Simplified Design Method

To use the simplified approach, the air freezing index (AFI) for the site location must be known. Insulation R-value and dimensions, and the depth of the footings are then determined from Table 2 and Figure 4. Horizontal insulation shall be bedded firmly on smooth ground or granular base. When foundation depths greater than 12 inches are required by Table 2, the increase in depth may be satisfied by substituting compacted gravel, crushed rock, sand, or approved non-frost susceptible materials.

Table 2. MINIMUM INSULATION REQUIREMENTS FOR FROST PROTECTED FOOTINGS IN HEATED BUILDINGS¹

Insulation requirements are for protection against frost damage in heated buildings. Greater values may be required to meet energy conservation standards. Interpolation between values is permissible.
See Appendix for Air Freezing Index values.
Insulation materials shall provide the stated minimum R-values under long-term exposure to moist, below-ground conditions in freezing climates. The following R-values shall be used to determine insulation thicknesses required for this application: Type II expanded polystyrene - 2.4R per inch; Types IV, V, VI, VII extruded polystyrene - 4.5R per inch; Type IX expanded polystyrene - 3.2R per inch. NR indicates that insulation is not required.
Vertical insulation shall be expanded polystyrene insulation or extruded polystyrene insulation.
Horizontal insulation shall be extruded polystyrene insulation.