Construction Design-Frost Protected Shallow Foundation (FPSF)

This post first appeared at the Green Building Advisor website.

One of the most popular foundation systems used in my market, and one I’ve been using for more than a decade, is the frost protected shallow foundation.  My very cold climate requires footing depths of five feet.  Digging, constructing and insulating a footing and foundation system that deep is time consuming and expensive.  A shallow foundation system can be a substantial savings for a new build.

The building codes covering frost protected shallow foundation systems (FPSF) is in section R403.3 of the International Residential Code.  There is a lot of information specific to this type of foundation that differs from other systems.  For instance, when using a FPSF, the average mean temperature of the building needs to be maintained at a minimum of 64°F (there are ways to design a frost protected foundation for unheated and semi-heated building, but this article will concentrate on continually conditioned dwellings).  There are also requirements for rigid insulation R-value derating and protection for buried insulation.

What is a FPSF?

A frost protected shallow foundation system is typically a monolithic concrete pour, sometimes called a turned down slab, where the footing and slab are constructed as one.  Insulation is placed against the slab edge as well as a “wing” of insulation that extends out away from the thickened edge to prevent frost from driving under the slab during the winter months. 

How much insulation is required at the slab edge and on the horizontal or “wing” and the minimum footing depth is determined by air freeze index for your location.  Figure R403.3(2) in the IRC (shown below) gives air freeze index numbers across the country.  The map is backed up in the code with tables showing the air freeze index for the individual counties in each state.  So, what is the air freeze index?

According to IRC R403.3(2), “the air-freezing index is defined as cumulative degree days below 32°F.  It is used as a measure of the combined magnitude and duration of air temperatures below freezing.  The index was computed over a 12-month period (July-June) for each of the 3,044 stations used in the above analysis.”  It is based on a 100-year average.

The air-freezing index is similar to the climate zone maps, (climate zones 1-8 with 1 being the warmest and 8 being the coldest), but instead of using 65°F as the starting temperature for the calculation, the freezing-air index uses 32°F.  The scale starts at 1,500 or less and has a high of 4,500.  My area of Northern Minnesota has an air-freezing index of 3,500, Northwestern Minnesota and Northeastern North Dakota have an air freezing index of 4,000.  Warmer areas, Georgia for example, have all counties listed at 1,500 or less.

Footing Depth and Insulation Levels

As I stated earlier, the air freezing index dictates footing depths and insulation levels for FPSFs.  (See table R403.3(1) in the IRC).  Starting with the footing, minimum footing depths, or the portion of the footing below grade, will be between 12” and 16” depending on the air freezing index of the area.  My area requires 16” minimum footing depth.  Any above grade portion of the footing will increase the footings overall thickness.  So, for instance, if I had an exposed slab edge that was 6” above grade, my overall footing thickness will be 22”.  (6” exposed slab edge + 16 minimum buried footing depth = 22” overall footing thickness.)

Next up is the slab edge or vertical insulation level, the insulation needs to extend from the top of the finished concrete slab to the bottom of the footing.  Again, depending on your freezing-air index, the minimum R-value of insulation needed will vary.  My area requires a minimum of R-9.  Here’s the kicker, all insulations used below grade in a FPSF are derated, in other words, you are not allowed to use the listed R-value of the product.  The reason being is it’s known that buried insulations will absorb some moisture.  Damp and wet insulation will not have the same resistance to heat flow as dry insulation.  For this reason, you need more of the same product to achieve the required minimum R-value.  The reduction in R-value depends on both the type of insulation (XPS or EPS are the only two allowed for prescriptive FPSF systems) and the insulation’s density.  Types IV, V, VI, VII and X extruded polystyrene (XPS) has derated value of R-4.5 per inch for vertical installation and R-4 per inch for horizontal installation.  EPS R-values are even lower, depending on the density.  (See the fine print note below the IRC table R403.3(1) for all the derated values.)

https://youtube.com/shorts/Euf4P43Qoo4

The horizontal or “wing” insulation will be derated to the values specified in R403.3(1).  The distance away from the footing edge and R-value requirements will differ depending on if the insulation is at a corner or along a wall.

