MN REn, will you be following the exterior insultn retro fit model of removing interior vapor barrier and installing a new one on the exterior of the R13 stud wall before spacing out impermeable exterior insulation with new exterior sheathing and siding? The R25 you plan to add should than put your new VPBR well inside the non condensing side of your new total insulation. It is important to remove old interior vapor barrier to not create a vapor pocket(if you believe it to be functional). When I reinsulated my current home the original interior vapor barrier was as nonfunctional as the wall insulation and the preventing of air infiltration around doors/windows.
You will be insulating to the point where doors, windows, air infiltration, fresh air needs will be far bigger factors for heat demand than more insulation. My house(modest sized 50's doll house) is about 25% lower R than your goal but it is high enough to really notice first the lose of two kids off to college and than even more the almost total conversion to LED lighting. The extra bodies and use of almost constant incandescent lighting did a lot of space heating that is now gone.
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