Greetings,
I live in Ontario Canada  (zone 6) and have an older 1950’s cape cod house that had renovations made to it, including creating a full second story on the back side.(attached pictures before and after)
For the second story the build wall is made of : 2x 6 frame with drywall, 6
mil vapour barrier, batt insulation, OSB covered with tyvek and finally
vinyl siding.  The interior is now completely finished, however,  the rim joist
between the first and second floor was completely ignored when the roof line changed . (picture looking at joist and between 1st and 2nd floor)
The picture was taken by me looking into the soffit space at the second
floor false roof detail.)  Additionally, a piece of sheathing was ignored in this
area as well and  no insulation or vapour barrier on the interior of the second floor rim joist.
Since I have access to the soffit area I am looking at insulating the rim joist area from the exterior. Unfortunately this approach does not allow a vapor barrier to be installed on the interior.  I am considering a few options and would appreciate thoughts and suggestions.
1.  attaching rigid fiberglass along the entire rim joist.  
2. Tape the seams between the rim joist and insulating with mineral wool .
The wool is easier to cut and work with in the cramped space
3.  hire a spray foam company to fill the rim joist area  from the exterior.

thanks
Norm