Welcome to the confusing and controversial world of building envelopes. To further simplify and confuse: Airtight is most important, and thermal bridging is more important than R value (to a point). Fine Homebuilding has a new Lstibureck article out on foam sheathing that is a must read for those interested in this topic. A couple takeaways: choose XPS over EPS. Strongly consider a rain screen. Upgrading to 2x6 gets you another 2" of CAVITY insulation. If youre considering foam sheathing then hopefully you know the many weaknesses of cavity insulation. I would prefer thicker foam sheathing staying with 2x4. I would also put the housewrap on the outside of the foam. Its not doing you much good between the sheathing and foam. If you get the right thickness of foam sheathing(as jlizer suggests is 1.5") then youre sheathing will generally be kept warmer than condensation temps. The real challenge of course is preventing what happens in worthy's pic. This is possible to do with tapes (ZIP system, others) and sealants but having a house wrap to integrate with flashing is a nice detail that doesnt rely on a particular chemical companies formulation. Havent done the double stud wall yet but to me it seems more trouble than its worth. It doesnt reduce thermal bridges at plates or your floor bands and eats a tremendous amount of interior floor space while reducing the amount of light (and heat for passive solar) that your windows let in. I would consider the ICFs for basement. @Robin; not everywhere else because they are expensive and dont take to changes very well. Better yet, look at pre-cast concrete walls (Superior is one brand) The complications listed above with wall thicknesses and furring for siding is one of the many reasons we have been using SIPS. Its great you are aware of the importance of air sealing. Strongly recommend you have a blower door test done at the framing stage. What you dont measure you cant improve. Here is a link that might be useful: Blower Door Test Explanation w pictures and diagrams

thought about it a bit more and realized that you may be wanting to put layers of ductboard for insulation. ah so! IF you remove the silver vapor barrier you can use it for this purpose. you don't want to have muptiple vapor barriers within the same assembly as it will condnesate. if you are hoping for a radiant barrier the ductboard would have to be on the undersides of the roofing assembly with the foil facing down. or you could take removed fsk (reflective duct board surface foil/scrim/kraft) and attach that to undersides of rafter bays. foil has to have a minimum of a 3/4" air space to work. if you put the foil on the 'floor' of the ceiling dust will stop reflectivity in a few years. I pay about $30 per 10x8 sheet of ductboard so I only use it for specific jobs. best of luck.

Hiring a competent licensed contractor to do the earthquake retrofit and other items that will be demanded of you during this permitted renovation would be the first order of business. He can guide you as to what would be the most cost effective approach. It won't be what you've indicated. Almost no return on the large amount of expense and labor to do what you're suggesting.

I'm remodeling our small bathroom and putting in a walk-in shower, where the longest side is on an exterior wall (no windows). Whomever built the house did not use plywood sheathing. The only sheathing on the house is foil faced foam board. When I redid the insulation in the bathroom, I put in ThermaFiber (mineral wool), not only for insulation but sound dampening. No vapor barrier has been installed yet. Well, a few weeks ago when it got really cold here, I noticed quite a bit of water at the bottom plate of that wall. When I checked the insulation, it was completely soaked at the bottom like sponge. Obviously, the condensation/dew point happened right where the mineral wool met the foam board. I ripped that out and have now taped the seam between the foam board and the studs, top plate, bottom plate with metal foil tape to eliminate any air gaps. What I am thinking of doing is this and I need advice on if this would work. Fill the remaining void with 3" pink foam board and seal any gaps with spray foam. Then, put a 6 mil plastic vapor barrier over that. The tile backer board is going to be 1/2" Schluter Kerdi board, so there is some more insulation. What I'm trying to do is minimize any air gaps in that space and prevent moist air from entering those stud bays. My other concern is if I got condensation in there, where would it go?