Using Insulating Concrete Forms To Make A Frost-Protected Slab Foundation

I devoted spare time this past year to building a new small office for our architecture practice. I thought of a simple way to use insulating concrete forms (ICF’s) to make a frost-protected, shallow slab foundation. It worked out well, so I’m sharing the idea, with hopes that other folks will improve upon it.

Here in Maine, foundation walls typically extend 4 to 5 feet (122 to 152 cm.) below finish grade to be safe from freezing soil. That uses a lot of concrete, an energy-intensive, carbon-intensive building material. Don’t get me wrong, concrete is great: plastic and cast-able when wet, strong and durable when cured. Treated correctly, a concrete slab can be a finished floor, and that floor can provide significant thermal mass within a structure.

Frost protected slabs help us use concrete for real advantages and avoid using it just to deal with freezing soil. The technique has been used for decades in the cold climates of Alaska and Scandinavia. The National Association of Homebuilders has researched and endorsed the technique. The idea is simple in principle, although the details are tricky. First, use well-drained material below and around the slab to keep water away. (Think about it: railroad tracks don’t “frost-heave” because they rest on about 3 feet (1 meter) of stone ballast.) Second, place a nearly-horizontal layer of rigid insulation around the perimeter of the slab, extending outward at least as far as the frost depth in your region.  This helps prevent freezing at the slab perimeter.

When making a frost-protected slab, one of the trickiest parts is forming the slab “haunch,” or the perimeter edge requiring thickening and reinforcement for structural loads. Another challenge is placing and leveling formwork on uneven ground. And a further challenge is providing ample insulation on outside of the slab edge – energy modeling reveals that slab edges are a large source of building heat loss.

That’s where ICF’s come in. They make handy, pre-insulated formwork. They’re easy to handle and place, and they have hard plastic splines embedded in them that can take fasteners – making it really easy to attach additional rigid insulation, expanded metal lath for stucco, and so on.

Here’s what I did. First, I had my neighbor Jim place clean, bank-run gravel to make a well-drained building pad. I compacted the gravel in 15-inch high “lifts,” then allowed a winter for the gravel to settle.

1st My buddy Stephen and I poured a footing, taking great care to make it as level as possible.




2nd3rd I coated the top with a cementitious “capillary break.” Then I snapped chalk lines for the slab perimeter and nailed steel track to the footing while the concrete was still “green.”




4th The next step was perhaps the most innovative. ICF’s usually come as 2 “panels” that are assembled using plastic connectors that double as holders for steel reinforcing bars in the haunch. I cut one of each pair of “panels” on a table saw, lowering it by about 4 1/2 inches (11.4 cm.), or the depth of the slab (except the haunch). Then I connected the panels, with the shorter ones to the inside. (The photo shows the corners placed first.) I could level the ICF’s and fasten them to the footing by screwing into their plastic splines through the steel track. Because we were careful with the footing, we only needed to level the ICF’s by 1/8 inch (3 mm.) over the whole perimeter.

The idea is then to place under-slab compacted fill and rigid insulation to the top of the interior “panel.” Then you’ve formed the interior and haunch of your slab. Here’s an illustration:



6th You’ll notice that you only benefit from half of the ICF’s insulation at the outside perimeter. The ICF’s I used provided 2 1/2 inches (6.4 cm.) of expanded polystyrene on the outside edge, with an R-value of about 9.5 (RSI-1.7). I added 3 inches of extruded polystyrene insulation (R-15, RSI-2.6) to the perimeter, fastening it with screws driven into the plastic splines. The total slab edge R-value is 24.5 (RSI-4.3). I also placed 3 inches of extruded polystyrene below the slab.





7th1 With the ICF’s all in place, it was time for under-slab fill, under-slab plumbing, insulation, vapor retarder and steel-bar reinforcement.





8th19th I have no skill with a screed or trowel, so I hired my friends Keith and Howard to pour the slab – they did a great job.  Once they finished, I sawed control joints to let the slab crack in a controlled way – I placed the saw joints in a pattern that mimics large tiles.






Later, I added the extra insulation at the slab edge. I protected the insulation by attaching expanded metal lath, using long screws to reach the plastic splines in the ICF’s, and applying two coats of cement stucco to the lath.


