Requesting comments on a retrofit double-wall design

Requesting comments on a retrofit double-wall design #green

I'm in the design stage of a whole-house remodel of a 1964 ranch in Evergreen, Colorado (high and dry, Zone 5B, but right on the edge of Zone 7) and would appreciate any comments on a proposed wall design. The existing home's walls are 2x4s with 1/2" fiberboard and plywood siding on the exterior and 1/2" gyp on the interior. Cavity insulation is old rockwool batts. What I'd like to do is remove the drywall and rockwool, fill the existing cavities with blown-in cellulose, sheath the interior of the existing walls with 1" XPS, frame a new 2x3 wall, 1" off the interior surface of the XPS and insulate that wall (and the space between the new 2x3s and XPS) with mineral wool batts. I've attached (I hope) a sketch of what I'm proposing. I'd like to leave the existing fiberboard and siding in place. The XPS would be the air barrier, taped at all seams, floor and ceiling air barrie...

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