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What about using XPS foam under the footing in a Shallow Frost-Protected Foundation system?

GBA Editor | Posted in Energy Efficiency and Durability on

I’m building a new building that will have radiant floor heat from a boiler. Using a type of foam insulated cement wall — foam is just on the outside. I’m using s shallow frost-protected foundation and wonder about using XPS foam under the footings to stop the thermal bridge that is there.

Any thoughts anyone?

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  1. GBA Editor
    Martin Holladay | | #1

    XPS is used all the time under footings for unheated buildings with frost-protected shallow foundations. See Figure 7 (on page 17) in the NAHB Research Center publication, “Revised Builder’s Guide to Frost Protected Shallow Foundations” ( Just be sure that you choose XPS with adequate density.

  2. Christopher Briley | | #2

    My thought - Consult your structural engineer.
    As Martin mentioned, you can do this with light buildings with monolithic slabs or thickened slabs, but I have yet to meet a structural engineer who would put his stamp on insulation under a typical spread footing. I'm not saying it can't be done, I'm saying unless you have someone who's cruched the numbers, the risk is probably not worth the reward. You can insulate from the interior and thermally isolate the concrete frost wall.

  3. Riversong | | #3


    Where are you located (what climate zone) and what do you mean by "a type of foam insulated cement wall"?

    Are you following the NAHB guidelines (accepted by code) for a shallow frost-protected foundation for your AFI zone? Unless you're in a warm climate, there should also be some horizontal "wing" insulation extending outside the perimeter of the foundation.

    With a radiant slab, you must have adequate subslab insulation, along with a vapor barrier and radon vent. You also must have slab edge insulation to isolate the slab from the grade beam. With this edge and bottom thermal isolation of the slab, there is no need to isolate the bottom of the foundation.

  4. Dave | | #4

    Thanks Guys -- I've been looking into this with an engineer -- he and I went through the Builders Guide in his office and the updated ASES guide. But he is a little shaky about doing it. None of the heated building guidelines talked about. As concerned about the load bear issue. I was looking at 100 psi XPS which is rated at a larger compression capacity than the ground is when it is compacted correctly. I've been looking at a product that Lite-Form is making for these type of footing -- It's all one part - shaped as a flat bottomed "U" -- just keep it level and pour with the bar inside. Then I'm using a product called Conform - pvc formed walls with 2" insulation on the outside. --- This a Vet hospital and it's interior wall in the kennels need to be dog proof. Nothing like a concrete wall wrapped in pvc. Oh I did talk to Owens Corning about using their XPS foam that is rated at 100 psi compression - and they weren't ready to recommended it!! So what does everyone think? Thanks

  5. Dave | | #5

    Robert I'm in Iowa - right at the 2000 degree heat line - so it didn't look like we will need any wings == the wall system is made by -- cool idea for animal facility's. There will be a subslab foam of 2 inchs - R-10. The Lite-Form company is here in town so it's very easy to get product. They make and deliver it on the same day! If I was using their product - this wouldn't be an issue - they have 2" of foam on both sides. But I can't find a decent way of protecting it from the dogs in Runs - the wall will be the rear and sides in some runs. Thanks for everyone's thoughts Dave

  6. Jesse Thompson | | #6

    Using rigid foam directly under concrete footings is much less difficult than one might assume, and 100 psi foam is almost never necessary. Buildings just aren't that heavy!

    Even with a very heavy point load in a house, the math isn't that bad: for example, a 20 kip point load on a 36" x 36" footing is 20,000 lbs spread over 9 sf, or 2,222 lbs / sf. That's only 15.5 PSI.

    XPS (Dow Styrofoam extruded) has a 25 psi capacity in their standard product, and they make 40, 60 and 100 psi foam as well. Your engineer is most likely being conservative because they haven't done this before, but your soils are much more likely to be a risk with heavy loads than rigid foam will.

