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Musings of an Energy Nerd

Foam Under Footings

Yes, you can build your house on rigid foam — as long as you can convince your local code official to let you do it

Thermal bridge alert. With the foundation wall at room temperature, the footing acts as a thermal bridge that allows heat to flow from the concrete block wall to the soil.
Image Credit: GreenBuildingAdvisor

UPDATED on January 26, 2018

A wide variety of residential foundation types, including monolithic slabs, crawl space foundations, and basement foundations, can lose heat due to poorly detailed insulation at the concrete footings. That’s because many construction details, including some details on the GBA Web site, fail to address thermal bridging through foundation footings.

There are several possible ways to address these thermal bridges, including:

  • ignoring the problem (based on the theory that the heat leaks are trivial);
  • on some types of foundations — those with stemwalls insulated on the exterior — switching to interior wall insulation to allow for an uninterrupted thermal barrier;
  • altering the construction details to include insulation under the footing.

In general, deep footings will lose less heat during the winter than shallow footings. Whether the amount of heat leaking through a concrete footing is enough to worry about depends on your climate and your performance goals; if you hope to achieve the Passivhaus standard, such a thermal bridge is clearly a no-no.

Is there any reason NOT to put foam under a footing?

Traditional wisdom taught builders to place footings on undisturbed soil below the frost line. If footings are properly designed for the soil at your building site — an exercise based on either soil testing and engineering calculations or rules of thumb and local knowledge — then such undisturbed soil should be able to support the weight of the building, with a healthy margin of safety.

Engineers explain that good soils should be able to support 3,000 lbs. per square foot (20.9 psi). As it turns out, common extruded polystyrene (XPS) insulation like Dow Styrofoam or Owens Corning Foamular has a compressive strength of 25 psi. That’s more than many soils that are routinely used to support a footing and a house. Moreover, it’s possible to order XPS…

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  1. Interested Onlooker | | #1

    Insulated Raft for Double-Wall Construction
    Presumably an insulated raft design for a double-wall building would need a ring beam with a conventional footing outside the edge insulation of the raft to support the loads from the external walls and the roof ?

  2. GBA Editor
    Martin Holladay | | #2

    Response to Insulated Onlooker
    Interested Onlooker,
    Q. "Presumably an insulated raft design for a double-wall building would need a ring beam with a conventional footing outside the edge insulation of the raft to support the loads from the external walls and the roof ?"

    A. Not necessarily. A wide variety of walls could be placed on an insulated raft foundation -- including an ICF wall, a wall framed with I-joist studs, a stud wall with exterior Larsen trusses, or even a conventional stud wall with thick exterior foam sheathing (i.e., a REMOTE or PERSIST wall) -- without a "ring beam with a conventional footing."

    As long as the roof and wall loads are transferred to the concrete foundation, the foundation will work.

  3. Interested Onlooker | | #3

    I was concerned about the thermal bridge which would occur if the outer frame were to sit directly on the raft. I am envisaging a construction in which the outer frame carries all the live and dead loads on (and of) the roof, the exterior walls and the windows.

  4. GBA Editor
    Martin Holladay | | #4

    Second response to Interested Onlooker
    Interested Onlooker,
    Obviously, the load path from the roof and walls needs to bear on the foundation. If you are choosing an insulated raft foundation, it only makes sense to plan your walls so that the load paths are well supported.

    If you want to build a double stud wall with the outer wall bearing the roof load, then obviously the outer wall needs to bear on concrete. That means that your siding won't be coplanar with the stucco coating used to protect the foundation foam, unless you install rigid foam of the same thickness on the exterior side of the outer stud wall.

    It's certainly possible to have the siding NOT be coplanar with the foundation stucco -- I've seen it. Whether or not the detail bothers you is a matter of aesthetics, not building science. Needless to say, durable metal flashing is required at such a transition.

    To me, it makes more sense to have the inner stud wall bear on the concrete footing, and to use Larsen trusses or exterior rigid foam to thicken your wall.

