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Slab-on-grade heat loss: the perimeter method
Slab-on-grade heat loss is different from wall or roof heat loss. Heat doesn't flow straight down through the soil — it flows outward through the slab edge, then down and back up through the soil to the outdoor air. This means the heat loss is proportional to the perimeter of the slab, not its area. A 40 x 30 ft slab and a 20 x 60 ft slab have the same area but very different perimeters and different heat losses.
ASHRAE F2 method
ASHRAE Fundamentals uses the F2 factor method: Q = F2 x P x DT. F2 is a heat loss coefficient in BTU/hr per linear foot of perimeter per degree F. ASHRAE Table 18.26 gives F2 values ranging from 0.73 for uninsulated slabs to 0.32 for heavily insulated slabs. The F2 factor accounts for soil thermal resistance, slab conductivity, frost depth, and insulation placement. For most Canadian homes, slab edge insulation of R-10 to R-15 to a depth of 2-4 ft is the standard approach.
Radiant floor slabs
A radiant floor heating slab operates at higher temperatures than a passive slab. The slab surface temperature might be 85-95°F (29-35°C) instead of room air temperature. This increases the effective DT at the slab edge and increases heat loss. For radiant slabs, use the slab surface temperature as your indoor reference temperature, not the air setpoint. Insulating a radiant slab edge is especially important — uninsulated radiant slabs waste 15-25% of heating output through the perimeter. See the heat load calculator to combine slab loss with other envelope components.
Frequently Asked Questions
Heat flows from warm to cold along the path of least resistance. Under a slab, the soil directly below is relatively warm (near the deep ground temperature of 8-12°C year-round). The dominant heat flow path is outward through the slab edge, then down through the soil near the surface where it's coldest, then back up to outdoor air. This edge-dominated flow pattern means perimeter controls heat loss, not area. A slab with double the area but the same perimeter loses the same amount of heat.
NBC 2020 requires minimum R-10 full perimeter or R-7.5 below grade plus R-5 above grade for slabs in most Canadian climate zones. Many energy programs require R-15 or higher. The energy code calculator gives province-specific requirements. From a heat loss perspective, R-10 to 4 ft depth reduces slab heat loss by about 38% compared to an uninsulated slab. R-20 to 4 ft reduces it by 56%.