I am building a small addition with a footprint of 12×10 in Northern Vermont, Climate zone 6, 2250 AFI, & 42 MAT. I would like to use a frost protected shallow foundation, without insulation “wings” due to the sloped site and resulting difficulty of excavation and backfill. The heated building FPSF design allows for “no wings” but the detail does not have insulation under the footings/ thickened edge, to allow for building heat to keep the soil above freezing. How big of net energy loss is this through the footings at 16″ below grade?

I did a very rough calculation using 0.2 Btu/(hr*sqft*deg F), a value I found for heat loss through basement walls, 14″ below grade, no insulation. So given a 12″ footing, 12+12+10′ perimeter, 30 deg F soil temperature, 70 degree interior temp I am getting ~270 BTU/hr or 6500 BTU/day. Since I heat with wood exclusively, I put that into terms I can understand -it’s an extra 2x2x16″ piece of maple firewood per day, or given 180 days per year I’m running the stove, an extra 0.05 cord per year. Seems insignificant, especially since I think these assumptions are quite generous in the “more energy” direction.

Any thoughts? Are my rough calculation and assumptions valid? See attached for my full heat loss hand-calc.