  5. Interested Onlooker | | #5

    Larsen trusses
    Thanks for a prompt reply. I have accepted Robert Riversong's advice against using Larsen trusses in seismic areas (the house is destined for Cascadia) and have followed his suggestion of using independent load paths for roof+exterior wall and floors+interior wall.

    Would it be sensible to build a Thermomass dwarf wall to carry the wall loads and sit this on the insulation with the inner wythe of the Thermomass wall acting as the form for the floor slab? Rebar protruding from the inner wythe would tie slab and dwarf wall together.

  6. GBA Editor
    Martin Holladay | | #6

    Third response to Interested Onlooker
    Interested Onlooker,
    Properly detailed, that would work.

    This isn't really a question of what is "sensible," however. There are hundreds of ways to build foundations and walls, and each way has advantages and disadvantages.

  7. [email protected] | | #7

    A Few Details + Larsen Trusses in Seismic Zones
    Pierce County didn't blink an eye on our Passivhaus permit application. Have a look here, if you like, at the envelope details from our permit set: On that project we used a bearing stud wall with rigid on the exterior--similar to REMOTE/Persist. On subsequent projects we've moved to a stud wall bearing conventionally on the foundation, structural sheathing as air barrier, with Larsen trusses blown with cellulose hung off the exterior. The foundation details remain essentially the same.

    An aside to Interested Onlooker: I'm curious as to why Robert Riversong advises against Larsen trusses in seismic zones. Can you (or he) point me to that discussion? Seattle is zone 3, and almost all Passivhaus building designs I've seen in this area use we we're calling Larsen trusses. (I recall that technically they are not actually "Larsen trusses"...)

  8. AJ Quinter | | #8

    Basement Slab over Rigid Insulation
    What is the advantage of having a basement slab over a heavy layer of rigid insulation? I would think if a layer of 25psi foam (3600 psf capacity) was laid over a well compacted layer of gravel, the slab could be deleted and a plywood floor could be used instead. Would you be concerned that the foam and plywood would not be able to span over soft spots in the underlying gravel and soil? Any thoughts?

  9. GBA Editor
    Martin Holladay | | #9

    Response to AJ Quinter
    Advantages of a slab compared to plywood:
    1. Greater durability
    2. Greater resistance to radon.

    I'm sure there are other advantages as well. But plenty of people have built wood foundations, including basement floors of wood:

  10. Doug McEvers | | #10

    Foam under footings as a capillary break
    It seems to me, one of the benefits of insulation on the outside of the building foundation is moisture control. Isolation of the building from the damp earth would help greatly in controlling humidity in an airtight building. If a conventional footing and foundation wall is used, would one have to wrap the footing with rigid insulation?

  11. aj builder | | #11

    rating a blog
    I rate this blog AAA

    Great topic.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Doug McEvers
    Q. "If a conventional footing and foundation wall is used, would one have to wrap the footing with rigid insulation?"

    A. I don't understand your question. No, you don't HAVE to wrap the footing with rigid foam. Most conventional homes do not place any rigid foam under the concrete footing.

  13. Doug McEvers | | #13

    Improvement to a conventional foundation
    Martin, I was thinking more along the lines of a modification to how the majority of new homes (full basement) are built in MN. I understand the warm side (inside of the foundation wall) and under the slab, rigid insulation system for a full basement, my question is on exterior foundation wall insulation. I like to insulate with rigid on the outside of the foundation wall and under the basement slab, could this system be improved by wrapping the footing with rigid insulation from a thermal and moisture control standpoint?

  14. GBA Editor
    Martin Holladay | | #14

    Response to Doug McEvers' second post

  15. Timmy O'Daniels | | #15

    Seismic Areas and Larsen Trusses
    Entering "seismic" and "Larsen" in the GBA Search Facility threw up :
    "Airtight Sheathing & Thermally Isolated Double-Stud Walls" and
    "Simple One on Thermal Bridging" as relevant hits.

    BTW I'm intrigued in what way are your Larsen trusses "not actually 'Larsen trusses'..." ?

