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Community and Q&A

Slab insulation using EPS

Chris Barnes | Posted in Green Building Techniques on


I am in the process of designing a new home construction project. Naturally, I am interested in minimizing costs and maximizing insulation/efficiency. The home is slated to be a ranch-style that is slab-on-grade. I have been planning on using high-density EPS foam (6″ thickness) underneath the slab, and 6″ foam on the exterior of the slab and footings. The EPS foam would also go underneath the footing as well so that there is a continuous layer of insulation.

I was recently reading that the majority of the energy is lost through the foundation’s exterior walls, so I am questioning the cost effectiveness and performance gain by doing a continuous layer of insulation. In other words, it seems that there is a higher cost versus efficiency gain using this scheme.

An alternative approach would be to insulate the exterior perimiter of the foundation down to the footer, and then install EPS foam over the slab. This would reduce the complexity of the foundation so there is nothing particular interesting going on that would scare (increase price) the foundation contractor (i.e., it would be a conventional foundation). We would, of course, need to increase the ceiling height to compensate for the thickness of the foam. The disadvantage that I see here is that the insulation is not continuous around the foundation footing so there would be some heat loss through the foundation wall.

In case this is relevantm we intend to insulate the walls to around R-35, and the cathedral ceilings to around R-45.

I would like to hear other’s thoughts on these two approaches.



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  1. GBA Editor
    Martin Holladay | | #1

    Where do you live? Climate matters.

  2. Chris Barnes | | #2

    This is in climate zone 4C. Sorry about that. :)

  3. GBA Editor
    Martin Holladay | | #3

    In a mild climate like yours, there is no doubt that sub-slab insulation has a longer payback period than it would in a colder climate. The length of the payback depends on how much you pay for fuel, as well as your estimates of future fuel price inflation.

    If you want a horizontal layer of insulation for your slab floor, it belongs under the slab, not over the slab. I find it hard to believe that there are any concrete contractors in the U.S. who haven't already learned how to place concrete over a horizontal layer of rigid foam. Is the technique that unusual in your area?

  4. Expert Member
    Dana Dorsett | | #4

    In a house with R35 whole-wall (all thermal bridging accounted for) there's little to no rationale for sub-slab R greater than R10 unless the slab is the heating radiator (radiant floors), in which case there's an argument for maybe R15. See Table 2, P 10:

    (Read the whole first chapter.)

    With R10 under the slab and R35 walls/R45 roof, with careful window selection it would be pretty easy to hit Net Zero Energy with a PV array that fits comfortably on the roof in zone 4C.

    There is also no rationale for using Type-IX (2lb density) or higher except under footings, and any foam under the footings and the footing design would have to be specified by an engineer. Under the slab Type-II (1.5lbs density) has sufficient compressive capacity to handle parking your Caterpiller D9 dozer on a 4" concrete slab- no matter how hard you're dancin' on top of the clothes washer on "spin" cycle you'll never see pressures that high in a residential slab.

    What's cheaper/easier and makes more sense in a zone 4 climate at your R-values it to use minimalist 2.5" + 2.5" ICFs (almost all are made with Type-II EPS) for the stem-walls, leaving the footing uninsulated, and 2.5-3" of Type-II EPS under the slab. If your design frost depths are shallow enough that the footing is essentially a grade-beam (no stem wall, or a stem-wall shorter than 12") then maybe 2.5-3" of higher density foam under the footing makes sense.

  5. Chris Barnes | | #5

    Great information, thanks for your thoughts!

    The county specifies footings that are 12" deep from grade to the bottom of the footing, and 6" clearance to the siding, so a total of 18" of footing+stem wall or a grade-beam. So I think I would fit in your condition of putting a layer of EPS underneath the grade-beam. Are you suggesting that because the bottom of the footer is more influenced by temperature fluctuations?

    We do not intend to heat the slab.

    Do you have any thoughts on a monolithic slab versus a grade-beam or stem wall method? It seems that the monolithic slab would require slightly less foam overall, and could potentially reduce the labor required. Though the ICF method seems to be the lead labor, but has a higher material expense.



  6. Expert Member
    Dana Dorsett | | #6

    To deal with the thermal bridge at the fairly shallow grade-beam/stemwall you can either use ~3" of higher-density EPS under the footings as you had planned, making the footings wide enough to distribute the weight, or pour the footings on compacted soil and add some "wing" insulation on the exterior similar to frost-protected foundation methods.

    If you have the house pretty much designed it's not a major engineering project to specify the footing widths and EPS density for the sub-footing insulation, but it's something that is best designed rather than field-hacked, because if you don't get it right the first time fixing it would be WAY more expensive than a few hours of certified professional-engineer time, and with the PE qualified & stamped detail drawings you won't have the schedule held up by inspectors on the variance either.

  7. Jin Kazama | | #7

    If you consider ICF walls, make sure you know what type of soils your building one.
    ICF walls are heavy, footings need to be large and engineered correctly..could bring in further expenses.

    Is the floor slab required to be attached to the footings in your code?

    You could also dig 12" deeper and separate the floor slab from the footings completely if allowed
    ( either with ICF or a small section of 1-2" foam )

    As Dana mentionned, your climate is the cheapest to build efficiently ..2-4" of EPS shold be more than enough under the slab unless you have specific energy goals.

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