A frost protected shallow foundation (FPSF) places footings above the frost line and protects them with insulation. Under 2021 IRC Section R403.3 it is allowed for buildings kept at a monthly mean temperature of not less than 64°F, with insulation per Figure R403.3(1) and Table R403.3(1).
An FPSF is a shallow footing, protected from frost by insulation placed around it instead of being carried below the frost line. The 2021 International Residential Code (IRC) covers it in Section R403.3. The code edition your jurisdiction has adopted, plus any local amendments, governs, and engineered or manufactured foundation systems follow the manufacturer's tables or instructions.
Key facts
- 2021 IRC Section R403.3 allows footings to stop above the frost line only where the building's monthly mean temperature is maintained at not less than 64°F (18°C).
- 2021 IRC Section R403.3 bars the method for unheated spaces such as porches, utility rooms, garages and carports.
- 2021 IRC Table R403.3(1) sets minimum footing depth at 12 inches for an air-freezing index of 1,500 or less and at 16 inches for an index of 2,500 to 4,000.
- 2021 IRC Table R403.3(1) footnotes d and e require vertical and horizontal insulation to be expanded polystyrene or extruded polystyrene.
- 2021 IRC Section R403.3.2 requires protection for horizontal insulation placed less than 12 inches below the ground surface or extending more than 24 inches from the foundation edge.
- 2021 IRC Section R403.3 requires insulation used below grade for frost protection to be labeled as complying with ASTM C578.
When is a frost protected shallow foundation allowed?
An FPSF is allowed only for heated buildings, where the monthly mean temperature of the building is maintained at not less than 64°F (2021 IRC Section R403.3).
The method depends on the building's heat, so it cannot be used for unheated spaces such as porches, utility rooms, garages and carports, and why it also cannot be attached to a basement or crawl space that is not maintained at a monthly mean of 64°F.
What is the minimum footing depth for an FPSF?
Table R403.3(1) gives the minimum footing depth D as 12 to 16 inches, depending on the air-freezing index for the site.
The air-freezing index is defined in the figure note as cumulative degree days below 32°F over a 12-month period, a measure of the combined size and length of freezing weather. Figure R403.3(2) maps it, and Table R403.3(2) lists values by location. Your local jurisdiction fills in the index in its adopted Table R301.2. Read the value for your site there; this article does not list the location table.
2021 IRC Table R403.3(1): minimum footing depth and insulation requirements for frost-protected footings in heated buildings. dimensions A, B and C are in inches and are located per Figure R403.3(1).
| Air-freezing index (°F days) | Minimum footing depth D (in.) | Vertical insulation R-value | Horizontal R-value along walls | Horizontal R-value at corners | A | B | C |
|---|---|---|---|---|---|---|---|
| 1,500 or less | 12 | 4.5 | Not required | Not required | Not required | Not required | Not required |
| 2,000 | 14 | 5.6 | Not required | Not required | Not required | Not required | Not required |
| 2,500 | 16 | 6.7 | 1.7 | 4.9 | 12 | 24 | 40 |
| 3,000 | 16 | 7.8 | 6.5 | 8.6 | 12 | 24 | 40 |
| 3,500 | 16 | 9.0 | 8.0 | 11.2 | 24 | 30 | 60 |
| 4,000 | 16 | 10.1 | 10.5 | 13.1 | 24 | 36 | 60 |
The table's note a says the insulation requirements protect against frost damage in heated buildings and that greater values could be required to meet energy conservation standards. Where the figure and the table are used together, Figure R403.3(1) shows where vertical insulation, horizontal insulation and dimensions A, B and C go.
What insulation can be used, and how is thickness found?
Vertical and horizontal insulation must be expanded polystyrene or extruded polystyrene, and the table's R-values are converted to thickness using per-inch values given in footnote c.
