In the Cold Ground -
Frost Protected Shallow Foundations

by Elizabeth M. Steiner
Construction Canada Magazine - March 2006

Frost-protected shallow foundation (FPSF) design provides cold weather durability without excavation below the frost line. Using this technique, a rigid polystyrene (i.e. expanded [EPS] or extruded [XPS]) insulation foam is placed around the outside of a foundation, directing heat loss from the building toward the foundation and taking advantage of natural geothermal energy.

It is a design concept that has gained acceptance due to its insulation benefits, energy efficiency, and cost effectiveness.

Frost-protected shallow foundations are based on the principal that heat loss through a building’s foundation keeps the ground beneath from freezing and heaving by raising the frost depth at the foundation’s perimeter. For example, the earth surrounding the foundation of a heated building does not freeze as deep as it would under a cleared road or on a windswept hill. Therefore it is unnecessary to extend the foundation as deep into the ground as for an unheated building. This is especially true for slab-on-grade foundations with slab-edge insulation where the primary path for heat loss is underneath the foundation wall or footing.

There are two types of frost heave: normal and tangential. The former is vertical—frost lenses build beneath the footings, expand upward, and then raise the building. Tangential heaving, on the other hand, occurs when the ground freezes to the foundation. As the ground rises, it adheres (adfreezes) to the foundation and raises the building with it. A strong tangential heaving force can far exceed the weight of a house and foundations. During a thaw, this adfreeze bond can weaken and break, releasing the building. In the United States and Canada, foundations are typically begun below the frost line. This method offers little protection against tangential frost heave, especially in unheated buildings.

Frost heave and settlement can cause severe structural damage to buildings. Some traditional foundation materials, which are low in tensile strength and resistance to shear forces, are particularly susceptible to cracking. Some other structures are more elastic than concrete—that is, they are more capable of being deformed by frost heave and settlement, and then returning to their original form after the frozen solid thaws and consolidates. However, irreversible damage can also occur to wood buildings when the foundation deforms the building it supports.

Since FPSF relies on the building foundation’s thermal interaction with the ground, the frost line near a foundation rises when the building is heated. This effect is magnified when plastic foam insulation is strategically placed around the foundation. For heated buildings, this insulation—along with the earth’s geothermal energy—can keep the soil temperature under the building above freezing temperatures, preventing frost heave.

In residential projects, garages are particularly susceptible to frost damage since they are usually unheated and typically have high rates of air infiltration. Cold air moving under the garage door can cool and freeze the ground beneath the slab—the floor will show a gradual rise toward the centre of the door opening. FPSF works on unheated buildings by conserving geothermal heat below them. Unheated areas of homes can also be constructed in this manner.

	In frost-protected shallow foundation (FPSF) design, polystyrene insulation is strategically positioned to raise the frost depth around the construction site, and direct the building's heat loss downward.

Design considerations
Changes to frost-protection requirements adopted in the 2003 International Codes (I-Codes) now allow for foundations constructed in accordance with Structural Engineering Institute/American Society of Civil Engineers (SEI/ASCE) 32-01, Design and Construction of Frost-Protected Shallow Foundations. The code recognizes FPSF use for applications above the design frost line or for those built on solid rock. Realizing benefits in heated, semi-heated, or unheated structures, FPSF can help improve frost protection for public access buildings, commercial and office buildings, and one or two-family dwellings.

While FPSF is an alternative to stem wall and floating slab foundations, it is most suitable for slab-on-grade homes on sites with moderate to low sloping grades. Frost-protected shallow foundation design can be used effectively with walk-out basements by insulating the foundation on the house’s downhill side, eliminating the need for a stepped footing. FPSF can also be useful in remodelling projects, as it helps minimize site disturbance. In addition to residential, commercial, and agricultural buildings, the technology has been applied to highways, dams, underground utilities, railroads, and earth embankments throughout the continent.

