Do I need to insulate under the footings of a frost protected shallow foundation?
I’m designing a FPSF in Southern Vermont (AFI 2300). I want to fully insulate the radiant slab by including 4″ XPS under the footings but this seems to conflict with the FPSF principle of relying on heat transferring through the footing to stabilize the ground temp. My next thought was to use the FPSF design guidance for an unheated building, however this is such a drastic increase in the horizontal insulation required (distance increased from 12″ to 63″ and R-value from 1.7 to 11.4.) That seems like overkill as there will still be heat transfer from the building to the ground, albeit at reduced levels. What I’m looking for is a hybrid approach.
I’m wondering, what are your thoughts about insulating under footing with a FSPF and if so, how did you determine the requirements for horizontal insulation?
Thanks so much in advance, appreciate you taking the time.