2021 IRC Table R403.3(1), footnote c: R-value per inch of insulation material used to determine thickness.
| Material | Vertical insulation, R per inch | Horizontal insulation, R per inch |
|---|---|---|
| Type II expanded polystyrene (EPS) | 3.2 | 2.6 |
| Type IX expanded polystyrene (EPS) | 3.4 | 2.8 |
| Types IV, V, VI, VII and X extruded polystyrene (XPS) | 4.5 | 4.0 |
Footnote c also says the insulation must provide the stated minimum R-values under long-term exposure to moist, below-ground conditions in freezing climates. Section R403.3 requires materials used below grade to insulate footings against frost to be labeled as complying with ASTM C578. Use the per-inch values from the footnote to find a thickness; the adopted code and the insulation manufacturer's published data govern.
How is horizontal insulation protected?
Horizontal insulation needs protection when it sits less than 12 inches below the ground surface or extends more than 24 inches from the foundation edge (2021 IRC Section R403.3.2).
The code lists the allowed protection: a concrete slab or asphalt paving on the ground surface directly above the insulation, or cementitious board, plywood rated for below-ground use, or other approved materials placed below ground, directly above the top surface of the insulation. For drainage, Section R403.3.3 says final grade is sloped per Section R401.3, and in soils other than Group I, gravel or crushed stone beneath horizontal insulation must drain to daylight or into an approved sewer system. Section R403.3.4 sends the use of foam plastic in areas of "very heavy" termite infestation probability to Section R318.4.
How does an FPSF connect to an adjoining foundation?
An adjoining foundation must itself be frost protected under Section R403.1.4, and the attachment rules differ for an unheated slab and a heated structure (2021 IRC Sections R403.3.1 to R403.3.1.2).
- Unheated slab-on-ground structure (Section R403.3.1.1). Vertical wall insulation and horizontal insulation follow Figure R403.3(3) and Table R403.3(1). Vertical insulation extends between the FPSF and the adjoining slab, and horizontal insulation is continuous under the adjoining slab and through any foundation walls. Where insulation passes through a foundation wall, it must either have bearing capacity equal to or greater than the building's structural loads, or the building must be designed with beams, lintels, cantilevers or other means so the loads do not bear on the insulation.
- Heated structure (Section R403.3.1.2). Horizontal and vertical insulation are not required between the FPSF and a structure maintained at a monthly mean of not less than 64°F. Where the FPSF abuts it, the insulation extends along the adjoining foundation, per Figure R403.3(4), a distance of not less than Dimension A in Table R403.3(1). Where the two form an inside corner, vertical insulation along the adjoining foundation is not required.
How does an FPSF compare with a conventional footing?
A conventional footing goes below the frost line, while an FPSF stays shallow and relies on heat from the building and insulation.
The base footing rule in Section R403.1.4 and Section R403.1.4.1 is covered with slab foundations in the monolithic slab article, which notes that a frost-protected shallow foundation under Section R403.3 is one of the allowed alternatives to extending below the frost line. The stem wall foundation article covers the raised-floor alternative, and the concrete block foundation article covers masonry foundation walls. An FPSF is not a crawl space or basement solution unless those spaces are kept at the 64°F monthly mean.
What should a framing plan show for an FPSF?
The foundation sits below the framing plan, but the floor system above must still bear on it. A floor framing plan should show bearing lines that land on the thickened slab edge, and the framing studio can draw those lines. The insulation, depth and dimensions A, B and C are foundation design items and belong on the foundation drawings, checked against the adopted code and your jurisdiction's air-freezing index.
Summary
The 2021 IRC allows a frost protected shallow foundation only for buildings kept at a monthly mean of not less than 64°F, never for unheated porches, garages or carports (Section R403.3). Table R403.3(1) sets footing depth at 12 to 16 inches and the insulation R-values and dimensions by air-freezing index, using expanded or extruded polystyrene labeled to ASTM C578. Horizontal insulation needs protection near the surface or beyond 24 inches from the edge (Section R403.3.2). Your adopted code and local amendments govern.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check foundations or any other code requirement: the editor works in millimeters and its sample profile uses demo span figures and metric member sizes, so treat its output as a preliminary drawing aid. To lay out the floor framing that bears on your foundation, open the framing studio. The code text is at codes.iccsafe.org.