10th When the slab had cured, I etched it with acid and applied a concrete stain to make the slab the finish floor of our office. (I “embellished” the stain color by adding mineral pigments of my own.) Several coats of sealer and buffed paste wax gave the floor a nice patina. After several months in the office, the floor is warm, durable and attractive.





If I were doing the job over, I’d calculate the loads coming through the bottom of the slab haunch onto the footing – if a dense foam insulation could support those loads without compressing, I would add it to the top of the footing, eliminating a significant path for heat conduction. I placed a 2-inch (5 cm.) layer of rigid polystyrene insulation horizontally 4 feet (122 cm.) around the slab perimeter. That’s an R-value of 10 (RSI-1.8). If I were doing it over, I’d use 4 inches of foam for an R-value of 20 (RSI-3.6).

It was fun to figure this out and do the work. Through the years, the inspiring work being done by folks in the NESEA community, and the great presentations I’ve attended at NESEA conferences, have given me the gumption to try new ideas. If someone with my meager skill levels can do this, then a lot of you skilled folks out there will be able to really improve upon the idea!


  1. David,

    Brilliant use of standard american components.

    Take a look at the slab-edge blocks they use in Sweden, we need to get a US or Canadian ICF company to rig something similar up:

    Swedish manufacturer:

  2. Jesse, thank you very much for the reference. Wow, I wish I had known about the Swedish slab-formwork components – and that they were available here! In your references, it’s very interesting to note that, at least for residences, they are bringing the structural loads directly down onto dense foam – that’s the change I wish I had made to my design.

    A quick, back-of-the-envelope calculation, using my little project as an example. I’m bringing 2 floor loads, a roof load and weights of materials down to ground on bearing walls, ultimately reaching compacted fill beneath a footing. The main floor load is 40 pounds per square foot live load and about 57 lb/sf dead load (the weight of the slab – this assumes all slab weight transferred to the perimeter, which is hardly likely). Upper floor is 40 lb/sf live and 10 lb/sf dead load, and the roof (with our climate’s snow loads) is about 50 lb/sf live and 10 lb/sf dead load. Then there’s about 80 lb/ lineal foot of material weight for the walls. In my little building, it works out that the bearing walls are transferring about 1736 lb/ lineal foot onto the footing. The concrete haunch is 8 inches wide, so the force at the bottom of the haunch/top of the footing is about 2604 lb/ lineal foot, or about 18 lb./ square inch. That’s well below the compressive strength of virtually all extruded polystyrene products I’ve looked at, so it’s obvious that I could have place foam on top of my footing to better insulate the slab. Until we can buy products like the Swedish ones, we should learn from that.

  3. Contact these folks they can calculate all of that for your when you build as they will onsite train you on using ICF. Effective R Value numbers seems low in your calculations, and also use ICFs for the entire house to really experience the benefits of this green tech, the uses are limitless for them.

  4. Robert Riversong says:


    If you had used the NAHB Shallow Frost-Protected Foundation Guide, you could have used far less vertical edge and horizontal wing insulation. We used to believe that horizontal insulation had to be as wide as the frost depth. This is not true for a heated structure, as decades of experience in Scandinavia, Canada and Alaska has demonstrated.

    The IRC accepts the SFP foundation system as specified in the NAHB manual. Here in north central VT (air freezing index of 2500, heating degree days of 8500), all that is required is a foundation depth of 12″, R-10 vertical edge insulation and 12″ of R-10 horizontal wing insulation (this is actually a bit excessive).

    You’re right that having no thermal break between slab and footing is a serious problem. Not only can 25 psi (3600 psf) XPS support residential building loads, on good bearing-capacity soil (min. 2000 psf), a spread footing is unnecessary.

    I make my SFP foundations with an exterior insulated reinforced grade beam that incorporates interior slab-edge insulation (R-10). And then I pour a separate (perhaps tinted) concrete slab over R-10 foam and Tu-Tuff vapor barrier. This creates a separate perimeter foundation to support building loads and a “floating” fully insulated slab.

    After installing wing insulation (slightly sloped for drainage), I staple 1/2″ hardware cloth to the wood sills, draped down over the perimeter insulation, and parge with 1/8″ or so of surface-bonding cement (fiberglass reinforced) for exterior finish.

    The lumber I use for grade beam forms is protected with 6 mil poly (which creates a capillary break underneath) so that it can be reused as structural lumber in the house.

