I have a finished attic space with HVAC ducting behind the kneewall in a vented, unconditioned area (I know, not ideal, but not my design either, and I can’t relocate all of this ductwork).  The vented roof (2×10 OC rafters with Kraft-faced R-30 batts filling the bays) makes for a drafty attic area, and I wonder if installing a vapor-variable air barrier along the underside of the rafters/insulation running from the floor at the exterior wall (where the rafter meets the exterior wall) all the way up to the back of the (uninsulated) knee wall will help keep the ducts at least a little more isolated from cold winter air (which really whips here in coastal NH /CZ 5).  

Since the kraft paper is a vapor retarder, should I worry about increased risk of condensation and related moisture issues if I install MemBrain or Intello directly over the existing paper-faced batts? 

I need to figure this out ASAP since upcoming work (we’re constructing a dormer) and for various reasons the front face of it will limit access to this space afterwards.  As the pictures show, we’ve removed batts where the dormer will be cut and can easily use unfaced batts or loose fill in this area.  But elsewhere, where no work is planned, can I keep the kraft-faced batts and install the VVAB or would that be risky?  Thanks so much!