Below Grade Foundations 

Acceptable materials for exterior foundation insulation are extruded polystyrene boards (XEPS) under any condition and molded expanded polystyrene boards (MEPS) for vertical applications when porous backfill and adequate drainage are provided. The use of expanded polystyrene (EPS) insulation for foundations and floors below grade has been consistently growing in popularity. EPS insulation is an efficient method of conserving heat energy in cold climates and air conditioning energy in warm climates. 

EPS is a lightweight, easy-to-handle and rigid thermal insulation. Its closed-cell structure assures long lasting thermal insulation properties and water-resistance. EPS insulation will not support the growth of bacteria or decay over time. It is available in several compressive strengths to withstand load and backfill forces. Usually Type I EPS material, as described in ASTM C 578-97, is adequate for foundation applications. 

A field study conducted by the Institute of Research in Construction based in Ottawa, Ontario, an adjunct of the National Research Council (NRC), found that EPS as exterior basement insulation is quite durable. After two complete freeze-thaw cycles, the insulation showed no degradation of its physical and thermal properties. The test results are especially impressive since the soil around the basement had heavy clay content, which retains moisture. 

For further information, visit the Institute of Research in Construction, an adjunct of the National Research Council (NRC). 

Frost-Protected Shallow Foundations 

The Construction, Codes and Standards Department of the National Association of Home Builders (NAHB) has developed a brochure entitled "Frost-Protected Shallow Foundations." It provides builders with a comprehensive review of the "how, where and why" of insulating shallow foundations. 

Frost-protected shallow foundations (FPSFs) are commonplace in Scandinavia. Within the past 45 years, more than one million structures with FPSF have been constructed. Builders in the United States have just recently started using this technology. It received a boost when approval was listed for slab-on-grade construction for heated buildings in the 1995 and 1998 editions of CABO/ICC One and Two Family Dwelling Code. The NAHB estimates that there are now approximately 3,000 buildings with FPSFs in the U.S. They also claim a relative savings of 40 percent in foundation costs. 

FPSF is a building technique that is used to protect slab-on-grade foundations. It can be utilized on a monolithic slab or a floating slab with concrete, concrete block or treated wood grade beams. Expanded polystyrene or extruded polystyrene rigid insulation is recommended because of its high resistance to moisture and consistent performance properties in severe freeze-thaw conditions. The insulation is used to "reduce slab-edge heat loss and hold heat from the house in the ground under the footings." This helps to prevent frost heave because the insulation raises the frost line around the foundation. 

The "Frost-Protected Shallow Foundations" brochure provides case studies and technical information on how FPSFs have been used in almost every type of residential housing, including single family, remodeling additions, apartments and even low-rise commercial buildings. For instance, New York builder, Bruno Schickel, has used the FPSF technique for 15 homes. "As you may know New York has its own building code, which simply requires foundations to be protected from frost,” said Schickel. “We found that the building inspectors had no problems with our designs and accepted them without hesitation." 

The U.S. Army has also used FPSF, specifically for an airport control tower in Galena, Alaska, where the frost depth is 13 feet and temperatures can get down to -60°F for weeks at a time. The tower has not experienced frost heave or differential settlement. 

The NAHB brochure provides basic installation instructions from determining the insulation dimensions to recommended protective finishes. General concerns, like termite protection, cold bridges and frost heave, are addressed. With detailed illustrations and clear directions, this is an excellent resource for builders looking to differentiate themselves. 

The brochure is available from the National Association of Home Builders. For in-depth information on FPSF, they also offer the “Design Guide for Frost-Protected Shallow Foundations, 2nd Edition.” To obtain a copy of either publication, call the NAHB Research Center at 800-638-8556. 


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