2×4 wall retrofit in CZ 5a

Looking to renovate an 80s rancher that has 2×4 walls with batt insulation. I am looking do a Mooney wall because of cost. This is gonna be a long term project as I redo rooms one or two at a time. Anyone have input or advice as far as air sealing or vapor moving through the wall? Should I put a vapor retarder between the existing wall and the 2×2 rips I’m adding? Is this worth it for an extra ~r 6.5 and some air sealing?
Is there anything I’m not considering?
proposed assembly
vinyl siding
building paper
sheathing
2×4 studs with r15 fiberglass
vapor retarder?
2×2 rips with r6.5 fiber glass
drywall and paint
GBA Detail Library
A collection of one thousand construction details organized by climate and house part


Replies
If you are pulling off the drywall, I think it absolutely is worthwhile to upgrade the wall's thermal performance.
Yes, include a vapor retarder. Standard 6 mil poly is fine; a variable vapor retarder like CertainTeed MemBrain is even better.
R-6,5 is better than nothing and you are at least getting a thermal break on the studs. But if you are going to all the effort, you might as well aim for more than that, assuming you have the space to spare inside.
If you are okay with losing 1" more, then strap the walls with 2x3s on edge and squeeze in an R-15 batt. Compressing the batt would reduce it to about R-11, but still a significant increase. The bonus is that R-15 batts cost less per square foot than the small rolls of R-6.5.
If you can spare 2" more, then you could take the approach Stephen Bonfiglioli uses: https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge. By making the cavities 7" deep, you could use a double layer of the existing R-15 batts in half the stud bays, and put new R-28 batts in the other half.
Consider the method here:
https://www.greenbuildingadvisor.com/article/furred-continuous-insulation-wall-assembly-to-meet-2021-codes#0
Use a rugged membrane to detail as an air barrier, such as those from Pro Clima, Siga, Rothoblaas. Do not install poly in an air conditioned building, or one that may be air conditioned someday.
You may wish to install (light gauge) metal flat strap vertically on top of the furring to make drywall installation simpler.