Except for the insulation placement and footing depth, FPSF is similar to traditional foundations. While traditional systems are protected from frost-heave damage by placing the footing below the frost line, FPSF can be placed just 305 to 406 mm (12 to 16 in.) below-grade in the most severe climates. FPSF insulation is strategically positioned to raise the frost depth around the construction site and direct the building’s heat loss downward. When required in colder climates, ‘wing’ insulation extends outward horizontally from the footing. The colder the climate, the further the wing insulation is extended. This can help reduce excavation depth and amount of concrete, making FPSF an economical alternative for frost-damage protection. (Wing insulation may be unnecessary in moderate climates.)

EPS products have been specified for more than 30 years as sub-slab insulation in residential, commercial, and industrial floor systems. With proper slab design, molded EPS insulation can deliver cost-effective thermal performance to the specifier and building owner. In Canada, the national standard specifying requirements for EPS insulation material is Underwriters Laboratories of Canada (CAN/ULC) S 701-01, Standard for Thermal Insulation, Polystyrene Boards and Pipe Covering. (South of the border, U.S. designers rely on ASTM International C 578-04, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, for information covering the types, physical properties, and dimensions of cellular polystyrene intended for use as thermal insulation.)

EPS insulation’s compressive stress/strain characteristics are determined using ASTM D 1621, Standard Test Method for Compressive Properties of Rigid Cellular Plastics, or ASTM C 165, Standard Test Method for Measuring Compressive Properties of Thermal Insulations. When selecting a proper density for preliminary design, compressive properties of EPS insulation are available from various resources. These properties vary, to some extent, depending on the specific manufacturing techniques and raw materials employed.

It is important to note material properties provided in product standards do not always allow for the correct material selection for every application when actual design conditions are taken into account. Product standards such as CAN/ULC S 701 are valuable for comparing different types of foam plastic insulation, and for evaluation or quality control purposes. However, North American product standards are rarely intended for use as the last word in addressing all end-use design conditions.

For instance, determining the thermal resistance in specific end-use application is an inexact science due to the numerous conditions experienced by the rigid foam insulation. However, EPS insulation is a rigid closed cell foam plastic that contains solely air within its cellular structure—a characteristic that translates into a stable long-term R-value. (R-value measures resistance to heat flow—the higher the R-value, the greater the insulating power.)

Advantages of frost-protecting shallow foundations
Thanks to rising energy costs (and an increasing sensibility towards sustainable design), energy efficiency and operating expenses are a prime consideration for owners of both residential and commercial structures.

Builders and code officials initially responded by providing more thermal insulation in the above-grade portions of the home. Uninsulated foundations no longer represent 10 to 15 per cent of a poorly insulated building’s total heat loss. Instead, an uninsulated, conditioned basement can account for up to 50 per cent of the heat loss in a house tightly sealed and well-insulated above grade. 1

Only in the late 1980s were U.S. and Canadian national building energy codes and standards revised to recommend foundation insulation in moderate to cold climates. The minimum requirements generally exceed existing energy code requirements. In addition to the initial cost savings achieved during construction, FPSF increases energy efficiency due to its superior insulation properties. Some practical and economic advantages of FPSF, when installed properly are described below.

Reduction in homeowner utility bills
Insulating any foundation results in warmer floors during winter in above-grade spaces, improved comfort levels, and reduced energy consumption. Since FPSF is insulated along the outside edges, it makes floors at the perimeter significantly warmer. It also helps reduce heat loss through the foundation.

A 1999 study prepared for the Kansas Corporation Commission’s (KCC’s) Energy Program by Joseph King, AIA, of Coriolis Architecture (Lawrence, Kansas), and Gene Meyer, PE (Kansas State University), showed slab insulation could reduce annual homeowner energy bills by $50 to $100, depending on the region (in Kansas) and type of heating system. (Since savings vary, one should consult the product manufacturer’s fact sheet on R-values.) 2

Creation of more usable, comfortable below-grade spaces
Insulating basement foundations creates more comfortable conditions, helping make the space more useable at less cost.

