A Case Study on the Frost-Protected Shallow Foundation Technology - 5/16/2004 - Home Foundation Structure Framing

A Case Study on the Frost-Protected Shallow Foundation Technology

This article was co-authored by Dick Morris, NAHB
and Jay Crandell, P.E., NAHB Research Center, Inc.

At one time in the not too distant past, design professionals were especially noted for their contributions to the advancement of construction technology and innovation, even in the housing industry. A great example of such contributions is architect Frank Lloyd Wright's pioneering use of Frost-Protected Shallow Foundations in his 1930s era affordable housing.

What is the role of a design professional in technology innovation?

Design professionals can provide additional assurance to builders, building owners and building departments that the FPSFs have been designed according to the applicable building code or standard. Moreover, in jurisdictions where codes allowing FPSFs have not yet been approved, it may be possible to build FPSFs under the "alternative means and methods" clause of practically any building code, especially if the plans are stamped by a design professional. A design professional's service may help builders and building owners to go beyond the normal practice of standard materials, standard methods, and standard practice. Basically, it's an opportunity to proactively address the "comfort zone" mentality that may sometimes reject innovation for no other reason than "that's not the way we do it around here." Thus, when an innovative builder or owner meets an innovative designer, the possibilities and benefits are almost limitless - provided the fundamental intents of the building code are indeed met.

Case Study - Frost Protected Shallow Foundations

Judy Niemeyer is a builder and President of Tierra Concrete Homes, Inc. in Pueblo, Colorado. She is also a four-time winner of the Energy Value Housing Award for her affordable, durable, and energy efficient pre-cast concrete homes. However, when Judy wanted to use the Frost-Protected Shallow Foundation (FPSF) concept to enhance the value of her product, she met significant resistance in the local plan review and permitting process. In fact, she has been required to use the appeals process in five different counties to obtain approvals. "I was left no other choice but to use the appeals process to do what I knew would improve the value of my homes," stated Ms. Niemeyer. Equipped with information available from NAHB, the NAHB Research Center, and a knowledgeable engineer, she was able to successfully obtain a building permit in each case. According to Ms. Niemeyer, "Now, the technology is gaining a foot-hold and others are beginning to use it in my area. My engineer was a valued resource in the appeals process."

"It is certainly not always necessary to use the appeals process. Many local code jurisdictions in the United States already accept the technology," according to Dick Morris of NAHB who has published an informative brochure to help gain appropriate recognition of the technology in the United States and has worked to have model building codes approve FPSFs. "In fact," states Morris, "the Council of American Building Officials approved the technology in the CABO 1&2-Family Dwelling Code in 1995, the American Society of Civil Engineers approved their new FPSF standard ASCE 32-01 in 2001, and the International Code Council adopted the standard for all residential and commercial construction in the 2003 International Building Code and International Residential Code." Mr. Morris estimates that more than 5,000 buildings have been constructed using FPSFs in the United States. One of these was a large affordable housing development that saved approximately $300,000 in foundation construction cost - it is not uncommon to realize cost savings upwards of $3,000 for individual homes. "This is a value-added technology that designers can bring forward to their builder and owner clients," says Mr. Morris.

What is an FPSF?

The Frost-Protected Shallow Foundation (FPSF) technology is not complex. Basically, the building code requires that footings be protected against frost heave. The conventional method is to place the footing below a locally prescribed frost depth. As an alternative, the building foundation can be made more energy efficient and the ground kept above freezing by insulating the foundation and soil (see Figure 1 below). By preventing the soil from freezing under the footing, frost heave is avoided. Thus, the intent of the code is met by using an energy and resource efficient foundation design.

Figure 1
FPSF Heat Flow Diagram for a Heated Building with Optional Floor Insulation

Who can design an FPSF and how?

The decision of who can design an FPSF is usually dependent on local regulations and the building department's position. In some cases, a simple FPSF design may be done by a home builder using the simplified CABO, IRC or similar code-approved prescriptive approach. In other cases, a more complicated design may be required. Or it may be used on a commercial building (e.g., strip mall) or multifamily building - both cases are usually required to carry the stamp of a licensed design professional.

The FPSF design process is really very straight-forward and involves a few simple steps:

1. Determine the local Air-freezing index from a U.S. map.
2. From design tables, determine the effective insulation thermal resistance (R-value) required to be applied to the foundation.
3. Calculate the required thickness of polystyrene insulation based on the effective below-ground R-value (not the dry value advertised by the manufacturer).
4. Apply an appropriate standard FPSF foundation construction detail (see Figure 2) to the construction blueprints and include appropriate insulation specifications and dimensions.

It is important to note that the FPSF technology can be used on a wide variety of foundation types provided the insulation and foundation is properly detailed. It is also important to consider the use and occupancy of the building that would govern the indoor temperature condition. Unheated buildings require a solid blanket of insulation under the building (to better conserve geothermal heat stores) whereas heated buildings may use only perimeter insulation. These and other important design considerations are covered in the design resource mentioned below.

Figure 2
FPSF Design Parameters and Standard Details for a Heated Building with a Slab Foundation

FPSF Design Resources

For a comprehensive design guide on Frost-Protected Shallow Foundations (2nd Edition), contact NAHB Research Center, Inc. at 1-800-638-8556 or visit the e-store at www.nahbrc.org for internet publication orders. The 2nd Edition guide is consistent with existing codes and provides greatly expanded information for improved design flexibility.

More informaiton about this topic can also be found at http://www.toolbase.org/fpsf and http://www.nahb.org/reference_list.aspx?sectionID=235.