Hello All,
I will be renovating an older 1926 platform-framed house in Central NY. The current wall construction is as follows (from outside to inside) 1)clapboard or shingle siding, 2)tar paper (likely), 3) 1x6 pine plank, 4) essentially no wall insulation (some settled cellulose), 5) lathe, 6) plaster.

Demolition will be from the inside out on the exterior walls. The outside clapboard is in great shape (as are the shingles, for the most part) and is quality 90+ year old old-growth wood - I don't want to risk the destruction of that by removing it, and also, I need complete wiring and plumbing upgrades anyways, and need to insulate in the window weight pockets. It kills me to tear out the old plaster and lathe, but this is just the most economical way to do it.

New wall assembly (from the outside in) will be as follows:
1) clapboard or shingles, 2)tar paper, 3) 1x6 pine plank, 4) 1/4" thick wall drainage membrane, 5) 2" closed-cell spray foam, 6) 3-1/2" rockwool, 7) 1/2" drywall. Studs and headers will be built out with 1" non-foil faced isocyanurate and 3/4" furring strips (to provide room for 3-1/2" of rockwool after installation of 2" CCSF).

I'd be interested in any feedback or comments. Thanks in advance.

where will your air barrier be? And how will it be done?

I'm not going to disassemble the outside wall to create a new air barrier - I'll seal the inside of the stud cavity in a reasonable way with at the joists and plank seams before this is sealed with the CCSF. The CCSF will be both a vapor and air barrier. The wall structure will dry to the inside with no additional special air or vapor barrier.

Not to worry, your house has red rosin paper under the siding not tar paper.

It's fine to use foil-faced goods for the edge strips on the framing. The major drying path is the sides of the framing, via the air & vapor permeable rock wool.

Red rosin paper is a new one for me - hadn't heard of that before. I'll have to keep an eye out for that when the construction begins...

Thanks for the advice - it sounds like I am on track. I've learned a lot from other postings here already.

Tar paper, once a common underlayment on sidewalls, is actually a good product for that spot. It absorbs moisture when necessary, then releases it slowly and dries out; helping to keep the back of the clapboards or shingles dry. Red rosin paper is lighter, without the asphalt component, and was also used but had different properties. It's more common as an underlayment under hardwood flooring.

In my insulation career it was the prevalent air barrier used in houses from the late 1800's until 1940's. Bob is right it's other purpose is between sub floor and finished floor. I would suggest before you start stripping plaster and lathe from you walls, take off a clapboard on the outside and drill a hole in the wall to check what you have. The biggest thing that happens with an interior gut is the finished trim that you now have will not reinstall very well. I can tell you from experience. Even simple ranch casing does not install in the same way when removed. Much easier to upgrade the insulation from the outside. Remove 1 clapboard below the windows and 1 above and blow in insulation with a tube in each stud bay. Full wiring and plumbing upgrades are done all the time without removing the plaster especially with PEX. I would not recommend the flash and bat system. I would consider all your options before you take a drastic step. Where are you located in NY? I'm also in upstate NY.

It is reluctantly that I remove the plaster and lathe from the outside walls, but the house is in need of all new electricals and a red squirrel infestation leaves me wondering what is left in the walls as there are still odors that I can't trace. The trim downstairs is dark stained pine and is pretty beat up and needs replacing. Ash can be procured at a pretty reasonable price and I have a great finish carpenter who will do an install on this for me. Upstairs, trim is in better shape and what can't be rescued can be reproduced at reasonable cost. Once I've opened up the outside walls, I'm going to take advantage of that to build a thicker wall and insulate better than I could otherwise, with a retrofit from the outside.

PEX is out of the question - pipes will be all copper. I'm not going to put PEX in the house - let's just say I am not a fan.

I've insulated a previous house from the outside and was very satisfied with the results. That house had an interior in great shape with beautiful chestnut trim - gutting that would have been a crime - but that is not the case with this new renovation.

If you have some details on why you don't recommend that insulation detail I am considering, I am all ears. Thanks for all the feedback and ideas thus far.

Spraying the cavity leaves the bay with a very irregular surface. I started my former insulation career with foam and can tell you that there is no way you can spray an even 2" of foam. The studs will have 3" and the center maybe an inch. The resulting cavity will not allow for a tight fit with the batt. Maybe it's just better to spray the full cavity and trim the over spray. It will be tighter.

I certainly recognize that I will not get an even coverage with the spray foam. But the company that foamed the basement above grade walls on my existing house did a pretty reasonable job of keeping the coverage pretty even. Yes, it is bumpy, but not a 2" difference between sides and middle. Spraying the full cavity has some pretty significant cost impacts, and also means that I can't complete phase 1 of this project (exterior shell changes and roof/dormer changes completed) over the summer with some insulation (i.e., 2" CCSF) so that I can complete the electricals, plumbing and mechanicals over the winter, after which I can go back and add the additional Roxul with my own labor.