    I think this is a simpler and more cost-effective and energy-efficient SFP foundation system than what you’ve tried with ICFs. It’s important not to overinsulate under the slab, since the SFP foundation relies on a combination of geo-thermal heat and heat loss from the house to maintain a “bubble” of warm earth underneath which not only prevents frost but also significantly lowers the delta-T and minimizes heat loss downward.

    You can see my foundation section at:

    • Robert, thanks for your comments. I’m familiar with the NAHB guide, but differ with them about how much to extend the “wing” insulation. The NAHB guide is a workable minimum, but my intention is to exceed that. Thanks too for your detail – always good to see how other folks are working on similar undertakings.

    • Robert, What is the point of the double wall? Why can’t a single 2×4 wall be built on the edge of the slab? With current spray foam technology, this should afford enough insulation value. Are you trying to balance the load on the grade beam? Thanks for your perspectives, which I found very helpful.

  5. Robert Riversong says:

    The NAHB guide is based on 40 years of experience and more than a million buildings in severe climates. It is the accepted standard for the IRC.

    What actual field experience has demonstrated is that the assumption that wing insulation needed to extend horizontally the same distance as frost depth is a fallacy (except for an unheated building such as garage).

    As I indicated, one can exceed the NAHB standards (as my foundations do) without going to extremes and using more petrochemical (ungreen) products than necessary.

  6. Jamie Wolf says:

    Robert said:

    “It’s important not to overinsulate under the slab, since the SFP foundation relies on a combination of geo-thermal heat and heat loss from the house to maintain a “bubble” of warm earth underneath which not only prevents frost but also significantly lowers the delta-T and minimizes heat loss downward.”

    You lost me here. You are suggesting that warming the earth (which would be accomplished by “heat loss downward”) is desired because it will then minimize that heat loss – huh? Why do we want to use the energy we are striving to retain to then heat the earth? Don’t we want no loss to earth, thus an SFP system more like a garage?

    …and yes, less foam is an important goal, but using less energy for the life of the building likely dwarfs the environmental impact of the additional foam

    And to David:

    Do your load calculations suggest that we could use foam as a thermal break between stem wall and wall plate – not just between footing and wall? I’m assuming the foam is not capable of this (or we’d be doing it).

    There is an imperative for radical load reduction (think Passivhaus) and with it the increased significance of thermal bridges and thus their elimination wherever we can find them (and these at the foundation/slab/wall plate junctions are the big ones) – so always on the lookout for strategies that can accomplish this while maintaining essential structural integrity.

    Also: How is your building wall supported by your foundation? Could you post the rest of the section?

  7. Hi Jamie – wonderful to hear from you,

    I believe that in certain cases, we could use foam at the bottom of the slab haunch as a thermal break between the haunch and a footing or compacted subsoil beneath. Check out the link provided above by Jesse Thompson:

    You’ll note that the Swedish product is purposefully designed to have the slab haunch isolated from subsoil by foam – it appears to be a dense EPS foam.

    I had regretted the thermal bridge through the slab haunch and footing, but I had assumed, in error, that foam could not bear the structural loads. I should have run the numbers to determine it that’s true, but as you note, we often assume that if something were possible, we’d already be doing it. Jesse’s post inspired me to look further – now I’m convinced that we should calculate structural loads in pounds per square inch (kg/M2) and check whether foam can bear those loads. There are several foam products available with surprisingly-high compression strengths – I’ve seen up to 100 psi.

    In my little building, the bearing walls are all on the perimeter, and thus are supported by reinforced concrete in the slab haunch. That load is transmitted to the reinforced footing which spreads it ultimately to bear on the compacted fill below. The footing is not necessary for load-bearing: I only cast it to provide a level and solid surface on which to place the ICF’s. One reason I extended the foam horizontally out 4 feet was to help elevate the soil temperature near the building=, thus reducing the delta-T. I think Robert makes some great points, but I feel that the NAHB guidelines are not gospel, and can be exceeded without guilt over a little more foam. The extra foam represents, at most, a few gallons of oil, yet the energy savings over the life of the building can potentially be thousands of times that. After our first season in the new office, we’re incredibly pleased by its energy performance: We have 384 square feet (36 M2) on the main floor, with the same area in a conditioned attic above, and despite a cold winter, we’re on track to burn about 1/4 cord (3.6 M3) of wood for space heating for the season.

