Possibly overkill, but the Building Science Corp white paper might help:
In Central NY we use 2" of XPS to keep the dew point at heating design temperature (-2°F) outside the wall assembly. These homes are quite comfortable, get excellent blower numbers, and are well prepared to fend off the universal solvent.
Still, my general policy and universal recommendation is always the same: "foam is good, more foam is better, and a s***load of foam is best of all." In my opinion, the concept of "overkill" does not apply until the building achieves passivhaus -level performance. Anything less is a compromise. Energy consumed now to save energy indefinitely into the future is a wise investment, although it is one tough sell to a nation that likes to buy lottery tickets.
By the way . . . speaking of dew point, I understand that there is an app available for psychrometric calculations for the iPhone and one on the way for Droid.
I discovered this comprehensive answer to the design question here:
thanks for all your suggestions!
Leo K. PE
I just finished a deep retrofit and used 2" polyisocyurate interiorly firectly over old rock, strapping, new rock.
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