My 1948 home  in Climate Zone 4 has a slate hip roof.  Part of the attic is conditioned, with 4″ original Rockwool  batts between the 2″ X 8″ rafters, with drywall applied to the bottoms of the rafters.  The roof triangle above the narrow ceiling drywall is very small with a single small copper vent at the roof peak and there are no soffit vents. There are knee walls in the conditioned area and then the ceiling slants, becoming flat.  I’ve read that the existing situation is to be considered unvented.  

We planned on removing the drywall, removing the Rockwool, leaving a 1.75′ ventilation space (with purchased baffles or home made), followed by R-21 fiberglass batts 5.5″ thick, followed by half inch rigid mirrored foam (RMax) and then by drywall.  

Despite what I’ve read here and elsewhere, it is still not clear to me that the ventilation space/channel is necessary in my house.  Instead of using cell spray foam, can we simply apply 1.5″ of mirrored rigid foam directly against the roof decking, then the 5.5″ fiberglass batting , followed by the half-inch rigid foam to stop thermal bridging and then the drywall?  Or is there now another alternative instead of closed cell spray foam?   Or is a ventilation channel of at least 1″ still necessary?  

Thanks for your help.