Frost-Protected Shallow Foundations

By Bennett's Concrete and Excavation, Norwich, NY. Updated October 2026.

The idea

A conventional footing goes below the frost line so frozen soil can't heave it. A frost-protected shallow foundation (FPSF) flips that around. The HUD-sponsored builder's guide by the NAHB Research Center explains that heat from a conditioned building warms the soil around the foundation, and rigid foam insulation placed along and out from the footing magnifies that effect, raising the frost depth right at the foundation.

It isn't new. The guide notes more than a million homes in Norway, Sweden and Finland built on insulated shallow footings, and several thousand in the US, including in Alaska.

What the code allows

New York's residential code is derived from the International Residential Code (the 2020 edition from the 2018 IRC), and the state's 2025 Uniform Code update took effect December 31, 2025. The IRC (R403.1.4.1) lists four ways to protect a foundation from frost: extend footings below the frost line, build a frost-protected shallow foundation under R403.3, design to the ASCE 32 standard, or bear on solid rock.

The prescriptive FPSF method in R403.3 applies to buildings kept at a monthly mean temperature of 64°F or higher. It can't be used for unheated spaces such as porches, utility rooms, garages and carports, and can't be attached to basements or crawl spaces that aren't kept at that temperature. Confirm with your code enforcement officer before designing around it.

How deep, and how much foam

The design depends on the air-freezing index (AFI), a measure of how cold a location's winters get. Table R403.3(1) sets minimum footing depth and vertical insulation by AFI:

At the colder rows, horizontal wing insulation extending out from the foundation is also required, especially at corners. The code assigns an AFI to each county in a companion table, and your designer works from that.

Insulation details that matter

• Material: foam must meet ASTM C578. The HUD guide says vertical insulation can be EPS or XPS, and horizontal insulation in its simplified method is XPS.
• Design values: the guide reduces foam's rated R-value for below-ground use, about 10 percent for XPS and 20 percent for EPS in vertical use.
• Protection: vertical foam needs a protective covering where exposed above grade and down to 6 inches below grade. Under R403.3.2, horizontal insulation less than 12 inches below grade, or extending more than 24 inches out, must be protected, for example by a slab or paving above it.
• Fuel: the guide warns that polystyrene breaks down in contact with gasoline or diesel.

Drainage is part of the design

Frost needs water. The HUD guide calls for a clear path for water to drain away from the foundation and for insulation to be placed above the groundwater table. Under R403.3.3, final grade must slope away, and in soils that don't drain well, gravel or crushed stone under horizontal insulation must drain to daylight or an approved sewer. On wet, clay-heavy Central New York sites, that drainage plan is as important as the foam.

Garages and unheated buildings

Because the prescriptive method depends on building heat, unheated buildings need a different approach. The HUD guide covers unheated garages with a detailed method, and the IRC points to ASCE 32 for semi-heated and unheated buildings. An attached garage next to a heated FPSF house needs its own frost protection. For small detached buildings, see the accessory structure exceptions in our shed pad guide.

How it compares here

For context, engineering firm KHH cites a design footing depth of 3 feet 6 inches for heated buildings in Syracuse, and says most local engineers use 4 feet. An FPSF trades much of that digging for insulation and careful drainage. Whether it makes sense depends on the site, the building and the designer. Bennett's builds monolithic slabs and ICF foundations to the plans you bring; see also our monolithic slab guide and frost heave explained.