Adding rigid foam to cathedral ceiling – GreenBuildingAdvisor

My living room has a cathedral ceiling with 2×8 rafters 16″ on center. It currently has fiberglass batts with Styrofoam baffles for air vents, soffit vents and a ridge vent. We constantly have issues with ice dams from heat loss and it being north facing so it never gets the heat of the sun. We are in zone 5, near zone 6.

My plan is to rip everything out from the interior and replace it with the following:

1) 1″ baffles made of 0.5″ EPS

2) R21 faced fiberglass (5.5″ thickness)

3) 2 layers of 2″ polyiso, staggered, with seams taped, edges spray foamed

4) 1×3 furring strips parallel to the rafters

5) 0.5″ drywall

I estimate this should get me close to R50. Is there any concern about moisture being trapped in the fiberglass batts with them being sandwiched between foam boards? Is there a better way to fix 4″ of polyiso to the rafters besides 5″ screws? Anything I may have overlooked? All advice is appreciated.