  16. M. Steven Dickerson | | #16

    How about something similar in the NE USA?
    I am building right now, and need foam products for a passive haus (home) which I am building. I would love to use this type foam products, but am finding it tough to get a supplier for something similar in the US (Massachusetts)..

  17. GBA Editor
    Martin Holladay | | #18

    Response to Skylar
    Thanks very much! What a great resource! I really appreciate all the links.

  18. Brad Buser | | #19

    What about termites!?
    I look at these systems, get really excited, then realize this is impossible to protect the stud wall from termites. I live in Oklahoma, one of those crazy mixed climate areas, that needs termite protection and could really benefit from slab edge protection on our monolithic edge foundations, but the termites keep us from doing it. Has anyone really determined a good way to address this on a shallow frost protected foundation?

  19. Dan Kolbert | | #20

    Suppliers - response to Dickerson
    Shouldn't be a problem getting foam. I found 25 psi EPS from several suppliers, including Foard in RI, and Beacon Sales in ME. Call around. XPS at 25 psi is available almost universally.

  20. user-788447 | | #21

    ICF manufacturers
    Thanks Martin.
    Timely topic for us also.
    A couple days ago we sat down with a local ICF manufacturer and asked about having custom formwork created for footings. According to the rep the ICF form manufacturer can make custom profiles to any design out of their base EPS stock which if I recall correctly comes in 4'x4'x8' sheets.
    We are currently working on another design to meet Passivhaus standards, this time planning on slab on grade. We may have an opportunity to cost compare between the systems you outlined vs. local costume manufactured formwork.

  21. user-912539 | | #22

    Sourcing EPS custom cut
    The physical properties of EPS are defined by ASTM C578 Standard Specification for Rigid, Cellular Polystyrene. This chart is found below at the EPS molders Association.

    The density of type IX EPS is referred to as two pound density. However, the specification allows a minimum of 1.8 pcf. This material has the 25lb compressive strength.
    The EPSMA site also lists its members who can provide a custom cut form of any shape you can draw. The computer cutting machine (CNC) uses a hot wire to cut out the special shape. Most block molders mold billets up to 16 feet. However, most computer cut shapes come in eight foot lengths.
    Most EPS block molders also have the capability of incorporating a borate solution during the manufacturing process to assist in the protection against carpenter ants and termites. The borate treated foam usually comes with a very small up-charge.

  22. Super Structure Builder | | #23

    Is there such a thing as
    Is there such a thing as getting too carried away? When I was a kid, one of the bible songs I learned was about the three guys who built three houses, with the foolish man building his on sand. Dow can make their foam 1000 PSI and I still wouldn't go near setting my house on foam. The ground is continuous, going straight down non stop, the foam is not. There could be some sliding or some settling, despite what the mfr might say. If you've built an air tight house, the heat loss through the slab should be negligible. I'm putting insulation underneath the Advantech that will go on the slab and that's gonna have to do. Foam that thick will prob run 30 a pop. For a 2500 SF foot print, you're talking $2,000 or more. How about just making your slab thicker with the money? Foam mfrs might claim the product is waterproof, but can it really withstand 10 years on moist soil? I'll pass on this technique.

  23. Aaron | | #24

    A Few Details + Larsen Trusses in Seismic Zones
    To Rob Harrison:
    It would have been nice to have a higher resolution drawings in your link.

  24. Skylar Swinford | | #25

    Not so super
    To Super Structure Builder:

    "If you've built an air tight house, the heat loss through the slab should be negligible." Yes, if you build a home that is airtight with no insulation, the amount of heat loss through the slab will be less than walls and ceilings exposed to ambient air. However, in an air-tight and super-insulated home, the heat loss through an uninsulated slab or perimeter thermal bridge could easily match or exceed the heat loss through the roof of wall system.

  25. hbxmRv2GbK | | #26

    level ground
    How important is it to have all the voids under the foam filled? I have not found an excavator who can dig perfectly flat and level. Would the strength of the faom be altered if the surface under it is not flat and level?

