Shallow, Insulated Foundations Lower Construction Costs
Most cold-climate building codes require you to place foundation footings below the frost line. That can be 3- to 4-feet deep in the northeastern United States. The goal is to protect foundations from frost heaving. There is an exception to this standard: many codes permit foundations to lie above the frost line as long as they're "protected from frost." However, approval depends on local code officials, and may require special engineering. The 1995 edition of the Council of American Building Officials (CABO) One and Two-Family Dwelling Code may simplify life for builders and code officials alike. The CABO code, which serves as the basis for the country's other three model codes, includes simplified guidelines for building slab-on-grade homes with shallow foundations that are protected from frost by rigid foam insulation.
The technology cited by CABO-called the Frost Protected Shallow Foundation (FPSF) not only saves energy, but slashes construction costs as well. FPSFs can be used beneath heated or unheated buildings. The technology itself is nothing new. It has been used in Scandinavia for more than 40 years (where it's now standard practice), and the National Association of Home Builders (NAHB) has aggressively promoted it in the U.S. for more than a decade. An FPSF often improves the energy efficiency of a typical home, because it requires more foundation insulation than many codes. (The exceptions are states with especially strong energy codes, like Washington and Oregon.)
How It Works
The concept is simple. Instead of placing footings below the frost line, the FPSF uses insulation and drainage techniques to raise the frost line to just below the surface. "We basically make the footings think they're in Florida," quips Bill Eich, a builder from Spirit Lake Iowa, and a nationally known proponent of the system. Even in the coldest climates, this permits footing depths as shallow as 12 inches.
Compressive strength is an important characteristic for these below-grade applications. Compressive strength is related to foam density. Extruded polystyrene used for sheathing above-grade walls is typically 1.5 pounds per cubic foot (pcf). Naturally, foundation applications require stouter material. CABO permits 2.0 pcf extruded polystyrene for horizontal insulation and 2.0 pcf extruded or expanded polystyrene for vertical insulation. The compressive strength of 2 pcf extruded is 40 pounds per square inch (psi) or 3600 pounds per square foot (psf). This exceeds the underlying soil's bearing capacity of around 2500 psf. If you prefer a margin of safety, higher density foams are available.
Designing Good Drainage
Insulation is only half the equation. The other half is drainage and moisture control. To keep surface water from soaking in around the foundation, all roof runoff must be directed away from the house. This means putting effective gutters all around the building and sloping the final grade away from the foundation at least 5 inches in the first 10 feet. (That's 1/2 inch per foot.) To protect the footing from subterranean water, it must bear on at least 4 inches of a non frost-susceptible material such as washed gravel or rock.
Tests Prove Efficiency
NAHB confirmed the system's efficiency by placing test probes around five homes in Vermont, Iowa, North Dakota and Alaska. Instruments recorded ground, foundation, slab, indoor and outdoor temperatures. The insulated footings kept the soil above freezing even in the coldest weather. When probes that Eich buried three feet below uninsulated ground measured temperatures below freezing, those at the base of nearby shallow foundations checked in at 37°F to 40°F.
A Win/Win System
An FPSF can benefit both builder and homeowner. Shallow foundation ditches are easier to work around. The FPSF uses less concrete than a 4-foot deep stemwall. Smaller ditches require less backfill material and the backfill settles less over time. Since a shallow ditch is less likely to disturb root systems, you can leave shade trees closer to the house. (Eich has built within three feet of large trees.)
Eich credits FPSF's with saving an average of $1500 in construction costs for a typical home. "Even homes with full basements usually have walkout portions or attached garages" he notes, "so we routinely plan on a shallow foundation for everything we do."
The technology has even made some customers reconsider what they want in a house. In Eich's market, everyone used to build full basements, since the footing had to go down 4 feet anyway. Now he finds more people building larger homes on the main level and forgetting about the basement. In a 131-unit Denver, Colorado, housing project, the U.S. Department of Housing and Urban Development was able to save $3000 per unit by substituting traditional stem walls with FPSFs. NAHB estimates that, given a realistic market penetration, the system could save nearly $300 million in annual construction costs.
How does insulation stop frost heave from occurring?
"Frost heave can only occur when all the following three conditions are present: 1) the soil is frost susceptible (meaning it contains more than 5 percent silt), 2) sufficient moisture is available (soil is above approximately 80 percent saturation) and 3) subfreezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of frost damage.
"Insulation as required in this design guide will prevent underlying soil from freezing. Soil has an insulating value ranging between R-1 per foot and R-3 per foot. (Yes, these values are in feet, not inches.) An inch of polystyrene insulation, R-4.5, has an equivalent R-value of about 4 feet of soil on average. The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation's soil-not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation. Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards."
-Source: Design Guide for Frost-Protected Shallow Foundations, National Association of Home Builders, 1995.
In a heated building, the frost protected shallow foundation (FPSF) relies on heat from the house to raise soil temperatures around the foundation. One layer of insulation covers the outside face of the foundation, while a second extends horizontally away from it. The rigid foam traps any heat that the ground absorbs from the building, keeping soil temperatures around the footing above freezing. The building's heating system can be safely turned off for a three week period in the winter because thermal lag in the concrete will maintain the soil temperature above freezing.
The vertical foam also protects the foundation wall from "ad freezing." Ad freezing occurs when expansive soils freeze to the outside of a foundation. In some cases, this can heave a foundation whose footing is below the frost line.
In a standard uninsulated foundation the footing must be buried deep below the frost line.
Unheated garages and other outbuildings require a different approach. A shallow foundation for one of these structures relies on heat from the earth. Ground temperature remains fairly constant around the local average annual temperature, which is between 40 and 50 degrees in the U.S. The footing bears directly on a layer of foam. The foam must extend beneath the entire structure, and it must bear on 6 to 12 inches of gravel.
This article appeared in Energy Source Builder #43 February 1996,