[Greenbuilding] thermal bridging--letting it go

[Alan Abrams]

<I'd say R13, but maybe less, if the facing is a better 
conductor than foamboard, eg drywall. In that case, there
could be significant 2-dimensional heatflow from the room 
air into the drywall, then sideways into the stud, ie the drywall
would act as a heat collecting fin for the stud, increasing its
effective area.>

implying that the foam sheathing or other t-break should be close to the
inner surface, rather than the exterior.  

This approach resolves numerous problems with foam on the exterior of a
frame wall--playing havoc with fastening siding, flashing windows, sealing
at the base of the wall, dithering over whether it goes over or under the
tyvek, etc.

Alan Abrams