Reduced construction costs
Raising the frost line can reduce construction costs by decreasing excavation depths and disturbing less soil on-site. Research findings from field evaluations in Denver, Colorado, and Freehold Township, New Jersey, show construction/excavation requirements and labour/material costs are 15 to 17 per cent less for FPSF than conventional foundations. The savings range from $800 to $6000, depending on local frost depths, as well as builder overhead and markups. 3

Avoidance of health risks due to soil gas
In areas where radon is present, the gas can reach high levels in buildings with poor outside air exchange. FPSF allows for sub-slab depressurization, helping permit the ventilation of accumulated radon beneath the slab. 4

	The National Building Code (NBC) contains information on frost-protected shallow foundations, and also includes an air freezing index (AFI) with specific design temperatures and wind pressures for many Canadian cities.

Proven performance under tough conditions

ASTM C 1512-01, Standard Test Method for Characterizing the Effect of Exposure to Environmental Cycling on Thermal Performance of Insulation Products, assesses the durability of insulation products. This test method began as a draft protocol, part of the 1995 exterior insulation basement systems (EIBS) joint research project conducted by the National Research Council of Canada/Institute for Research in Construction (NRC/IRC) and funded by the Expanded Polystyrene Association of Canada (EPAC).

The report confirms the thermal advantages of EPS exterior foundation insulation. Expanded polystyrene was attached to the foundation wall’s exterior, which was exposed to soil backfill for 30 months. The EPS insulation samples’ moisture content removed after this length of exposure was in the range of 0.01 to 0.96 per cent by volume. The project also instrumented and monitored specimen thermal performance, site weather conditions, and soil moisture content.

While thorough analysis detected water at the foam’s outer surface during periods of heavy rain and major thaws, the concrete basement wall surface showed no evidence of water penetration through most of its height. The thermal performance of EPS was found to remain stable and largely be unaffected by water movement.

Additionally, EPS’ durability in a below-grade application was demonstrated as part of the joint research project. The in-situ thermal performance of the insulation material was monitored continuously over the 30-month exposure and found to be constant. Thermal and mechanical properties of material samples tested after removal from the application were also unchanged. Testing confirmed all types of EPS insulation retained their specified material properties even after being subjected to freeze-thaw cycling.

Test parameters
1. The EPS insulation was directly exposed to high moisture content solid conditions, yet the moisture content after the two-year exposure period was found to be less than 0.5 per cent by volume on average.

2. The in-situ thermal performance of the EPS was monitored over the two-year exposure and found to remain constant (i.e. there was no loss in thermal resistance).

3. Samples taken from the field exposure underwent laboratory testing to confirm thermal performance and durability. Test results indicated there was no change in material properties after the two-year field exposure.

4. The research project included development of a durability test protocol to provide a means of assessing performance of all insulation types subjected to extreme thermal gradient and environmental cycling. Testing performed by NRC confirmed all types of EPS insulation retained their specified material properties even after being subjected to freeze-thaw cycling.

Testing conducted as part of the project confirmed the method provided valid comparative ratings for the products tested versus field performance. The draft protocol received final approval in October 2001. Measurements of moisture content after long-term exposure in below-grade applications confirm the performance of EPS insulation. Numerous published reports demonstrate water absorption by EPS insulation exposed in actual applications over extended periods of time is much less than values indicated by laboratory tests. 5 These studies reconfirm the ability of expanded polystyrene to provide frost protection in shallow foundations for Canadian projects.

Notes
1. For more information, visit the U.S. Department of Energy (DoE) energy efficiency and renewable energy website, www.energycodes.gov, and search for Residential Insulation Foundation.

2. See “Frost Protected Shallow Foundations” by Robert Fuller in Builder Magazine. (May 2004).

3. See the aforementioned Fuller article.

4. See Richard A. Morris’ “Frost Protected Shallow Foundations” in the Society of the Plastics Industry’s NAHB/NRC Project No. 2070 (August 1998).

5. For example, see NRC-IRC’s In-situ Performance Evaluation of Exterior Insulation Basement Systems—EPS Specimens, published in March 1999.

Elizabeth M. Steiner is executive director of the Expanded Polystyrene Molders Association (EPSMA) and has worked with the EPS industry for more than 15 years. She facilitates leadership and educational outreach to specifiers, contractors, and other building professionals on the material’s technical and performance advancements. Steiner is an active member of Construction Specifications Canada (CSC), Construction Specifications Institute (CSI), and ASTM International. She can be reached via e-mail at emsteiner@epscentral.org.