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Community and Q&A

Under Slab Foam Strength

Mgwinter | Posted in Green Building Techniques on


I am in the process of getting some plans together for an addition to a 1860’s New England Colonial House that will be slab on grade construction. I have been reading about under slab foam and am wondering if anyone has been concerned about exceeding the weight load of the foam as to cause the slab to fail.

I have gotten one quote so far from a company called “Universal Foam Products” and a product called “Geofoam” that they have available in several different densities ranging from 12 to 46 PSI.

Is there a standard density that people use for 6 inch thick or 12 inch thick under slab foam (I am still calculating what thickness I can afford).

Thank you, this is my first post, and I hope it confirms to forum rules and expectations of posters,


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  1. Bigrig | | #1
  2. Expert Member
    Dana Dorsett | | #2

    Unless you're installing it under a footing and it needs to support the weight of the house over just the footprint of the footing, 12-15psi EPS is more than sufficient for a residential slab. With a 4" slab you can even park your 5 yard dump-truck on it without cracking the slab.

    Type-II (1.5lb nominal density, 1.3lb min) is the most common density used for slab applications. Going as high as 12" (R50) would be overkill for almost any application, unless you were going for PassiveHouse certification.

    In US climate zones 5-7 anything more than 3" (R12.5) might end up saving less energy than spending the money for inch 4 (or higher) on rooftop photovoltaic solar would produce over any reasonable lifecycle for either. In US climate zones 2-4 that may be true for anything over 2".

    In central Alaska or the Yukon territory there may be a rationale for 5", but 6" might still be a stretch.

    So, it kind of depends on your goals here. If this New England Colonial is in fact IN New England, 4" makes sense if it's a radiant slab, otherwise 3" is plenty. See table 2, p10 of this document:

    IRC 2012 code-min would be even less, but putting at least 1.5- 2" under the slab does a world of good for keeping the slab temps above the summertime dew point in summer, reducing the risk of mold growing on the bottom of your rugs.

  3. STEPHEN SHEEHY | | #3

    Miles. You can get reclaimed foam from Green Insulation group in Worcester MA. I purchased 4" and 2" XPS from them several months ago. They shipped to me in Maine, so I'm sure they can ship to you. The price was great and the product was fine, just dirty, which obviously doesn't matter.

  4. Mgwinter | | #4

    Nathan thanks for this link, this is great information. I see my worry was misplaced.


    Thank you for your very informative and thoughtful answer. One of the reasons I was concerned was that I was planning on constructing one of the interior walls out of a insulated concrete form with a 5.75 inch or thicker core of concrete. I also have a large recording mixer that weighs 1500 lbs, but I see from your post that I should easily be covered with the lowest psi foam that I mentioned.

    The house is in Southern Connecticut as a matter of fact, and as interested as I am in PassiveHouse construction I’m not sure the added upfront expense is in the budget. Radiant Slab is interesting as well and I would love to do it, but being that I am building this more or less by myself as a first time builder, I don’t think it is something I am able to take on with any confidence.

    I think I will take your advice and go with the lower amount of foam of around 3-4 inches. Is there a specific product or vendor you recommend for this application?

  5. propeller | | #5

    Here's a product that may interest you.

    I wish I would have known this product before we build a super insulated slab last fall...

    This article help to better understand and sell to other on your project the bearing capacity of type II EPS foam.

  6. Expert Member
    Dana Dorsett | | #6

    As Stephen Sheehy points out there are several vendors of reclaimed foam in MA/RI/CT. It's definitely the right way to go. The cost is typically 1/4-1/3 that of virgin stock goods. That means you can install 9-12" for about the same material cost of 3" of virgin stock EPS. In addition to Green Insulation group, the other larger outfit is in Nationwide Foam in Framingham MA: But there are some smaller-time operators out there too- search the materials section of your local craigslist for "rigid insulation" or "foam insulation", since many of them use that as a free/low-cost advertising channel.

    For under-slab applications, stick with EPS & XPS, but NOT polyiso, which is prone to wicking up water, losing a large fraction of it's insulative properties when saturated.

  7. Mgwinter | | #7

    Thanks everyone, great resources here. These reclaimed foam options are fantastic, I will certainly be purchasing the under slab foam from one of these vendors. All this help is much appreciated, I didn’t realize this is such a supportive community here - Thank you!

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