In the above illustration, using my climate area as an example, the insulation at the corners will need to extend away from the slab edge a minimum of 30” (B) for a distance of 60” (C) from the foundation corner.  The R-value of the corner insulation is a minimum of R-11.2.  If the insulation is any of the above listed XPS densities, the horizontal derating is R-4 per inch, so we would end up using 3” (R-12) on the corners.  The horizontal insulation along the wall is listed at R-8 and needs to extend away from the slab edge for a minimum of 24” (A).

What I typically see in my market is the use of XPS used as both the horizontal and vertical insulation.  Usually, the insulation is not cut for the horizontal or “wing”, it is used in its full 4’ width.  I have yet to see a higher R-value insulation used in corners as is required by code.  It’s always 2” or R-10 (R-8 after derating).

Insulation Protection

Both the horizontal and vertical insulation needs to be protected.  Any exposed vertical insulation will begin to deteriorate with exposure to sunlight and atmospheric pollutants.  I typically cover any exposed insulation with a bent metal flashing.

The below grade horizontal or “wing” insulation also needs to be protected against damage if it’s buried less than 12” deep or extends more than 24” way from the slab edge (R403.3.2 Protection of horizontal insulation below ground).  The code allows a concrete slab or asphalt paving on the surface or cement board, below grade plywood or other approved materials placed directly above the surface of the insulation when it’s buried.

Manufactured Systems

There are a couple manufactured frost protected shallow foundations I am aware of.  One I have personally used is the Mono Slab EZ Form system.  The forms consist of an inner and outer wedge-shaped form made from EPS foam.  The form is staked in place and a reinforcing 2x form board fits into a precast slot built into the form.  The outer form becomes both the slab edge and horizontal frost wing.  The inner form becomes the back or inside portion of the thickened edge of the slab.  The inner form is optional, we chose not to use in the above project.  There are a few different versions of the Mono Slab EZ Form, a standard form, which measures 16” x 16” is used for more moderate temperature locations.  Another version is called the Arctic Form, which measures 18” x 30”.  We used this version for our very cold climate builds.  There is also a commercial form that measures 24” x 24”.

Another company called WarmFörm has a design with its roots based in Europe, though the forms are manufactured in the US.  Another EPS insulated form; this product looks closer to an insulated raft slab than frost protected shallow foundation.  You can read more about this type of frost protected shallow foundation in a Fine HomeBuilding Article by Scott Gibson.  Prefabricated Foam Forms for Slab Foundations – Fine Homebuilding

Other considerations

There can be a few design challenges when using FPSF systems.  First, there is no place to shelter during severe weather.  I have been in a few homes where the homeowner had a concrete shelter constructed either inside the home or in an attached garage as protection during severe weather.

Heating and cooling systems can also be more complicated.  My market uses primarily hot water in-floor heat system in FPSF homes.  These systems are very popular but can be somewhat of problem during the spring and fall when overnight temperatures are cool enough to warrant heating, but daytime temperatures are warm.  Slab heat tends to be very slow to react to temperature changes, overheating of living spaces is common during these times of the year.  What I’ve been designing in slab on grade projects I’m involved in is a combination of hot water slab heat and air source heat pump mini-split systems.  Summertime cooling and spring/fall heating is supplied by the heat pump system with wintertime heating supplied by electric or gas boilers feeding the radiant in-floor tubing.

There are also some limitations for floor coverings.  Nail-down wood floorings are not an option.  I am seeing more glue down engineered wood products being used and of course tile, vinyl and carpet are common choices.  Be aware that using a carpet over a heated floor will require a special carpet pad.

The frost protected shallow foundation has been my go-to slab on grade foundation system for the past decade.  Though there are a few challenges with the system, when built correctly, I have yet to see one have any type of failure.  The worst I’ve seen is an occasional concrete crack.  The biggest benefit is the reduced cost over other foundation systems.  Affordability in new construction over the past couple years has been an issue, cost savings while delivering a reliable end product can make this system a good choice.  A great resource for more information on FPSF systems can be found at Revised Builder’s Guide to Frost Protected Shallow Foundations (homeinnovation.com).