    I can’t post a picture of the whole cross section in this comment, but perhaps I’ll add another post about our little building another time.

  8. Jamie Wolf says:

    I’d like to know more about the building envelope. I did some presumptive math (1/4 cord = 5MBTU/768SF=4557BTU/SF/YR) – pretty impressive.

    So these questions:
    What did you model the heat loss at?
    Did you test ACH?
    What about ventilation?
    What’s the rest of the energy profile?

    OK – I guess I’m asking for that second post!

    Also – what I was suggesting was using foam between the wood frame and the slab.

    • Jamie, I’ll work on that new post. I suppose that one could place foam at the bottom of the wall plates, if the load per unit of surface area were less than the compressive strength of the foam. However, that wouldn’t block the flow of heat by conduction from the slab interior to the bottom of the slab haunch. That’s why I think it would be better to place foam there.

  9. jason erb says:

    wow! i can’t believe the amount of time i have spent researching the proper ways to construct a detached garage concrete foundation with hydronic (pex tubing) heating. from what i understand, after excavating the organic soil, etc. i will want to build a form to accomodate 12″x12″ footing. backfill 5″ gravel and compact with tamper/bobcat. this will be a monolythic (single stage) pour so my question is this:
    when i lay down my vapor barrier on top of the gravel, do i extend it down around the outside edge of my wooden form so it will be completely covered by the slab as well as footing? i will also be installing 2″ polystyrene on top of the vapor barrier. however i am a bit confused on how to continue (with the polystyrene) towards the edges of the forms, as the gravel is elevated higher than the bottom of the footing and also sloping downwards in the direction of the bottom of footing, begiining a minimum of 14″ away. any suggestions or links i can go to to see an illustration of the proper technique in accomplishing this?

    rebar will be installed on top of the polystyrene and pex tubing attached on top of the rebar, with tubing being approximately two and a half inches from the 5″ slabs’ surface.
    any difference between rebar and the wire mesh for reinforcement of the slab? other than rebar will be in the footing and maybe a few rows around inside perimeter of the slab.

    thanks for any comments on my dilema. any better suggestions would be greatly appreciated. btw, i live in canada and it gets quite cold here in the winters. garage will be 22’x42′ with 2″x6″x10′ walls.

    • Jason, I’m reluctant to give you specific advice, because I don’t know all the details of how you’re forming the slab, using the building, and so on. But a few observations:

      – Place the vapor retarder last in your sub-slab assembly, above the foam and immediately below the concrete you’re going to pour. You don’t want to accumulate water from the wet concrete in the space below the slab – that water should stay in the concrete mix, and dry to the top of the slab.

      – If you intend to heat this slab, and you’re in a cold Canadian climate, it’s imperative that you use abundant insulation below the slab and especially along the slab edge. Try not to repeat my mistake – see if you can devise a way to have high-compression-strength foam at the bottom of your slab haunch.

      Anyone else care to chime in?

      – Yes you want the vapor retarder to extend to the slab haunch, and underneath it, if possible. The way I did my slab, that wasn’t feasible, so I coated the top of my footing with “Thoroseal,” which acts as a capillary break and a vapor retarder.

