Foam board on the exterior is best, but with your situation- - you can improve it greatly. Good that you are adding foam board on the interior to stop the thermal bridging of the studs/plates- R-19 reduced to R-13.7 whole wall R-value. Good for you for thinking correctly! And good you came here for confirmation. An un-faced f.b. (foam board) is best, either closed/open cell, mainly
it needs to be thick enough to stop cavity condensation. This is just the opposite of cold climate, exterior
f.b., yet both keep the cavity warm (by being thick enough), not to cause moisture deposits on the wood framing.
The moisture drive is from exterior to interior most of the year and with/if a fan-fold sheeting (1.3-1.7 perms, drying to the exterior the rest of year) is installed well, you shouldn’t have trouble. This will act as your WRB to deflect water down and out from the cavity. IF concerned by numerous tears/cuts/reverse laps, you could caulk/tape/mastic cavity pieces of housewrap at the studs exterior perimeters, with the bottom edge out past the bottom plate to drain. You might try tapping the bottom course of siding out gently for the room needed, as some face-nail it or to hold it tight to the starter strip (if used), and the bottom plate to stop bug entry there. Re-set any hidden nails with a flat bar and hammer, careful not to damage the cement board.
The corner sheathing is all that’s required to meet shear code for your location, no need to add more. If worried, add some flat “X” strapping anywhere inside for a little more
shear resistance on the bare studs; http://www.strongtie.com/products/co...c-twb-rcwb.asp
The existing sheathing is much better; http://bct.eco.umass.edu/publication...n-the-outside/
The EPS is about R-3.6 per inch and XPS is R-5, I’d use 2” of cc EPS (Type 1) or 1-1/2” of closed cell XPS. It depends on the Type/density of EPS, if comparing the higher perms for letting moisture through easier/quicker, or water absortion.https://docs.google.com/viewer?a=v&q...t5jlOsa_kZRRgw
BSC has recommended foil-faced
and later changed that to un-faced
f.b. to let any moisture go through slowly to the inside room, letting the HVAC re-circulate the air and dry it;
1993; impermeable f.b.Figs.1, 2; http://www.buildingscience.com/docum...e-humid-south/
2009; semi-permeable f.b. Fig.3; http://www.buildingscience.com/docum...tial-buildings
Did you notice both figures showed wood furring before the drywall?
Two reasons; keep an air space to dry any moisture on the paper-facing of drywall and to have the water vapor in a gaseous state after passing through the f.b.; second; stop drywall/foam in direct contact
to limit conduction which would constantly cool it and compromise its purpose of preventing cavity condensation.
Canned foam/caulk the bottom edges of the foam board AND drywall for an air seal/capillary break, and the top edges against the new ceiling drywall for the same reasons. ADA the drywall, especially at the electrical boxes/switches, including the ceiling ones; http://www.buildingscience.com/docum...wall-approach/
Remember, you are making the room perimeter your thermal/air barrier, take care to do it correctly. A 1/16” gap can compromise an air barrier, yet you can perforate a vapor barrier and not compromise it because it is surface measured.