Hard to say without being there to see it.
Recommended Levels of Insulation : ENERGY STAR
Not sure why you would want to use blue foam, far more expencive then just fiberglass batts, and more work.
There has to be an air flow from the soffits to the peck of the roof to keep the shingles and sheathing from over heating so you would have to add foam baffles if you insulate. By adding them you will reduce the amount of inslation that will fit uncompressed in the bays.
PS, joist are used to build the floor and ceiling, rafters are what's holding up the roof.