Re: Insulating non-standard depth walls
Howdy. If you leave an air gap using 3.5" insulation then you will have convection currants in the wall and lowering the insulation value of the wall. I would opt for 5.5" as compacted with less air is better. i would staple the paper to the outside edge of the stud makes it a little harder to drywall but less issues then when stapling to the inside edge of the studs. Great time to add caulking to reduce air infiltration. Caulking the top and bottom plates of the wall as you drywall reduce air infiltration. and adding non expanding foam around the windows also a great improvement . Dap caulk works real good and does not cure rigid so no damage to the window seals.
Any an all of my comments are just my opinion and not to be confused with facts.