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.)

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 (

13 Replies to “Construction Design-Frost Protected Shallow Foundation (FPSF)”

  1. This is a great article and I love the detail. Thanks! We have a property in Federal Dam, MN and are in the planning stage of a smaller house/cabin. The soil is primarily clay. Have you had experience with FPSF in this type of soil, and if so, was it successful?

    1. Hi Mike,
      I have used the FPSF on clay without any issues. There are codes to the bearing capacity of soil, these codes cover all foundation systems. If you are in question or think you have poor soil conditions, you may want to have a discussion with a civil engineer.

      Something the more rural areas often value engineered out of residential building projects is the clean crushed stone under slabs and FPSF systems. The stone has two purposes, first is for the required radon mitigation systems, the second as a capillary break for any sub-slab insulation and the concrete slab itself. The sub-slab foams can saturate with surface moisture, this changes the resistance to heat flow of the insulation, reducing the R-value. I see concrete slabs poured directly on sand, or the foam insulation placed on the sand with the vapor retarder in the wrong location (under the foam) all the time. Don’t assume the concrete crew will get all the details correct, make sure there are some drawings shown how these systems are to be constructed.

      Hope this helps,

      1. I had a slab poured for a small cabin last fall, our concrete guy put vapor retarder under the 2″ xps, which was on top of sand. Of course it rained before the concrete was poured. How worried should I be about that rain water being trapped and compromising my under slab foam?

        1. Hi Brett,

          Why we want the polyethylene sheet above the foam is to prevent the assembly from absorbing moisture from the concrete during the slab pour. Any water that was present before the slab pour was probably displaced or absorbed by the concrete as it was placed. What will happen when the vapor barrier ends up on the wrong side is that moisture will slowly dry to the interior and there will be a slight decrease in insulation performance until drying is complete. I wouldn’t lose any sleep over it, but I would suggest monitoring and controlling interior humidity levels, especially for the first year or two after the home is completed.

          Good question!

  2. Hi Randy,
    Thanks for the article on “Construction Design-Frost Protected Shallow Foundation (FPSF).”
    I recently bought some property off the Gunflint Trail in northern MN and I plan to build a small cabin there using an FPSF. My excavator installed a driveway and he said most people build with an FPSF in this area.
    I want to have a porch attached to the cabin and I’m curious as to how one would integrate shallow porch footings with the FPSF.
    Would one keep the porch narrow and extend the horizontal “wing” insulation out to the porch footings? Or can you insulate the porch footing independently of the main structure foundation (seems risky)?
    Thanks for any additional insights you might add on this subject.

    1. Hi Rick,

      Thanks for the question.

      Frost protected shallow foundations are listed in the residential building code under R403.3. The codes cover heated structures maintained at a minimum temperature of 64°F. Adding a 3-season porch or other attached structure that is either not heated or heated to less than 64°F will require engineering. ASCE 32 (American Society of Civil Engineers) is the document covering frost protected shallow foundations. The soil type is going to have a big factor on what can be done. In the past, we’ve installed a single round concrete footing below the frost line, the porch roof was supported from the footing and the slab for the porch floor was poured independent of the house foundation. This was an open porch, no walls, so far no issues. I think if I were you, I’d pay the few hundred dollars for the engineering, just to be safe.

      Hope this helps,

  3. How would a covered concrete porch or patio be factored into the design?
    Mechanically attached (rebar) or isolated from the homes FPSF?

    1. Hi Dan,

      We typically pour the porch slab independent of the house slab. We treat the porch slab just like the house slab, install the wing foam and include foam under the slab, but do not mechanically attach the two. If you need a pier to support a roof load, that will be poured independent of the slab down to frost depth, we typically just use a Sonotube sized for the load. ASCE 32-01 is the engineering specifications dealing with FPSF, there is more info for unheated slab protection in that publication.

      Hope this helps,

    1. Hi Jay,

      I only work in Minnesota. I stopped being a general contractor a few years ago, I work part time with another contractor on his projects (I still enjoy the job) and work with homeowners and other builders, helping them with improving both new and existing homes.


  4. Hello Sir- Scary stuff finding out your flatwork guy doesn’t know which side to put the vapor barrier on? I’ve become obsessed with thermal performance as of late. Now that I’ve removed the trees and tree trunks/roots and lazered my site flat, I’m at that point of discovering I’ve moved into an area beyond my skills/knowledge and/or maybe my capabilities. I’ve done this on purpose. Its crucial I don’t screw this up. I currently have been issued a ‘foundation only’ permit- on the assumption that I will be constructing a home in the future. So, Im building my little 30×40 fpsf ‘to code’ – and doing it myself. I told the inspector this, and he is aware. (its common) The permit- allows me to install plumbing and in-floor heat (pex) HVAC, which will be closely scrutinized for inspection/approval. My question to you sir- is, If you were in my shoes, where would you be looking for/obtaining solid, credible, code for my zone/county information on an exploded view or schematic details of a frost protected monolithic slab?

  5. forgot to mention my county/town websites do not have building code regulations on them. As far as my build, I have the latitude to build anything- the only thing the inspector told me was to make sure the outer 12′ perimeter is 18′ deep. The vertical insulation must be as well, and- 4 feet out blanketing the footprint. im just after some possibilities of what I can and cant do. Does the official residential building code/s vary from county to county?

    1. Hi Michael,

      The codes do vary based on what is called the Air-Freezing Index, which can be found in chapter 4 of the International Residential Code. The code link is free online though you cannot print or copy from this site without a paid subscription. You will need to look up the air freezing index number for your location and apply that info to your design. Another great resource is: file:///C:/Users/willc/Desktop/Frost-Protected%20Shallow%20Foundations%20_%20UpCodes_files/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf. There’s a ton of additional information in that publication.

      Hope this helps,

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