A resource efficient design for foundations in all U.S. climates
Frost-Protected Shallow Foundations (FPSF) provides protection against frost damage without the need for excavating below the frost line. An FPSF has insulation placed strategically around the outside of a foundation to direct heat loss from the building toward the foundation, and also to use the earth's natural geothermal energy.
Traditionally, foundations are protected from frost-heaving damage by placing the footing below the frost line. Because FPSF are protected from freezing by thermal insulation, bottoms of footings can be just twelve to sixteen inches below grade. This reduces excavation costs, making this an economical alternative for protecting foundations against frost damage.
Insulated footings had been used as early as the 1930s by Frank Lloyd Wright in the Chicago area. There are now over one million homes in Norway, Sweden, and Finland with insulated shallow footings, recognized by their building codes as standard practice. It is estimated that there are over 5,000 buildings in the United States that have successfully used frost protected shallow foundations.
Frost-protected shallow foundations are similar to conventional foundations except for insulation placement and footing depth. Bottoms of FPSF footings are placed about 12 to 16 inches below grade. FPSF have vertical insulation placed at the outside edge of the foundation extending from above grade to the bottom of the footing. When required in colder climates, "Wing" insulation extends outward horizontally from the footing. The colder the climate, the further the wing insulation is extended. Wing insulation is unnecessary in moderate climates.
The insulation used in FPSF is commonly rigid expanded or extruded polystyrene foam suitable for below grade application, and it must be in compliance with ASTM C 578 Standard. FPSF can be used for both heated and unheated portions of a building.
Frost-protected shallow foundations (FPSF) allow bottoms of footings to be shallower than the normal code-required frost depth. This reduces the amount of excavation and concrete or masonry required, which reduces labor and material costs. Heating energy costs are also reduced because of increased perimeter foundation insulation.
Insulation around the perimeter of the foundation reduces the amount of heat loss from the warm interior to the cold exterior in the winter, which would normally pass through the slab or floor, and out through the foundation. Heating energy costs are thus reduced.
Frost-protected shallow foundations rely on building and geothermal heat to keep the earth around the foundation warm enough to preclude frost formation that might heave the structure. The foundation is wrapped with polystyrene foam that provides a thermal barrier and channels the escaping heat to warm the earth beneath the foundation. The method allows footing or foundation depth to be reduced from 48" to 16" in colder climates; representing a 2/3 savings in the amount of concrete needed, and with less excavation, disturbing less of the surrounding earth.
Ease of Implementation
Frost protected shallow foundations are simpler to install than traditional foundations. Because they are shallower than conventional footings/foundations in a given region and rely on polystyrene foam for insulation, they can be installed with simpler equipment, like a trenching machine instead of a backhoe. The rigid polystyrene foam is a natural as a leave-in-place form, so once the style is mastered, crews find FPSFs easier and faster to install. Detailing the outside edge of the foam with a durable, attractive finish is the biggest challenge to a FPSF.
Cost is highly dependent on building size and climate. Savings of $635-4750 have been reported.
Provisions for FPSF are included in the 2000 and 2003 edition of the International Residential Code (IRC).The American Society of Civil Engineers developed a standard, "Design and Construction of Frost-Protected Shallow Foundations", ASCE/SEI 32-01. This standard is referenced by the 2000 (by amendment) and 2003 International Residential Code (IRC). ASCE/SEI 32-01 is also adopted by the 2003 International Building Code (IBC) which allows FPSF to be used on multifamily buildings, houses four stories and above, very large houses, malls, schools, and offices.
In areas of heavy termite infestation, local codes may restrict the use of rigid foam insulation on the outside of the foundation, or may require termite resistant insulation.
K. Hovnanian Inc: College Park Estates. Freehold Township, New Jersey
Oakwood Homes: Green Valley Ranch. Denver, Colorado
A FPSF can be incorporated into slab-on-ground construction, but it can also be used for stemwall, floating slab, and unvented crawlspace foundations.
The NAHB Research Center published a "Design Guide for Frost Protected Shallow Foundations" to help determine proper foundation design and insulation details. This design guide contains detailed drawings of the prescribed methods of construction. In addition, the National Association of Homebuilders (NAHB) has published brochures describing FPSF.
Care should be taken that the foundation edge is completely insulated with no uninsulated area through which heat can pass. As with any foundation type good drainage is important so that water does not collect below the footing and freeze.
Manufacturers do not provide warranties on an in-place system like a foundation. The foam used should perform to the ASTM standards contained in that manufacturer’s literature. Builders will typically provide a ten year limited warranty to the home buyer on foundations, so careful execution of the details should be conformed to.
Construction and excavation, labor and material costs are less for FPSF than for conventional foundations. In a 1993 NAHB Research Center demonstration project, the cost of an FPSF was compared to an equivalent slab-on-grade foundation with footings extending to the local design frost depth. A 1988 study by the NAHB Research Center, Inc., showed a 15 to 21 percent cost savings with FPSF over conventional foundations. Another benefit is with the slab insulated from the outside, energy efficiency and comfort factor of the occupants is increased.
Exterior foundation insulation requires detailing for an exterior durable finish to protect the insulation from ultraviolet damage and mechanical damage, this can add $2 per square foot (of exposed foundation) to the cost of the FPSF and should be factored in to the cost decision.
Copyright 2012 Ronald Sauve All Rights Reserved
This page was last modified on April 06, 2012
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