Twenty years ago, I built my first frost-protected shallow foundation here in Maine. It cost me less than $500 to install and has performed flawlessly ever since; I still live on it today. Admittedly, this type of foundation is eyed somewhat warily by those unfamiliar with its design principles. But, in fact, as foundations go, it’s quick and cost-effective and provides an excellent base for the super-insulated homes we build.
Compared with a full foundation, a frost-protected slab can reduce construction costs in my market by about $20,000 for a house with 850 square feet on the first floor. Part of that economy comes from the fact that the base prep and formwork for a shallow foundation can be done by a crew of carpenters instead of a foundation contractor. That’s one less sub to manage and pay. Other savings come from reductions in the amount of excavation and concrete needed. Also, there’s no first-floor deck to frame.
The Shallow Concept
A frost-protected shallow foundation (FPSF) relies on rigid foam insulation to protect the foundation from frost. (One-inch-thick extruded polystyrene foam — XPS — has an R-value of 4.5 and an insulating effect equal to 4 feet of soil.) On a typical FPSF, a layer of rigid insulation is placed vertically around the edge of the slab and extended horizontally for a given distance, depending on the severity of the climate. This “wing” insulation traps the heat of the earth under the foundation, ensuring that the soil there doesn’t freeze.
Although FPSFs can be configured as crawlspace foundations, I’ve always favored the slab-on-grade approach, both for the economy of using the slab as the finish floor and for the thermal mass it provides in a passive solar design. Of course, this limits me to relatively flat sites. I’ve built FPSFs on slopes with a 3-foot change in elevation from one corner of the slab to another, but if the site is much steeper than that, the cost of elevating the low side with gravel fill becomes an issue. At that point, a frost wall or full basement probably makes better sense.
(Also keep in mind that not every locality approves FPSFs; regions with high termite infestations are less likely to allow buried foam. Check with your local building department for applicability.)
There’s probably no single best way to design and detail an FPSF. A lot of information is available on the Web to guide the installation of these foundations. I’ve found the NAHB document “Revised Builder’s Guide to Frost Protected Shallow Foundations” the most useful (toolbase.org). This document divides FPSFs into two categories, those for heated buildings and those for unheated buildings, with different details for each.
Heated buildings. The theory behind FPSF design for heated buildings is that the indoor heat will move through the slab to the ground below and prevent it from freezing. In mild climates, only the slab’s vertical edge has to be insulated. In colder regions, wing insulation of a given thickness and width is specified according to the specific regional air freezing index (AFI). You can find AFI data for 3,110 cities in the U.S. and Puerto Rico at the National Climatic Data Center (ncdc.noaa.gov). And IRC Table R403.3(2) categorizes the index for all 50 states by county.
Unheated buildings. Though garages, barns, and other outbuildings may not be heated, they can still be built on FPSFs. At a certain depth, which varies by geographic locale, the ground temperature remains at a constant temperature well above the freezing point all year round. By insulating the surface of the ground beneath the foundation, sufficient heat can be trapped to prevent freezing. That’s the principle behind FPSF design for unheated buildings. Insulation is placed beneath the entire slab and out beyond its edges, again to a distance determined according to the regional AFI. Vertical perimeter insulation is unnecessary. In all but the most severe climates, 2- or 3-inch-thick sub-slab foam is all it takes to protect the foundation.
Super-insulated slabs. In coastal Maine, we have an AFI of less than 2,250. Here, according to the NAHB design guide, a slab foundation for a heated building requires neither horizontal rigid insulation underneath nor insulation extending beyond the slab’s perimeter. But losing heat to the ground below is not consistent with the super-insulated approach our company uses. So even though the homes we build are heated, our shallow foundation design resembles that used for unheated buildings, except with much greater insulation levels. When insulating a slab, we aim for R-values as high as 30 or even greater (see “Sub-Slab Insulation: How Much Is Enough?”).
Which Foam is Best?
Unlike a conventional foundation that bears directly on the ground, an FPSF foundation bears on a platform of foam. Our structural engineer specifies the appropriate foam density to support the weight of the building. Typically, for a two-story wood-framed house or barn, we’ve used foam with a density of 2.4 pounds per cubic foot and a compressive strength of 15 psi.
Depending on the energy goals of the project, we’ve placed between 2 inches and 6 inches of XPS or expanded polystyrene (EPS) under slabs, giving us sub-slab insulation values between R-10 and R-30. XPS is generally rated at R-5 per inch and EPS at R-4.5. The traditional view has been that XPS is best for burial because it absorbs less moisture over time. However, a more recent 15-year study indicates that EPS may actually absorb less water and retain a higher R-value over time than XPS.
One important difference between the two types is that EPS is more environmentally friendly. Both foams are produced with blowing agents, but EPS is expanded using a relatively benign hydrocarbon such as pentane or butane, while XPS production uses HCFCs, which have implications for ozone depletion and global warming.
Available material thickness is another consideration. Although some sources imply that it’s okay to place multiple layers of sub-slab foam, our structural engineer advises us to use a single layer to avoid the chance of sideways slippage between sheets, or of voids occurring between them that could cause eventual settling. We can readily get 6-inch-thick EPS but have had no luck finding XPS in sheets thicker than 2 inches. For these reasons, we typically use 4-foot-by-16-foot sheets of 6-inch-thick Geofoam EPS (401/232-0270, branchriver.com), which has a nominal R-value of 4.6 per inch, or R-27.6 overall. These sheets can be easily handled by two workers.