How to Design a Frost Protected Shallow Foundation System

My brother has an upcoming construction project on a property he owns that he’s asked for my help in both the design and construction of the structure.  There are portions of the project that are planned to be covered by Fine HomeBuilding Magazine plus I will be writing several blog posts for Green Building Advisor, Rockwool, and for my blog, Northern Built.  One of the first tasks is to plan the foundation system, in this case it will be a frost protected shallow foundation (FPSF).  The illustration below shows both the foundation and wall construction ideas.

FPSF are very common in my market but are rarely constructed to the code specifications.  The International Residential Code’s requirements are found in section R403.  CHAPTER 4 FOUNDATIONS – 2021 INTERNATIONAL RESIDENTIAL CODE (IRC) (iccsafe.org).  One of the first items to know is the monthly mean temperature cannot be below 64°F (18°C).  This requirement will limit this type of foundation to homes that are continuously heated, seasonal properties where wintertime temps are maintained below 64°F will need to have a different type of foundation.

Another common error, the R-values of the insulation used in both vertical (slab edge) and horizontal (wing) locations.  This insulation is at or below grade, meaning the product must be able to maintain their R-values when exposed to moist conditions in freezing climates.  Because all rigid insulation (at least the stuff I know about) will absorb at least some moisture when buried, the manufacturer’s listed R-values are de-rated in the FPSF code.  For instance, when using extruded polystyrene (XPS) insulation to insulate the edge of a slab, the manufacturer’s listed R-value will be de-rated from R-5 to R-4.5.  If your area requires R-10 for the slab edge, you will be required to install a minimum of 2.5″ of XPS to meet this derated value.  You can find this code reference in Table R403.3(1), footnote c.

Getting back to my brother’s project, how do I plan for the frost protected shallow foundation?  To start, I need to identify what the code calls the air-freezing index for my location.  The code has a map of the United States showing this information that is impossible to read, so any project’s location will need to be determined using Table R403.3(2), which breaks down locations based on state and county.  The location for this project has 3,500-air-freezing index (the counties adjacent to mine have a 4,000-air-freezing index).  Based on that information, we move to table R403.3(1), which gives us the required information for the design.

The table indicates that the minimum buried footing depth for the 3,500-air-freezing index is 16-inches.  This is the thickness of the footing required below grade.  Code reference R404.1.6 states that we also need to have a minimum of 6″ of exposed, above grade foundation.  This separation helps protect any wood building materials from exposure to ground moisture.  The illustration in R403.3 also state that the maximum foundation exposed in the above grade, exposed footing is 12-inches.  My overall thickened edge footing design will be between 22 and 28-inches.

The thickened footing width is specified in table R403.1(1) and is based on snow and/or live roof loads, building height, the load bearing capacity of the soil, and any seismic considerations.  This structure is a single story, snow load of 70 psf, in 3,000 psf soil.  I could get away with a 12-inch-wide footing, but we will be constructing an 18″ wide footing to give us a little more capacity, just in case its needed.

Next, we need to plan the R-values based on the air-freezing index of 3,500.  The vertical, or slab edge insulation according to table R403.3(1) requires a minimum of R-9, but the energy code requires a minimum of R-10.  (Building codes and energy codes don’t always agree.)  How thick the insulation will be is dependent on which insulation is chosen.  Both EPS and XPS are options, which have a de-rated R-value between R-3.2 and R-3.4 for EPS, depending on density and R-4.5 for XPS.  I’m hoping to use a Rockwool product.  ComfortBoard is not one of the insulations listed by code, so I’ve asked for help from the building science department at Rockwool, I’m waiting to find out.  I have a suspicion that I will be using the minimum R-values listed for Expanded Polystyrene (EPS), which is R-3.2 per inch when used in a vertical application.  I will need a little more than 3″ total for the slab edge to hit R-10.

The wing or horizontal insulation is planned to be XPS (simply because it’s a stocked item at all the area lumberyards, EPS is special order).  XPS’s listed de-rated R-value when used in the horizontal application is R-4.  The horizontal insulation detail is shown in a diagram near the beginning of the code section (R403.3).

Back to Table R403.3(1), the corner insulation is required to be a minimum of R-11.2 for a length of 60-inches (C) off each corner.  The insulation width off the building is a minimum of 30-inches (B).  This will result in a minimum of 3-inch thick XPS along each corner.  The center area of the slab edge, the insulation R-value is R-8 with a width off the foundation of 24-inches (A).  2-inch thick XPS is needed in this location.

I’ve gone over some of the basic design information for my brother’s frost protected shallow foundation.  There’s more info in the code section (link above), and in a publication called the Revised Builders Guide to Frost Protected Shallow Foundations.  I also have another blog on the topic that can be found here. 

There is more foundation planning needed but will be discussed in a future blog post.  This structure will be heated using in-floor hot water tubing.  Stay tuned for that future post.  For now, the foundation planning is complete, if only building it was as easy.  Stay tuned for more content on this project in the coming weeks and months.