I’m having a hard time finding/deciphering the best insulation solution for this scenario for Climate Zone 5, NW Indiana:

I bought an existing 1940’s mixed-use building.  I have sections of the building with low sloped unvented roof with recently added (3-5 yrs?) rubber membrane roofing above.  I opened up the ceiling inside below in these flat roof areas to expose 2×12 wood joists by removing 3/8″ drywall, that was in bad shape, along with some very thin (1″?) “crepe paper” insulation that just fell right out.  I was pleased to also discover by investigating around a roof penetration that the roofer added 1.5” of foam board above the ¾” roof decking.

I want to add new insulation to the joist bays to be as efficient as possible, but if I go too thick with the fiberglass without sufficient interior vapor barrier, I’m concerned that I will not have enough exterior side foam to prevent moisture build up on the roof deck 1x material.  Here are how I see my options:

Option 1: Add 2” more foam against the underside of the roof deck and fill with fiberglass (R-30) and kraft facing and drywall.  (Table 806.4 wants R-20 but would there be any issue with the roof deck sandwiched in between foam layers?)

Option 2: (much easier) Fill with fiberglass all the way to the roof deck (R-38) and use a foil scrim on the interior side or otherwise try to prevent moisture migration outward, but at the risk of not drying to the inside very well.  (This could carry risks for future roof leaks having a hard time to dry as well, right?)  I cannot just leave kraft facing exposed for flame spread code concerns.

Option 3: Same as Option 2, but using drywall and paint below.  The use of the space is yet undetermined, but it will likely not need a finished ceiling, or if finished space, would use a drop ceiling grid, so drywall is not really necessary; and adding drywall would add a fairly significant expense, but could provide some additional fire protection also.  The building is sprinklered as well, so I’m not convinced of much added value by adding drywall except for its maybe being the best solution for this question.

Option 4: Fill with open cell foam and cover with a vapor barrier?: A. kraft paper and drywall? B: Foil scrim? C: drywall and paint as the only vapor barrier? D: Can I get by without a vapor barrier depending on permeability of the foam?

Option 5:  Would you recommend something else??

 With much gratitude for your time if you are so kind!