  10. Jason Erb says:

    Hey David, thanks for the reply. as i am not much for jumping in on forums (mostly just reading and absorbing info), i figured i’d try it this one time to see if i could figure out a few details i am still seeking. from what i can tell, there doesn’t seem to be any national standard on how exactly to assemble all the components of a concrete slab/foundation with hydronic heating. some say concrete will crack as rigid foam can not handle the weight of the concrete on top, others say use wire mesh and not rebar (msybe becauae it is easier to lay out/install and attach tubing to), some say vapor barrier under the foam – or over the foam (as you’ve indicated), some say attach tubing to foam (via clips) with the rebar on top, others say tubing on top of rebar/wire mesh (via wire or tie straps).
    i guess it may all depend on what the laws/codes suggest in certain areas, but it seems to me that common sense along with decades of engineering and testing that US Energy Department, Hydronic Heating Association, Radiant Panel Association, National Association of Home Builders, etc… would come together with a uniform practice for this type of installation. they all (atleast as far as these so-called Associations are concerned) seem to pass the buck on to “your local contractor”. every forum i’ve read has this same reference to “local contractor” somewhere in it, yet even those people conversing in the forums get different answers from different contractors in the same/similar locations!!
    Sorry, i kinda went off on a rant there.
    as i’m clearly not an engineer (however i do work with engineers in the oil/gas industry, and have come across many in these forums who claim to be one; and they are mostly not all that brilliant when it comes to practical applications versus on-paper/calculated applications, nor are they in any kind of agreement on this specific topic) it seems to me that a monolithic slab is maybe not the best foundation for such a large sq/ft (22’x42′) garage. so, i’m kinda second guessing myself and may pour my 12″ footings first and then the slab after. this way i can insulate the footings a little better and the slab can float, in the event that i experience heaving from the underside of concrete/slab. as for the vapor barrier i don’t really see it making a difference whether it was on top or bottom of foam except in the event that one was to have their tubing on the foam and secure it with those clips directly to the foam, thus puncturing the foam. however i don’t understand this method as, from what i understand, the tubing is most efective when it is about 2 – 2 1/2 inches away from the top surface and the rebar about 1/3 from the bottom of slab. thus, in a 5′ slab i would have rebar (suspended with chairs or blocking of some kind on top of the foam)approximately 2 inches from the bottom of slab with the tubing on top of that, which would be about center of slab, leaving about 2 inches of concrete above the tubing. it just doesn’t make any sense to me to have the tubing so far down in the slab unless maybe if the slab was only 3-4 inches thick i guess. not to mention the entire configuration of the slab is somewhat compromised by the fact that the tubing cannot be incorporated directly into the concrete itself thus making the concrete only 4 inches thick in the areas that the tubing is; assuming it is a 5 inch slab and one was using 7/8′ tubing (basically 1″). i guess it could be argued that if the tubing is in the center of the slab that this slab is also only 4 inches thick in those places that the tubing is. however, i just refer to what i know about building construction and make the assumption that concrete might be the same as floor joists in the sense that when you run wiring through them, you are suppose to drill the holes directly in the center of the joist, thus making a 12″ floor joist into a supposid 6″ + 6″ which apparently is stronger than a suppposid 10″ + 2″. maybe i am wrong but this is what makes sense to me.
    i would also note that if one was to use 1/2″ rebar and secure the tubing directly in a linear line on top of the rebar (say 12″ spacing for both rebar and tubing) rather than spacing the tubing between the rebar (thus having the rebar and tubing 6″‘s apart from one another and not actually on top of the other) one would actually have 1 1/2 inches of space within the slab which is not concrete. maybe i have thought this through too much, but if i go to my “local contractors” before i have this slab poured and i get 3 different answers on how to do this thing correctly, then i’m probably gonna lose my mind!
    as for the foooting insulation, i dont get the idea of 2″foam on the outside of footing wall. i do understand how it will work in keeping the heat in the building, i just don’t see how one brings it 6″ above ground level (which building specs require my footings to be) or leaving it just below ground level so you cant see it coming out of the earth, without it actually either having rain/moisture getting between it and the footing (which is bad) or coming up to where the sheathing meets the footing and it is protruding 2 inches wider at the base of the footing compared to the edge of the outside wall (making the perimeter base of footings 4 inches wider end-to-end). not to mention, that in this scenario, my weed wacker will be in constant contact with the foam when i am trimming the grass up against the garage. however, i do plan on a sidewalk around the entire garage eventually, but in the meantime i dont need moisture getting in there nor do i want to have heat loss either. seems like lose/lose to me on this one.
    anyways, obviously i will be having discussions with the contractors, plumbers, concrete and city guys about everything before i go ahead with this once the ground thaws here in the next month or so. however, seeing that this is going to be a very expensive project i figured i’d better do some research as i don’t need to happen to me what has happened to so many of the others i have read about in so many forums who made some awful decisions or took some terrible advice. Ouch, it almost makes me not want to even attempt this project! but then again, i still never did come across anyone who claimed to have built the perfect garage so… mostly “very pleased with the results” to “the concrete cracked in a few spots, to “i wished i’d have went with shorter lengths of tubing/ more zones with larger manifold”, or probably the worst one “i wished i would have put the 2″ rigid foam under the slab”. still haven’t heard of anyone puncturing their tubing during the install nor at any time after system is running so that’s good NOT TO HEAR!

    probably the wrong forum to have jumped in on but it seemed like you guys were some of the more intelligent fellows when it came to the things i was most wanting to know about. was a really good read. thx.