  26. GBA Editor
    Martin Holladay | | #27

    Response to Tim Geiger
    It isn't necessary to have an excavator who can prepare your site perfectly. You have a layer of crushed stone under your foam, right? If you use 3/4-in. crushed stone -- not larger -- it should be easy to rake. An ordinary garden rake works fine.

    Use a long, straight 2x4 and a level to check your work.

  27. Kevin Dickson | | #28

    Heated buildings need insulated footings too
    "“Revised Builder’s Guide to Frost Protected Shallow Foundations,” recommends the use of XPS under the footings of unheated buildings."

    This is a major problem because it implies you can't do it for heated buildings. Who can we bug to get this omission corrected?

  28. GBA Editor
    Martin Holladay | | #29

    Response to Kevin Dickson
    I suppose you could try to find an interested researcher at the NAHB Research Center, which is responsible for the document.

  29. Cory Johnston, PE | | #30

    Insulated Foundations and Slabs
    As usual with these types of discussions, many subjects get involved and ideally will all be integrated to get the best overall performance. Placing foundations on insulation is structurally viable but has some issues to be aware of. Concrete placed in a standard excavation automatically forms to the unavoidable iregularities of an excavation. Foam will not, or at least not as well. Excavations and foam placement must therefore be done more carefully if the full bearing capacity of the soil and foam is to be used. On a raft foundation, this is not quite as critical as wall loads are generally spread over a larger area, with a corresponding lower bearing pressure, assuming the raft foundation is designed correctly. However, building code required frost depth can come into play and make a raft type foundation problematic unless it is part of a basement, versus a standard slab at grade. One also has to consider the thermal mass of a raft type slab/foundaton as it can easily change the overall thermal performance characteristics of the structure.

    I am currently working on a passivhaus design in Michigan and looking at all these considerations with the architect. I appreciate the article and comments to see how these issues are being addressed by others.

  30. Kevin Dickson, MSME, P.E. | | #31

    Comments on Frost Heave
    Some very interesting research from the manufactured home industry is worth mentioning here. It wasn't intended for slab foundations, but I believe their findings are applicable to on grade slabs.

    It shows us that the "wing insulation" discussed in the FPSF guidelines can be ignored, you don't have to dig down at all, (except to remove organic material), and any gravel intended to drain water away is unnecessary.

    Here's the reasoning:

    Depending on the type of soil, frost heave will only occur when the moisture content is above roughly 15%. It turns out that it is really easy to keep the soil under your home this dry. The home itself protects the soil underneath it from water, and the surface water at the perimeter must be directed to a point at least 10 feet away.

    Since the wing insulation is only to keep the soil below the shallow footing or slab edge warmer than 32F, if the footing or edge rests on on dry soil, you don't need to keep it warm.

    If all the perimeter surface water is directed away from the house, no water gets underneath to freeze. Gutters help, but aren't required. Since you don't have any water under your foundation, it means you don't need any gravel and french drains to take it away.

    This approach also means you don't have to rely on the heat from inside the home to prevent foundation freezing.

    Note that you should still completely insulate the bottom and sides of the slab for energy reasons.

    Here's the press release for the report, but I guess you have to pay for the actual report.

  31. Doug McEvers | | #32

    Soil Moisture

    Lot's of "ifs" in your post, I like gutters on all of my projects with long extensions to get rainwater away from the building. Proper grading is also a top priority as you have stated. In the Midwest we have had some hellish rains in the last few years so I would say whatever foundation type you choose, install gutters with 3" x 4" downspouts and extensions.

  32. maison-passive | | #33

    Insulated foundations
    I've seen some visitors from your site on my blog ;-)
    I am glad it may interest. But it's in french.

    So if you read french, there is a special category here for insulated foundations products

    As I wrote two years ago (with a lot of hope): Welcome to the passive house movement!

    Luckily, there are people like you who show the right way to go: "passive house once, passive house for ever" as our austrian friends from Vorarlberg say.

