F-Factor
F-factor is the heat-loss coefficient for a slab-on-grade foundation, expressed in BTU per linear foot of perimeter per °F of temperature difference. Slab-on-grade heat loss is dominated by the perimeter (where the slab edge meets cold soil), not the slab area, which is why F-factor uses linear-foot units. Lower F-factor means better-insulated slab edge. Uninsulated slabs run F-0.73; R-10 vertical edge insulation 24 inches deep drops to F-0.52; frost-protected shallow foundations can hit F-0.38.