There are two good sources for designing frost-protected shallow foundations (FPSFs): The National Association of Home Builders (NAHB) Research Center's Revised Builder's Guide to Frost Protected Shallow Foundations has basic information and offers a simplified FPSF design method for heated buildings that avoids crunching numbers, as well as a detailed design method for those who want to dig deeper. The American Society of Civil Engineers' publication, Design and Construction of Frost-Protected Shallow Foundations, offers greater detail. Both publications provide maps, tables, and charts necessary for the calculations.
The calculations below are for the sloped-site monolithic slab described in the article and follow the steps described in the NAHB's detailed method. The tables referenced are from NAHB's Builder's Guide (PDF).
Step 1: Determine the site's Design Air Freezing Index.
- Closest datapoint to jobsite: 1,683
Step 2: Calculate the R-value of the floor system cross section (Table 9, Nominal Resistance of Common Materials)
- 4-inch concrete slab with R-0.05 per inch 0.20
- 2-inch XPS rigid foam with R-5.0 per inch 10.00
- No floor covering 0.00
- Total floor system R-value 10.20
Step 3: Determine the required R-value of vertical wall insulation (Table 4. Minimum Thermal Resistance of Vertical Wall Insulation)
- Foundation height above grade: 12 inches
- Vertical wall insulation: R-5.7
Step 4: Select vertical wall insulation (Table 2, Design Values for FPSF Insulation Materials)
- EPS Type II with an R-value of 3.4 per inch
- Thickness of insulation required: 5.7 ÷ 3.4 = 1.67 inches
- Wall thickness of Reward ICFs is 2.5, more than enough to meet FPSFdesign criteria
Step 5: Select foundation depth or horizontal insulation
- Not required as AFI (Air Freezing Index) is less than 2,250
- If needed, use Table 5 (Foundation Depths) with AFI >2,250
Step 6: Select thickness of horizontal insulation for walls (Table 2, Design Values for FPSF Insulation Materials)
If design calls for horizontal insulation, a minimum of 12 inches of ground cover is needed, and the horizontal insulation must securely abut the vertical wall insulation.
Step 7: Select foundation depth or horizontal insulation at corners (Table 6, Minimum Thermal Resistance of Horizontal Insulation Along Walls; and Table 7, Minimum Thermal Resistance of Horizontal Insulation at Corners)
- Not required
- If horizontal insulation is required for the design but is not desired, the foundation depth at corners can be increased to offset the need for horizontal insulation. More heat loss occurs at slab corners than through mid-wall sections.
Lee McGinley is a Certified Passive House Tradesperson who designs and builds high-performance homes. He lives in Addison, Vt. His testimony on the effectiveness of FPSFs was submitted by the National Association of Home Builders to the Council of American Building Officials (precursor to the ICC codes) in a successful effort to retain FPSFs as code-compliant building systems.