Frost-protected shallow foundation: foam instead of a deep footing

IRC R403.3 lets heated buildings (monthly mean ≥64°F) use a 12–16″ footing wrapped in rigid foam instead of digging below the frost line. Depth, R-values and wing width come from Table R403.3(1) by Air Freezing Index.

Cross-section of a heated building’s slab-on-grade edge. The thickened slab edge and footing reach only 12 to 16 inches below finish grade. Rigid foam insulation covers the outside face of the foundation from the slab down to the footing, and a horizontal foam wing extends 12 to 24 inches outward at the footing level. Heat from the building and the ground is held in under the footing, so the soil there stays unfrozen, while in the open ground beyond the insulation the frost line dips much deeper. Notes state the vertical foam is R-4.5 to R-10.1, the wing is needed only where the Air Freezing Index is 2,500 or more, and the method is for heated buildings only, not unheated garages, porches or carports. — Source: 2021 IRC R403.3 and Table R403.3(1); ASTM C578 foam

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Cross-section of a heated building’s slab-on-grade edge. The thickened slab edge and footing reach only 12 to 16 inches below finish grade. Rigid foam insulation covers the outside face of the foundation from the slab down to the footing, and a horizontal foam wing extends 12 to 24 inches outward at the footing level. Heat from the building and the ground is held in under the footing, so the soil there stays unfrozen, while in the open ground beyond the insulation the frost line dips much deeper. Notes state the vertical foam is R-4.5 to R-10.1, the wing is needed only where the Air Freezing Index is 2,500 or more, and the method is for heated buildings only, not unheated garages, porches or carports.

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