I’m doing some remodelling and have old 2×4 walls.  I’m willing to give up some interior space to improve the performance of the wall.  Currently I have 3/4″ plywood sheathing and (I think) black felt paper on the exterior side.  Taking that down and putting foam on is not a practical option for me at this time.  That being said, is there a recommended way to retrofit the wall?  I’m considering two options:

 – The Bonfigloli method (https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge)

 – Framing a second staggered 2×4 wall on inside, effectively creating a 2×8-thickness double-stud wall

Are there any moisture concerns in the Seattle area with regard to the second option?  I couldn’t find a good detail from the GBA detail library for such a wall retrofit.

If I’m going with rockwool inside the wall (regardless of the method used), should I put on a vapor retarder / air barrier like ProClima’s INTELLO on the interior side?  At the foursevenfive blog, which seems to push the product a lot, they have a post recommending Intello in the between the two studs (https://foursevenfive.com/blog/the-double-stud-wall-simplified-low-cost-high-performance/).

My house is pretty leaky (build in 1972) and I would also like to improve its airtightness.  Does intello work well as an air barrier?  I understand for it to work as an air barrier I’ll need to apply it to my ceilings as well.

I will be doing a re-siding of my house in a couple years, at which time I’ll probably plan on applying a peel-and-stick (like Henry VP100) to the sheathing to further improve the envelope.