My question relates to the need for an air barrier only vs an air+vapor barrier in a knee wall application.

I have a 1.5 story cape-style house in Zone 5, Massachusetts (mid-50s).  My goal is to make the space behind the knee wall a conditioned space. 

The roof assembly from outside in is Asphalt shingle -> Vapor/water/air barrier -> Wood tongue and groove roof (not OSB or plywood) sheathing -> Rafter vent in a 2 x 6 rafter -> R-21 fiberglass batt insulation.  (In one newer area the roof sheathing is plywood.)  The rafter bay is air sealed at the soffit end from the bottom of the rafter vent to the wall plate using a 1” foam block with the perimeter of the foam block sealed.

What’s Next

Some posts on this site have stated that a Class I vapor retarder is needed where some have stated that a Class III is better for this application.   The 2020 DOE Ueno/Lstiburek report (p. 98) suggested the need for an air+vapor barrier, but it wasn’t clear on rating, or what sort of barrier was beneath the shingles.  My obvious concern is condensation. 

Summary questions:  Given that a Class I vapor retarder is under the roofing shingles, should I use a Class I or Class III vapor retarder under the rafters?  

I appreciate your guidance.