If you hadn't already bought the batts I'd recommend something else, like 1" of closed cell foam against your plaster/lath or whatever as a modest vapor retarder and air-seal, followed by wet-spray cellulose. Alternatively, dense-packing dry-blown cellulose to 3-3.5 lbs density while the old sheathing is still in place, it will stand up well enough on it's own as you strip it a section at a time for the new sheathing, and fill every nook, cranny & missing splinter, making it a near air-barrier.
Batts never fit perfectly even in new construction, and cutting/compressing stretching to fit antique studs with existing wiring/plumbing etc is impossible to make perfect, but sprayed/blown goods conform to every detail, and (unlike batts) tend to perform very close to the rated R.
Cellulose products that contain only borate fire retardents (no sulfates- you have to ask, but it's not particularly more expensive) is toxic to ants & termites, but not mammals. It's an eye-irritant to mammals though, and you don't tend to find mouse-nests in cellulose insulated walls (even with products that use a mix of sulfate & borates.)
Assuming you have 3.5" cavity depth (or are they full-dimension fours, like my 90 year old antique in Worcester?) you're looking at a center-cavity R of ~ R13 in fiber, and with 1.5" of exterior foam R7.5-R10. In order to avoid wintertime moisture accumulation on the structural sheathing from interior air leaks & vapor drives it needs to have a January mean-temperature in the 37-40F range (the dew point of 68-70F, 30-35% relative humidity air.) The mean temperature in Hartford in January is ~ 25F ( see
http://www.climate-zone.com/climate/united-states/connecticut/hartford/ ) If you went with 1.5" XPS the sheathing would be ~37% of the total R, and assuming a 68F interior temp the sheathing would average ~41F, and if you went with is it would be even warmer/drier. In either instance an interior vapor barrier is neither necessary or desirable (particularly undesirable if foil-faced iso is used.)
If you went with only 1" iso and de-rated it to R5.6 (it's 25F performance), you'd be looking at only ~30% of the R exterior to the sheathing, which would then run at ~38F- still fine, but considerably more condensing hours than if you went with the 1.5" goods. 1" of XPS would only be ~28% of the R on the exterior leaving the mean-temp of the sheathing at 37F- marginal, but with a considerable amount of exterior drying capacity through the foam- you'd still be at very low risk without an interior vapor retarder, especially if you're leaving the 3/4" furring gap between the foam and the siding.
The best case is 1.5" of foil-faced iso extending down below the foundation sill, with a copper flashing on the exterior of the foundation sill and new sheathing as termite & ant deterrence (copper is toxic to them too), with a cellulose cavity fill. The whole-wall R (thermal bridging of the studs & plates factored in) of a 2x4 fiber-filled wood-clad wall is ~ R10, and putting 1.5" of iso DOUBLES that to about R20. (By contrast the whole-wall performance of a 2x6" wall with batts/cellulose, no exterior foam is only R14-ish. Play around with this tool a bit:
http://www.ornl.gov/sci/roofs+walls...S/Calc.htm Exterior foam adds R linearly from the no-foam whole-wall R calcs, so you can get pretty good numbers adding R5/inch for XPS, or R5.6/inch for iso.) The annual condensing hours at the structural sheathing drops to near-zero, so the fact that the foil facer pretty much bars drying toward the exetrior is inconsequential, and setting it up to dry toward the interior is fine- just avoid things like vinyl or foil wallpapers that would impede interior drying of potential occasional flashing leaks, etc. Foil faced iso is fairly easy to turn into a primary air-barrier with FSK tape on the seams and foam-sealing the edges. If you caulk/glue the plywood sheathing to the studs & framing as you go that too can be pretty effective. Sealing both the best you can, and staggering the seams of the foam with those of the plywood further improves overall air-tightness.
You might want to
read this before deciding whether to put the housewrap over the plywood vs. putting it over the foam.