I have a 2nd story with a garrison overhang which is a huge air leak.  It currently has 1 batt of fiberglass acting as the barrier between the exterior and interior.  I want to tear down the soffit and block off the overhang with rigid foam in a cut and cobble method.  I am going to cut and cobble on the rim joist and bottom of the soffit because the overhang has duct work going down a few of the joists…I am trying to keep the conditioned air inside the “envelope” so that limits my 

My question is should I/can I attach an air barrier (like Henry Blueskin) to the bottom of the joists before I cover it with soffit?  I can’t furr out the soffit because the 2nd floor siding only comes down so much, so I thought a self adhered air barrier would be a good air seal without adding much thickness to the assembly.

I would pull down the 0.25″ thick plywood soffit, stuff Rockwool into the overhang, cut and cobble a piece of rigid foam to the rim joist and into the joist to cover the bottom 1″ of the joist from the exterior wall top plate to the rim joist, attach air barrier, install Hardie soffit material

Thoughts?