Insulating from the Outside
We are currently doing an addition and renovation, part of which includes new siding and windows on the existing portion of the house. Because the existing structure had celotex sheathing and no insulation in the stud cavities, we are removing the celotex, furring out the studs to the full wall depth so that they flush out with the new walls and spray foaming from the outside. While the wall cavities are open, we are also pulling new three-wire for the outlets in the existing part of the house to get them grounded.
Note that since we furred the wall out by 1.5″, we were actually able to add 1.5″ of foam over the band board, as well, which will greatly improve insulation in the floor cavity.
It is taking lots of coordination between my guys, the electricians and the insulators, but it has gone smoothly thus far and I expect to have the whole house finished with this phase of demo/rewire/insulation/re-sheath in the span of only a week.