First, let me say that this NAHB publication is very nice. Very nice work. Here is a link where it can be downloaded if you have not seen it.

Revised Builder’s Guide to Frost Protected Shallow Foundations | Home Innovation Research Labs

I am not a heat-transfer engineer, but IMO, you don’t need to be in order to look at the recommendations in this document from a qualitative, but questioning, point of view, which is what I have been doing. There are two scenarios presented. Frost-proof shallow foundations in heated buildings and the same thing in unheated buildings. The primary focus is heated buildings and for this scenario, everything adds up nicely, at least in my eyes. However, for unheated buildings, things get murky to me. I hope someone can guide me through this scenario under the following simple and reasonable example. Note first, that for unheated buildings, the only stipulation in the guide is that the minimum average monthly indoor temperature of the building be above 41F.

OK, imagine a reasonable cold winter situation in which the outside air temperature does not rise above 32F for a straight week and the sky is also cloudy for this period. Under these conditions, how could our unheated building temperature be above 41F? How can we keep the foundation from freezing? To me, the only way is if the building is able to tap into the geothermal energy stored in the ground. What other heat source is available? That is my premise-1, which is the need to gain as much stored geothermal energy as possible and to minimize as much as needed, energy loss to the cold outside air. If you look at Figure-7 of this guide, things do not add up. The outside horizontal insulation, Dg, is logical. However, as with the heated building scenario, what about including outside vertical insulation to shield the footing from the outside cold air? More importantly, what is the purpose of insulation under the footing and under the slab? To my eye, that is shooting yourself in the foot by shielding the footing and interior slab from the only potential heat source, the ground.

I must be missing something obvious here, but what?