Confirmation – sheathing air gap needed for insulating existing wall? – GreenBuildingAdvisor

Hi, I’m insulating a few triangular exterior walls as part of my “devils triangle” retrofits. 1922 house, zone 5B, almost 6B, northern CO. Relatively small interior fixes so not involving AHJ for the moment, but want to do it right. Have to go in from the inside – outside is too big of a project right now and has historic district issues. This is a conditioned utility space without electrical wiring or windows on the exterior wall.

Exterior wall is:

Old but solid stucco (painted many times, including last year)

Metal Lath

Old but intact felt (probably original)

1 x horizontal pine sheathing

2×4 16OC bays (true 2″ x 4″)

1/4″ fiberboard (doin’ nuthin’)

Plan is to fill the bays with mineral wool batts, then do 2″ of continuous foil-faced polyiso on the inside. Fur out with 1x4s, then 1/2″ drywall. That gets me to ~R26 without much complexity.

My questions are:

– Do I need to leave a gap against the existing sheathing? I see that is recommended for old walls where there is no housewrap, but is the original felt good enough?

– Do I need a smart vapor barrier like Intello or Membrain over the mineral wool? Or does the taped foil faced polyiso perform that task? Winter indoor RH is ~25% at best, no humidification.

– Anything else dumb I’m missing?