I have a two story, half circle, bay window where I am insulating the framing on the ground floor of the bay. The exterior is stucco, when removing the old drywall and insulation I found tar paper attached to the framing of the bay, and with a little more investigation found that metal lath was then attached to the framing over the tar paper and then stucco on top. No sheathing or exterior insulation. 

My plan is to use closed cell spray foam(I am open to other options if spray foam won’t work in this situation) in the stud bays(2×4 walls) for maximum R-value. I’m unsure of the proper way to insulate while having long term air, water and vapor control. Is it okay to spray foam up against the tar paper? or removing the tar paper and insulate against the backside of stucco? This side of the house is north facing so direct sun is very limited, if any depending on the season, and no gutters above(as of now). My worries are long term moisture control of the stucco, and if there would be any cause of degradation to the spray foam if I were to go ahead and spray against the tar paper or the stucco. should I somehow create an air gap between the foam and the backside of the exterior wall assembly?

I am in central NY. My county is in climate zone 6 but am on the edge where it is zone 5.