  33. GBA Editor
    Martin Holladay | | #34

    Merci bien!
    Merci pour l'information sur le système d’isolation périphérique.

    Merci aussi pour cette perspective historique: "En outre, nos amis américains ont été semble-t-il, impressionnés par l’équipe allemande lors du dernier Solar Decathlon. Il faut dire, qu’historiquement, ils ne sont pas étrangers au concept de maison passive. Car le mouvement vers plus d’efficacité énergétique a été amorcé au USA à la fin de années 1970 et au début des années 1980, grâce à des pionniers tel que William A. Shurcliff ou encore de la construction du Rocky Mountains Institute par Amory & Hunter Lovins. Depuis, la Waldsee Biohaus a vu le jour dans le Minesota, le standard prend racine et les sites d’informations se multiplient aux USA."

    Aux deux bords de l'océan atlantique, nous travaillons ensemble pour le même but. Bonne chance!

  34. Kevin Dickson, MSME, P.E. | | #35

    Soil Moisture

    Under the SBRA guidelines, water management maintenance remains extremely import for long term success. I fixed a home last year that had sunk 5" because the missing downspout extension had been ignored for about 40 years. The foundation was down 3' per code, and there was never any frost heave, but it failed nonetheless because it was on mud.

    Wing insulation may have helped deflect that water out a couple more feet and prevented the damage.

    Another advantage of a raft-style slab is that it is high up on the site, above the mud that might occur from long-term bad drainage. That mud, frost heaving or not, my damage a stem wall or basement wall.

  35. Dave | | #36

    Polystyrene under footings is goodness
    Excellent article Martin. I totally agree with putting polystyrene under the entire foundation (including the footings) to reduce heat loss. I am doing this with ICF construction (ie under a very heavy concrete house). My rationale for it is written up at and in the surrounding implementation pages. I have just added a link to your excellent article to help persuade people that I am not crazy.

  36. Gavin Farrell | | #37

    Lateral/seismic testing
    Well, I'm sold that the load bearing capacity is no problem for foam, but how about lateral behavior? This would only be a concern in seismically active areas. As noted above, concrete fills all the little nooks and crannies a little better, setting up load transfer nicely. I see one of the pics of the insulation system shows a nice egg crate interlock between layers of foam which would prevent any lateral movement, but how about the bond between the foam and concrete above and the foam and gravel below? Is there enough keying or friction developed to prevent any lateral movment problems in, say, an earthquake? I would bet the answer is 'yes', but I am intrigued if it has been tested as such.

  37. wjrobinson | | #38

    Tim Geiger, grading
    I grade tennis courts to 1/8" tolerances. Look up laser grading. We know how to perfect planar surfaces preconcrete or in this case foam.

  38. Jim Bannon | | #39

    question on basement interior foam
    Well, it's too late to consider foam under the footings for our 18X21' bump out addition, we poured them about two weeks ago. Site is located in Southern NH, climate zone 5. We are planning 2" of XPS under the slab, and a continuous layer of 2" ridgid foam on the interior side of the new foundation walls (as shown in the first illustration at the end of the article). The rigid foam on the interior walls will run up to the band joist which will sprayed with closed cell foam. I was originally planning to use 2" of foil faced Thermax on the walls, but note that the illustration at the end of the above article specifies "vapor semi-permeable insulation". I presume this is to allow vapor from the concrete walls dry to the inside. But wouldn't it be just as well to keep the vapor in the concrete walls?

    The foundation wall has conventional damproofing applied to the exterior (Premier brand masonry coating applied with a roller) and of course footing drains on both sides of the footings (exterior and under slab). Site is properly graded, soils are well drained sandy gravel, roof runoff is managed via gutters and downspouts that discharge well away from the foundation. The existing basement is as dry as any that I've ever seen, and has never had any moisture issues.

    I was planning on foil faced polyiso (Thermax) vs. XPS for the walls simply because of the higher R-value, and to keep any atmospheric derived vapor (e.g. say someone left the bulkhead door open on a humid summer day) off the concrete walls. Is this asking for trouble?

