Hi all! 
I’m doing a kitchen remodel in Central VT (zone 6a). The 18′ exterior wall was in terrible shape with extensive sill rot under the two windows, so we gutted the room down to the timbers. Oh, these walls are also 3″ out of plumb in 8′ for reasons only the long arc of time knows; I can theorize but I won’t here.
The clients are very keen to make their home more comfortable and we’ve settled on double stud walls, which we would have pretty much had to do anyway to get plumb walls for the cabinetry.
Along the bottom 4′ we replaced the old board sheathing with 7/16 Zip after we repaired the rotten sills. I am having trouble wrapping my head around the upper 4′ of the wall which is still board sheathing and has only rosin paper in places between the clapboards and the old sheathing. The clapboards are in a widely variable state of repair – some okay, some marginal, some toast – but the clients really would prefer not to strip them and let me install a self-adhering drainable housewrap over everything. I feel like I remember folks talking about carefully installing tar paper into the stud bays and sealing this to each timber (in this case), but I cannot find the detail anywhere. The details I do find entail spacing the tar paper off the sheathing at least 1/2″, but the insulation plan is for dense pack cellulose, which of course the tar paper can’t hold it’s shape against. The double stud wall is currently built and varies in thickness from 11.75″ at the bottom and about 9″ at the top, on average, sort of… the old wall is wonky. My sense is the only way to do this well is to strip the clapboards and apply a weather resistive barrier (WRB), but does anyone have other ideas that have proven successful? 
Thanks so much to anyone who takes to time to help me navigate this! This forum has been a real lifesaver for these complicated situations I find myself with since I work on a lot of old houses…
Cheers!
Dave