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Community and Q&A

Sub-Slab Insulation Density

thrifttrust | Posted in Green Products and Materials on

In another post I calculated the dead load of a four inch concrete slab to be .35 PSI. Why do I always seem to see foam of 25 PSI or greater recommended? While the price differential between 15 and 25 PSI XPS isn’t that great, 25 PSI EPS is more than double the cost of garden variety 10 PSI. I realize that the ratings are for an unacceptable 10% deformation but still, 25 PSI seems overkill.

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  1. Expert Member
    Michael Maines | | #1

    Foam is rated at 10% deformation at maximum load, as you note, per ASTM C518. If you have 1" of sub-slab insulation that may be ok, but if you have 4-6", or more, it starts to add up quickly.

    Perhaps more importantly, that number does not take into account long-term creep, which is hard to define, but one company, Foam-Control, recommends a 3:1 safety factor to guard against creep.

    I've had structural engineers look at unconventional foundation details many times and they are always uncomfortable with any structural loads on foam; no matter what I spec they seem to push it up a notch. Understandable, since their license is on the line.

    That said, 15 psi is very likely safe, and what I usually specify, and 10 psi might even work. But if the project is going to need an engineer's stamp I brace myself for the upgrade to 25 psi.

    That's all for slabs with no loads on them. When there is going to be a structural load, the same concepts apply, but 25 to 40 psi foam is recommended, depending on the situation--mainly, how big is the load, how well is it distributed and what is the risk of the concrete failing.

  2. ranson | | #2

    Engineers are requiring the foam to have at least the same compressive strength as mediocre, soft soil. ASTM D6817 says that EPS29 (Same density as Type IX but made for load bearing use) can withstand 10.9psi with no more than 1% deformation. That's slightly more than 1500psf, the lowest soil bearing capacity that still has prescriptive footings in the IRC.


  3. thrifttrust | | #3

    Ah, so in residential applications the compressive strength of the foam has less to do with the load placed upon it than mimicing the strength of build-able soil?

    John, you state that the IRC specifies a minimum value for footings. Is there a different value for slabs that need carry only their own weight plus the live load?

    Anyway, I thank you guys for your insight. I'm gearing up for a fight over my desire for insulated footings in my project. I see a lot of insulated monolithic slab foundations on the interweb but few with insulated basement footings.

    Douglas Higden

  4. GBA Editor
    Martin Holladay | | #4

    Thanks to everyone who has contributed to this informative thread. Good stuff!

    I have incorporated several comments from this thread into an updated version of my 2010 article, Foam Under Footings.

  5. Jon_R | | #5

    There are procedures for calculating foam under concrete safe loads. For example:

  6. Expert Member
    Michael Maines | | #6

    Douglas, it's hard to know which came first, but it is true that the worst soil type listed in the IRC codes--clay, silty clay, etc., have a "presumptive load-bearing value" of 1500 psf, which of course works out to 10.9 psi. Engineers don't like to built on that kind of soil any more than they like to use low-density foam. Sand, silty gravel, etc. has a presumptive load-bearing pressure of 2000 psf, and soil that builders and engineers like to see, sandy gravel and/or gravel, is listed at 3000 psf, or 20.8 psi.

    I often use the analogy that type 2 EPS is like building on sand, and type 9 EPS is like building on gravel. Builders know that sand is ok under a slab and gravel is preferred under a footing. The code does not state what the expected deformation of soil is at the presumptive (their word) load-bearing pressure, but maybe it appears in references elsewhere.

  7. thrifttrust | | #7

    Great article Martin, and thanks for the Owens Corning link Jon. Michael, your soil bearing information seems to apply directly to my situation. I haven't had the soil on my site tested, but I owned a home a mile away and it was on solid clay, (or maybe not so solid ;). My plan for dealing with such poor draining soil is to build on a bed of compacted gravel that drains to a sump. It is my understanding that gravel is necessary under an insulated footing anyway, if only to provide a level bed for the foam to rest upon. I can't imagine how one could get undisturbed soil flat enough. My question is, does adding a gravel bed improve the soil bearing capacity of clay, and If so, how thick must the bed be to achieve a benefit?

  8. Expert Member
    Michael Maines | | #8

    Douglas, true, I spec a 4-8" bed of clean, angular crushed stone, compacted to refusal with a vibratory plate, below foam under footings. In reality it probably does increase the bearing capacity of the soil but that's beyond my knowledge. If you have clay it's always a good idea to involve a soils engineer, in case the soil is expansive. Some structural engineers can do a soils analysis as well.

  9. Expert Member


    One thing I have noticed working on old buildings is that almost invariably they are not level because there has been some substance in the foundation over time. Designing footings with some amount of redundancy seems to me to be a good idea.

  10. Jon_Lawrence | | #10

    The poured concrete foundations around here typically have at least 4" inches of compacted stone to create a level and dry base, and I would presume a base with a higher compressive strength. The attached picture shows a foundation I did last spring. There is a a decent amount of clay around here so if you get some rain before a pour and don't have stone, it becomes a muddy mess. The slab area is also filled with stone up to the top of the footings and then compacted.

    I thought about insulating the footings and the outside of the foundation walls on this job and I am glad I did not as we had a rainy spring and it would have delayed things. I have plenty of time to insulate from the inside.

    So while I did not use foam under the footings, I did use foam under spot footings and where the concrete columns attach to the top of foundation walls to create thermal breaks. In the latter case, I used 1000 psi columns bearing blocks between the foundation wall and the base plates. For the spot footings, I used Foamular 600 - I am trying to avoid XPS as much as possible and only needed a few sheets for this application. The maximum point load on these spot footings in 16,000 lbs and with a 3'x3' footings, that works out to around 12 psi which is below the 20 psi maximum dead load recommended by Formular in the link Jon provided. I could have gotten locally sourced 3.0 lbs density EPS, but I wanted to make sure I had a product that was clearly labeled so I could prove to the local inspector that the compressive strength was more than adequate. Even at 10% compression on a 2" piece of foam we are talking less than an 1/8 of compression.

    For under the slab I am using 1.5lbs density EPS sheets, 2 layers of 2".

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