  39. GBA Editor
    Martin Holladay | | #40

    Response to Jim Bannon
    I think you'll be fine. You're right that the experts at the Building Science Corp. believe that a concrete basement wall should be able to dry to the interior, but I think that the importance of this detail is exaggerated. After all, the soil on the outside of your basement wall is usually damp. If you encourage a basement wall to dry to the interior, you are inviting a lot of moisture into your home.

    Lots of people have insulated the interior of basement walls with Thermax, and found that the walls perform very well. I haven't ever heard of any problems arising from the use of Thermax in that location.

    The minimum code requirement for basement insulation in your climate zone is R-10. Just a few miles north, in climate zone 6, the code minimum is R-15. If you end up using 2 inches of XPS, your wall will barely meet code. A little extra insulation never hurts.

  40. user-632871 | | #41

    Does anyone know of any studies about ants tunneling in below-grade foam? I have seen amazing infestations in XPS in contact with wood framing (flat roof used as balcony). The ants don't eat XPS but they find it awesome for tunnels. Perhaps below-grade, against concrete, is no issue, and way below-grade even less of an issue. I hope so, as I've used XPS in that application many times, including my own house.

  41. jklingel | | #42

    Misleading numbers
    Be careful w/ the "25 psi foam". For EPS, I believe that advertised number is at 10% compressive resistance, ie, deformation. Beyond 5% compression is where you start to get out of the elastic limits of the foam, and by 10% the foam will not rebound. The "creep" referred to above is mentioned on the DOW web site, and they recommend a 3:1 ratio to avoid it (static loads; 5:1 for dynamic loads). I got info on this from the Insulfoam (EPS) web site, and by talking to a cat who does compression testing of foam. For example: EPS foam with a 40 psi compressive resistance (I believe that is what is called "40 psi foam") supports 35 psi at 5% but only 15 psi at 1%. I would not be happy building on something that I know will creep 10%. Just "bear" that in mind when you build and pick a compressive value that you are comfortable with.

  42. cfse | | #43

    Re: Misleading numbers
    Thank you, John, for stating an important point. It is crucial to not run with the "x psi" numbers on the packaging. These are very rarely the appropriate design values. There are numerous issues to take into account when considering placing foam below footings. Take care to hire an experienced structural engineer.

  43. jklingel | | #44

    Gavin Farrell
    Gavin: Lateral may not be a problem, according to an engineer here. If I recall the numbers he quoted, a house pushes sideways w/ about 10% of it's weight during seismic activity, and the shear plane between the foam and a slab would w/stand about 20% of the house's weight. However, I did not ask about the consequences of putting the vapor barrier right under the slab. I can't see the shear plane taking much load in that case. I'll have to ask about that next visit.

  44. Josh Golek | | #45

    Foam and Foundations more thoughts
    John: Thank you for pointing out something not many people realize. The listed capacity on foam is usually not a design value-- it is the load the foam can take right before it fails or deforms to an unacceptable limit. Design values may be much lower depending on how much settlement the engineer deems acceptable, which will be driven largely by the thickness and density of the foam used as well as the buildings framing scheme. We sometimes change densities under a foundation depending on location in an effort to minimize differential settlement. On the other hand, bearing capacities of soil such as the ones listed in IRC Chapter 4 are design values whose full capacity can be used when sizing a footing. So, a bearing capacity of 2,000 psf (common in VT where silty soils are prevalent) has a 14 psi design capacity or allowable load. "25 psi" foam after dividing by 3 has about 8 psi. Unless you consider snow, wind and live load to be cyclical loads. Then a factor of safety of 5 is commonly used , leaving us with a 5 psi design value for those loads. By the way I have no problem putting foam under a foundation I do it all the time at the firm I work for with several different foundation types. If you are going to build with thick foam under a foundation, it needs attention and forethought. Think about your utilities a bit, if the house settles 3/4" uniformly and suffers no harm, will the septic tank still be hooked up? Maybe, maybe not. Bottom line: until building codes can correlate all the variables for you in an understandable table for the most common foundation types, I recommend that you have your design checked by an engineer.