  7. Coyo | | #7

    From my experience engineers like to see 25 PSI foam under footings - Jesse's math shows how little load there is in reality. Trouble is that most engineers haven't dealt with this and it seems "wrong" to them to build on top of foam. There isn't anything "wrong" with it and your edge loss is always your greatest heat loss - so it is worthwhile arguing for. We use 12" of 25 PSI foam under the footing and then use 15 PSI under the slab...lower density, so lower R-value but also less heat loss then at the building edge. But in your climate and your project you are probably more concerned about keeping your radiant heat in versus your heat loss via the ground. Robert's advice to isolate your slab from your thickened footing area with a vertical thermal break is probably your best bet to start off from. I recommend using EPS vs the XPS as it handles moisture much better. I have seen many heavy water lodged XPS sheets of foam which weight a ton and a half. Of course you keep water away from your foundation anyways.

  8. Graham Mink | | #8

    What happens if some type of insect or animal decides to make a home in the sub-footing foam? That could affect the structural integrity of the foam. Although the chances of this happening are pretty slim, the potential downside risk would make me look for other options.

  9. GBA Editor
    Martin Holladay | | #9

    In cold climates, footings are usually 4 to 6 feet below grade. You don't get ants that deep.

  10. Kevin Dickson, MSME | | #10


    But FPSF is typically only 12" deep, still probably too deep for most ants.

  11. Dave | | #11

    Had a good visit with the Lite-Form people yesterday. We did review the Asce standards for FPSF. They do use foam under footings that are in unheated buildings. They don't use it in heated buildings but that doesn't mean it can't be used. We would use XPS not EPS - much less moisture penetration and with high compression numbers. Engineer has been working on the numbers - Compression of soils around here are in the 3000 lbs per square foot which is just over 20 lbs per square inch -- well with in performance levels of the higher density XPS foam. Lite-Forms system is in 8ft sections with cement channel being the height and width of the engineer choosing. Then 2" of foam. Then back fill about 6" above the footings top once the new walls are place on the footing. Looks like we will be using a 24" wide cement footing - a 8" wall on top of that. If we place the wall a bit off center - then we will have a 12" wing insulation to place on top of the footing. That will leave a 4" ledge on the inside for the slab to be place on. There will be 2" of foam under the slab and it will have radiant heat installed in it. So that will make the 8" high cement footing, 2" of foam under and 2" of foam on top than 6+" of back fill on top of that so there is plenty of slope away from the building. anyone seeing anything that is not making sense? Or other thought? Thanks Dave

  12. Riversong | | #12


    A couple things don't make sense. You're doing a shallow foundation in three separate pours: footings, stem wall and slab. If you really want to insulate below the perimeter, than why not a thickened-edge slab in a single pour?

    I prefer isolating the perimeter grade beam from the slab by doing two pours, particularly since I use a different mix for the slab (with fiber and, often, color), and because I build a 12" thick wall system I can use a 12" wide grade beam (which is more than enough bearing for a two-storey house on all but the worst soils). And I always design a floating slab. Bearing the edges on a footing requires that the subslab fill be so well compacted that there can not possibly be differential settling to cause cracking at the soil/footing junction.

    But, if you're doing this offset wall-on-footing design, then all you need for insulation is the exterior vertical board required for your AFI zone and sub-slab and slab-edge insulation to isolate the heated concrete from the ground and from the footing. No sub-footing insulation is necessary as long as you have a good capillary break on top.

  13. Dave | | #13

    Robert - thanks for your thoughts. The reason for the mulitable pours - the Wall of the building is a single pour pvc form with 2" of foam on the exterior but still in the pvc. This is for a Vet clinic. And a lot of the interior wall need a very durable exterior because dogs are equal to hogs when it comes to destroying things. These walls will be 14 to 21 (front of building) feet tall. The entire exterior wall will be done in one pour. So the footing have to be done before the walls are built and poured. As for the slab -- it gets pretty complex with all the dog runs having some slope to in floor trench drains. Multiple 12ft by 20ft rooms (separation of animals reduces barking). I'm consernered about your comment about the slab -- if it is place on the inside 4" of the footing and the area under the floor is not compressed really well -- then the slab will crack and settle at those edges --- that will not be good because that is right where the trench drains are!! -- thanks for your thoughts Dave

  14. Riversong | | #14


    Edge-supported slabs are commonly done and, with proper sub-slab crushed stone fill, sub-slab vapor barrier, sufficient steel reinforcement, appropriately-located control joints, expansion joints and bond-breakers at edges, are not problematic. I design my slabs to be both thermally and structurally isolated from the foundation because it reduces the potential for failure, but my strategies often vary from conventional approaches.

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