Vapor Issue w/ EPS & Mineral Wool in Attic – GreenBuildingAdvisor

Hello!

I have a question about air/vapor but I have to explain my project a bit for context:

I am trying to carve out a small office space in the attic of a c. 1950 colonial in Massachusetts (Climate Zone 5a). Due to limited space under the rafters (and limited money) this will come up well short of the energy code but I want to do the best that I can within reason. The plan for the homeowner is to add outboard insulation and a dormer when the roof is replaced, which is +/- 5 years away. Until then, I need a place for a desk and a guitar.

The attic is unconditioned and I am well aware that it will not always be comfortable without a window AC or space heater. Unfortunately there is no ridge vent but there are 6 box vents about 24″ from the ridge. I have already installed 2 soffit vents to make sure that the space outside of this room will remain open with decent but not ideal ventilation.

The general plan is to frame the room with new 2×4 knee walls w/ mineral wool batt. I don’t have much room to insulate inboard of the rafters above but theres just enough space to install furring strips and cover with 1″ foamboard and still have room for a 6’6″ door. Above that would be 3.5″ of rockwool batt and 2″ of space at the underside of the roof deck. I could use baffles to maintain the 2″ air space but I would hate to pass up the opportunity to install another 1″ of foamboard and leave 1″ of air space to allow the roof to dry. I have attached drawings that show a section of these materials.

The red hot question is regarding the mineral wool being sandwiched by foamboard. It seems like almost all of the foamboard available in small quantities at box stores is faced with foil and I want to be careful not to install vapor retarders on both sides of the mineral wool. In my drawings, I call out for EPS foam (it being the most vapor open) but with a facer its still below 1.0 perm. I could forgo the outermost foam closest to the roof deck and use plastic baffles, decreasing R-value but creating a nice big 2″ air space? Or am I not using enough insulation to truly worry about vapor anyways?

Any advice is appreciated.

Edit: Im adding a closeup of the detail at the roof rafters since my attachment seems small and hard to read.