  45. anderslewendal | | #46

    FPSF with foam blocks
    Martin: I volunteer with Bozeman's affordable housing advisory board. Our goal is to create more affordable homes partly by using creative new building science. FPSF's are not new. Can we make a case for using 12" thick -2# density foam block extended 3' past the structure to act as the foundation? No concrete.

    My engineer thinks we can. Thoughts?

  46. GBA Editor
    Martin Holladay | | #47

    Response to Anders Lewendal
    I'm not an engineer. You have apparently already consulted with a real one. Believe him or her.

  47. anderslewendal | | #48

    You are right, we can likely get an engineer to approve it, but that does not address the economics and energy efficiency of the idea. In zone six we mostly install footings and four foot walls. If everyone is doing it, it must be the best solution. I am not so sure. I'll keep working on it and report back.

  48. PaducahKy | | #49

    Termite Protection
    I have 15 years field experience in the pest management industry as well as owning my own pest management business. I really like GreenBuildingAdvisor articles, and have for a good twoplus years, especially the insulation methodology embraced. Having said that, soils in most areas should be poisoned chemically at the proper times, volumes, & rates with termiticide. The entire process could be another article. Suffice it to say, subterranean termites can easily bore through foam to wood or other cellulose items, whether it be wood studs, beams, moldings, shelves, comic book collections on a shelf, pictures on a wall, or bibles next to their activity. I say that because we, myself included, often forget it's not just the wall studs, and not just the material value but time and emitiin correcting the problem. Most termite "pretreats" are done by the lowest bidder, and often that price doesn't even include treating the exterior backfill which may come months down the road. As well, it rarely includes treating before the footer but only after. There are some in my industry against foam near or at the soil, and encapsulated crawlspaces as well, who will not warranty work around foam. Myself consider it like any other inaccessible areas and slab homes, and feel it simply requires additional care and regular monitoring. If you decide to use foam, we have today exceptional products to control termites. Unfortunately not only product choice, but volume of total diluted should be very high, perhaps in hundred or hundreds of gallons, at the right stages, and to do absolutely best, on repeat visits after different soil disturbance (pipes, backfill, mulch, etc) periods and construction phases (additional backfill in porches and garages before pour). If, like 99% of the calls I get, the question is how many cents per square I charge, and can you match a penny difference with xyz, you're not asking the right questions. Tell them what products you might like, how many gallonsyou think it might take after you've read a product label (most are the same), inquire about followup needs, explain about your structure with foam, and tell them you're not made of money but you're willing to pay as long as it is done by the book. Okay, all that and one more thing: Borates. Extremely low toxicity and when applied to wood or porous building materials, and kept dry from rain and moisture, can last 20 to 30 years or more. You can inquire to have this done or do yourself, though like everything in life even the simplist things aren't so simple. The borates, in most of my area, would be supplemental to liquid termiticides, though for some uses can technically qualify alone. The concern I have, which can be mostly mitigated, would be the foam being "crushed=removed" because it was eaten by termites foraging looking for cellulose for nutrition. If you poisoned the soil prior this should protect you for a good 5, 10, 15 years depending on variables. After that you could supplement with a retreat/booster/bringing back to standards at ground level, inject more below the foam with a carefully designed system of injection pipes (must be done before construction), and/or a baiting/monitoring system service (stations in the ground that have to be checked and serviced typically every quarter (some 3x and some yearly). Some of the better baiting systems are Sentricon (not my choice because of being tied as a biz owner to DOW), and Advance system, among a few others. Patrick Perry, Perry's Pest Control.

  49. Buzz_Burger | | #50

    Termite Protection
    Question for Patrick Perry (or anyone). How far below grade will termites/ants go? With a full basement and interior foam on poured concrete walls that is continuous with sub slab foam, would there be any reason to be concerned? Assuming the poured concrete wall is sound an insect would have to descend 8' or so to reach any foam insulation. Or, again with a full depth basement, could exterior rigid insulation be protected using a membrane such as Bituthene?