  11. David wrote:
    – If you intend to heat this slab, and you’re in a cold Canadian climate, it’s imperative that you use abundant insulation below the slab and especially along the slab edge. Try not to repeat my mistake – see if you can devise a way to have high-compression-strength foam at the bottom of your slab haunch.

    Does anybody have a basic diagram of how this foam might fit into the foundation? I’m having a bit of trouble picturing it!

  12. Steven, if you check out the links in Jesse Thompson’s comment (#1 above), you’ll see some examples.

  13. One design alternative I’m considering is to use structural EPS+steel walls for the foundation wall. I don’t want to pay for an engineer to sign off on it, so I would just make it to code, which calls for a 4′ foundation wall.

    These things don’t suffer from thermal bridging like concrete and are R36. So… what I’m wondering is, do I still need to insulate under my heated slab? Perhaps it is unnecessary because the whole area under the slab would be thermal mass; the heat energy under the slab would be contained by the foam wall foundation.

    Can someone see a reason to insulate under the slab with this design?

    • Steven, there are good reasons to insulate under the slab. First, although the delta-T is not high, it will still be a preventable heat loss. Second, if the slab is directly in contact with relatively-cool earth temperatures, there’s a risk of the slab reaching dew point temperatures at times, leading to condensation on the slab surface. Third, it’s very unlikely that you can thermally-charge all of the thermal mass under that slab – you’d be lucky to store significant heat in the top few inches of the concrete.

  14. I live in Maine and am considering buying a house that was built on a slab foundation in 2008. Is this a bad idea? What questions should I ask before purchasing?

  15. Jim Ridge says:

    This blog inspires me that maybe an easy fix for my slab house may be possible. I live in a region where my house should have been built with a foundation wall but was not, It is only a 4″ slab and the slab is partially exposed on the exterior. I have done a lot of reading about shallow foundations and was thinking about how I could do some sort of retro fit, while it would not be perfect it would certainly minimize the heat loss that is occurring now. The house is about 40 years old and has been heated every winter and exhibits no signs of structural deformation despite the lack of of a foundation wall. The only exception to this is the unheated garage and frost heave cracking is present in the outermost corner. Would anybody have any strictly hypothetical ideas on how to best insulate my existing slab using ICFs, Foam Panels, or something else? I am also planning on wrapping the bottom section of the wood siding home in something more durable than wood such as stone or brick facing as well as it is too exposed to moisture, perhaps this factor needs to be considered/incorporated in what is hopefully a easy insulation fix.

  16. What does frost or freezing soil do to a foundation’s concrete stabilization?

    • Freezing can wreak havoc on concrete. That is, freezing water can. Concrete itself can withstand freezing temperatures, as any walk down a sidewalk in winter will demonstrate. But freezing water expands with a force that can shatter concrete. So one has to make sure that water doesn’t freeze in the vicinity of the concrete. A great way to do this is with drainage – making sure water is diverted elsewhere. Another way is to extend an apron of insulation horizontally out from a foundation, to help keep earth temperatures by the foundation above freezing. But the main thing is the drainage.

  17. Does anyone know if they have comeup with a form just for this thickening or haunch design planning to do one in the spring here in new brunswick canada

  18. I would skip the footing and put foam under the thickened area. Ie, a frost protected shallow foundation monolithic slab. Away from the edges, you can put sand over the foam to reduce the concrete thickness. Ie, the foam is all on the same plane.

  19. Hi David,

    It has been a couple of years since you built this foundation. I am considering building a new house on similar foundation construction.

    May I ask how your foundation has stood up over the last couple winters?

    Thank you,

  20. Andrew, we’re into our fourth winter on the foundation, and it has held up fine. No signs of any movement, cracking, or otherwise. We have three years now with exceptional energy performance for this little building. This fourth winter is strangely warm, so fuel use may be even lower than years past

  21. Tommy hakomaki says:

    you could make your own slab-edge elements. It dont look too hard imho
    Look at this youtube clip from swedish accell
    Skip the fibre cement board part. Stucco or cement render it instead.

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