My wife and I moved into a 1955 cape that I am looking to renovate bit by bit, room by room in most cases- as budget allows and all work done by myself. We’re on the northern border of climate zone 4 and very close to the highway.

I’m currently in the den. The drywall is in such bad repair that I would rather replace it and then paint than chase all the nail pops, failed repairs and abysmal butt joints. So after some “exploration” I see that the walls are insulated with shriveled old cellulose batts that do next to nothing anymore.

The exterior wall assembly is as follows: 1/2” drywall (8’ ceilings), 2×4 studs (insulated as previously mentioned), 1×6 T&G sheathing on the diagonal, tar paper, 1×10 cedar clapboards (original cladding), 3/8” fanfold Foamular, vinyl siding (80’s residing). The two big window units are 1986-vintage Andersen PermaShield double casements in ~57”Wx60”H rough openings.

I’m considering replacing the double casement units with single euro-type triple glazed tilt & turn windows for the views, the efficiency, the sound reduction.

My plan is to gut all the old drywall and old insulation. Then replace the janky receptacle wiring, insulate the band joist with Kraken spray foam and 3.5” Comfort Batts, insulate the wall cavities with 3.5” Comfort Batts, MemBrain vapor retarder, install sound isolation clips and hat channels, 2x 5/8” drywall with Green Glue between, THEN paint.

 My question is the efficacy of installing the MemBrain vapor retarder on the walls when there is no purposeful modern WRB or moisture barrier on the exterior (currently, I will reside and insulate etc at some later stage I imagine). The wall assembly being what it is currently, will the MemBrain cause problems? Or will the planned use of rock wool coupled with the air gap behind the drywall afforded by the hat channels make it all work, even in phases?

Sorry for the overly complete details, but I’d rather give all the info upfront than answer tangential questions by being too vague.