams taped with foil tape

–            Furring strips and 1×6 tongue-and-groove boards

Questions:

1. Is the taped, foil-faced polyiso an adequate air barrier for this assembly, or do I need a layer of drywall under the T&G boards?

2. Should I try to add another inch or two of iso? Obviously more is better, but it’ll be a tight squeeze to get much more in there.

3. Is the fiberglass-faced iso an acceptable baffle material for this setup?