  50. Buzz_Burger | | #51

    Termite Protection
    Question for Patrick Perry (or anyone). How far below grade will termites/ants go? With a full basement and interior foam on poured concrete walls that is continuous with sub slab foam, would there be any reason to be concerned? Assuming the poured concrete wall is sound an insect would have to descend 8' or so to reach any foam insulation. Or, again with a full depth basement, could exterior rigid insulation be protected using a fully adhered water proofing membrane such as Bituthane?

  51. Kennethmaher3 | | #52

    Does this also hold true for permanent wood foundations? I am interested in using a PWF and am wondering if XPS can be used under the 2x8 footing plate, above the crushed stone. I suspect this won't work with such a narrow footing plate, because the system really uses the crushed stone to spread the load further before bearing on the soil, but I am wondering if XPS could withstand the dead load of a single story house under the 2x8 footing plate.

  52. GBA Editor
    Martin Holladay | | #53

    Q. "Does this also hold true for permanent wood foundations?"

    A. There are two hurdles: the engineering hurdle (which, like you, I suspect is insurmountable) and the code hurdle.

    A permanent wood foundation is a foundation system with a package of specifications. I don't think that the building code or building inspectors will allow you to detour from the standard package of specifications. Even if there is a path forward -- in other words, even if you find an engineer who says you can do it -- I doubt if it's an easy path.

  53. Kennethmaher3 | | #54


    Thanks for the response. I am building in a rural area, where the code and inspector is not an obstacle. I am building a small off grid house with an insulated mass floor. The problem I'm pondering is optimizing the heat loss to cost and environmental impact of a wooden foundation. I am leaning toward using a 2x12 footing plate over 4 inches of xps and rock wool under the rest of the floor. But I'm wondering whether there will even be much heat loss through the thermal bridge of 4 inches of wood at the bottom of the PWF, which would mean it might be better to stick to that package of specifications, as you describe it, and omit the XPS and just use the 2x8 footing plate. I could simply insulate the floor and leave the load bearing walls as thermal bridges. Do you have an instinct either way?

  54. ecdunn | | #55

    Has anyone done cost/benefit studies on foundation insulation systems? I am building passive solar homes with 2" exterior foam board on stem to top of footing and 4' from stem wall under slab with good results. Easy, and much cheaper than the author's design.

    1. GBA Editor
      Martin Holladay | | #56

      This GBA article takes a stab at answering your question: "Determining Sub-Slab Rigid Foam Thickness."

  55. Kendog52361 | | #57

    I'm wondering whether the "insulated raft foundations" could be set/poured as the slab/footing part of a basement, with the basement walls being built on top of the "insulated raft foundation"?

    1. GBA Editor
      Martin Holladay | | #58

      Q. "I'm wondering whether the insulated raft foundation could be set/poured as the slab/footing part of a basement, with the basement walls being built on top of the insulated raft foundation?"

      A. Possibly. But here's my advice: (1) You shouldn't attempt this without the involvement of an engineer; and (2) If an engineer agrees, make sure that you also include exterior insulation for the basement walls.

      1. this_page_left_blank | | #59

        It would make sense to simply continue the raft all the way up the basement walls.

        Assuming the basement walls are concrete, they will add quite a considerable load to the slab. As such, it will likely have to be substantially bigger with more reinforcing than a raft slab would be, all other things being equal (same number of floors above ground, etc.).

  56. KKlouzal | | #60

    This article came out 9 years ago. I am currently searching for products like this available in North America. Are there any new manufacturers available since this article was written? Maybe it's time for an update? Pairing an Insulated Raft Foundation with Insulated Concrete Forms has the potential of creating a fantastic 360 degree building envelope.

    1. Expert Member
      MALCOLM TAYLOR | | #61


      The article was updated in 2018.

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