I have an existing space above the garage that I want to fully insulate and condition. The floor is insulated as is the part that will become a crawl space. It’s currently a vented roof deck with soffit vents, ridge vents, and baffles. 

The biggest hurdle is that I already have 24″ pf blown insulation in the outboard spaces that I’d prefer to not to have to redo/undo. Any suggestions on how to insulate the knee wall and roof deck  prior to installing drywall? 

One thought is to leave it as a vented assembly and install 2″ rigid foam on the outside of the kneewall. Unfortunately the kneewall is 2×4 construction so I can’t get a whole lot of R value out of just packing it with fiberglass and it’s not likely to be very tight. 

The rest of the house isflash and batt with closed cell foam on the underside of the roof deck. I’m not adverse to doing that here but getting spray foam out to the soffit vents will be difficult without tramping down the existing blown insulation. 

Any recommendations are appreciated.