Frost Protected Shallow Foundations - Frequently Asked Questions - 8/12/2000 - Home Foundation Structure Framing

Frost Protected Shallow Foundations - Frequently Asked Questions

Frost Protected Shallow Foundations (FPSF) protect foundations against frost damage without the need for excavating below the frost line and are in use by builders in cold-climate areas. FPSF were introduced to the United States through the efforts of builders, the NAHB and the NAHB Research Center.

Q: What is the standard for Frost Protected Shallow Foundations?

A: The standard in use is known as SEI/ASCE 32-01 - American Society of Civil Engineers, "Design and Construction of Frost-Protected Shallow Foundations." This standard addresses the design and construction of FPSF in areas subject to ground freezing. For more information on ordering a copy of this standard, visit ASCE's website at http://www.asce.org/ or the NAHB Research Center Bookstore.

Q: Can I use a FPSF in Grand Rapids, Mich. (or anywhere in the United States)?

A: That depends upon building code acceptance in your specific jurisdiction. FPSF is a technology developed in Scandinavia, and now is common practice in the United States with over 5,000 buildings already installed with FPSFs. This technology began to be adopted in the United States during the 1990s and the National Association of Home Builders (NAHB) has aggressively promoted its use throughout the U.S. Today many local building code officials approve this type of foundation in their jurisdiction. The International Residential Code (IRC) 2000 and 2003 editions also include this technology starting at section R403.3. Of course, if you build in a hot climate region, such as Florida, you may not need this type of foundation.

Q: What is the Air Freezing Index (AFI) and how does it relate to FPSFs?

A: The AFI is a measure of the combined magnitude and duration of air temperatures above and below freezing during any given winter. The data is collected each year by the National Climatic Data Center (NCDC) and averaged for various time intervals. For FPSF, the 100 year average data is used. AFIs should not be confused with HDDs (heating degree days). The AFI assists the builder with proper design of the FPSF for any given location; in fact the building code uses the AFI to set the required amount of insulation for a FPSF.

Q: I've looked on the map on Figure R404.2(2) of the IRC 2000 and can't tell if the location in my state is within the 3,500 AFI or the 3,300 AFI. Where can I find additional details?

A: Table A.3 of the SEI/ASCE 32-01 provides the AFI for 30 U.S. locations. A more complete listing is provided by the National Climatic Data Center (NCDC) with the AFI for 3,110 cities in the United States. You can search or download the NCDC's table in either PDF format, or as a spreadsheet. To get this data, visit http://lwf.ncdc.noaa.gov/oa/fpsf/fpsfpublications.html. Use the 100-year AFI in the far right column.

Prior to construction, make sure you check with your local building code official to see if you are both in agreement concerning the AFI to use at your site.

Q: While reviewing the information about AFI, I saw something called MAT. What is MAT?

A: MAT is mean annual temperature. You will need to know your MAT and AFI to design FPSFs for unheated buildings. The National Climatic Data Center provides MATs for each weather station in the Air Freezing Index table and an MAT map at http://lwf.ncdc.noaa.gov/oa/fpsf.

Q: Would a vapor barrier keep moisture under the slab and increase the probability of frost heave?

A: Heat moves from hot to cold, from the warm house into the ground, and especially toward the cold perimeter of the foundation where slab edge insulation retards its flow. The temperature directly under the vapor barrier will be close to that of the house. A vapor barrier has virtually no heat resistance and will not affect the probability of frost heave (the lifting of a soil surface or concrete slab due to the freezing of moisture in the underlying soil).

Q: My customer wants me to install a hydronic radiant floor heating system in a home with FPSF. Will this type of heating work with FPSF?

A: Radiant floor applications will work just as efficiently with FPSF as with other slab/foundation types. Under ASCE 32-01 Section 5, insulation placed below the floor slab shall not exceed a nominal R-value of 10, and under Section 6, an R-value of 28. Keep this in mind when following advice from radiant floor designers. Make sure your heating contractor has all the specifications including the R-value of the insulation you will use. The amount of insulation will have an impact upon the sizing of equipment and materials used for radiant heating.

Q: What are the requirements for constructing an unheated garage appurtenance connected to a FPSF for a heated residence?

A: There are now four ways in the 2003 IRC to design garages attached to homes built on FPSFs:

1. Use a conventional deep foundation. Place the footings below frost line, but do not disrupt the house insulation with the garage walls or slab. Vertical house slab edge insulation must continue uninterrupted between the house slab and the garage floor slab and garage walls. Horizontal wing insulation for the house must continue under the garage floor and through the garage foundation walls (use high density foam insulation with sufficient compressive strength to carry the load or take the load off the insulation with a lintel, beam or cantilever). Also, the top of the FPSF house slab can be no more than 12 inches above garage floor, and the bottom of the FPSF house slab can be no less then 12 inches below the top of the garage floor.
2. Insulate and heat the garage to 63 degrees. Use an FPSF for heated buildings for the garage foundation and insulate the perimeter. Do not circulate garage air into the house. Use a separate heating system designed to turn on at 63 degrees.
3. Insulate and heat the garage to 41 degrees. Use an FPSF for a semi-heated building (make the foundation 8 inches deeper than required for a heated building) found in ASCE 32-01. Use a separate heating system designed to turn on at 41 degrees.
4. Use the unheated building system in ASCE 32-01. Insulate under the entire garage floor, under the bearing wall, and beyond the perimeter 3 to 6 feet, horizontally. Or use the continuous foundation wall design (ASCE 32-01 Section 7.2) around the garage perimeter, insulating under the wall and each direction the required distance horizontally (this method allows the garage floor to float up and down with frost as in a conventional unheated garage with a deep foundation).
   
   
    

   Q: How do I insulate an addition built on an FPSF?

   A: Insulate the addition the same way you would any other FPSF, with two exceptions. If you are constructing a heated addition on a FPSF and attaching it to a heated building, you do not need to install insulation between the building and the addition. Also, if the addition meets the outside corner of the building, continue the vertical and horizontal FPSF insulation from the addition onto the building foundation past the corner of the building a distance equal to the width of any required horizontal sidewall wing insulation. If no wing insulation is required (AFI of 2,000 or less), you can stop the FPSF insulation at the corner of the building. If the addition meets the building at an inside corner, no insulation needs to be installed on the building at the inside corner.

   
   
    

   Q: What if a home with an FPSF is left unheated for several weeks during the winter?

   A: A heat bulb of warm, moist soil with a very large heat capacity builds up under the FPSF. The heat stored in this soil will protect the foundation for several weeks. The design assumptions for FPSFs are extremely conservative. FPSFs are designed to be protected from frost during the coldest winter in 100 years, with no sod or snow cover to insulate the ground, and worst-case soil conditions with a moderate amount of moisture (high moisture levels retard freezing with a high heat capacity and low moisture levels reduce conductivity of heat and therefore retard movement of heat from the ground to the cold ambient air).

   
    

   Q: What if my local building official refuses to allow installation of an FPSF?

   A: The best practice is to discuss any new technology with your local building code official during the planning stages. Chances are you will have greater acceptance and will have an opportunity to introduce new ideas without the time constraint or pressures of your construction schedule. Show the code official the requirements accepted by the International Code Council, the American Society of Civil Engineers, NAHB and NAHB Research Center. Direct him to information available online at http://www.toolbase.org/fpsf, and http://www.nahb.org/reference_list.aspx?sectionID=235. If necessary, get an engineer to stamp your plans. For further assistance, contact Dick Morris at NAHB (dmorris@nahb.com).