I have been googling and reading about flat roofs for hours.

I recently had a leak in my flat rubber roof (term bar was installed on top of roof instead of sides before I bought house) which caused me to replaced the 60 mm rubber roof. They built a taper out of 2x4s after removing the old rubber (there was another layer of roofing installed that was left on).

I am looking to insulate from the inside. Essentially I have my 2x6 joists, then plywood from the original roof, then the 2x4 installed perpendicular to the joists to pitch the new roof about 6 inches, then the new OSB and 60mm.

Since there is an air gap between the original roof and new OSB/EPDM, can I insulate with batt to get my R value, then drywall? There are also soffitt vents installed under the flat roof's overhang.

I currently have the drywall ripped out of the ceiling because it was wet from the leaking. The room is small, about 10x20. I was hoping to roll R30 batt, then drywall, or staple up a vapor barrier between the insulation and drywall. There are a few windows in this room, but it is not a kitchen or bathroom, so the moisture shouldn't be too much of an issue.

Live in Wisconsin, so I need R25-30 from what I've read.

Thoughts?