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Community and Q&A

Using EPS for sub-slab insulation

J Chesnut | Posted in Green Building Techniques on

My understanding is that XPS has better ‘water repellent’ properties than EPS so is superior for sub basement slab insulation. I would prefer to spec EPS for its lower Global Warming Potential and I know there are several densities for EPS.
Do I need to spec a certain type of EPS for below grade applications?

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  1. Riversong | | #1

    This used to be the prevailing wisdom, and I still tend to subscribe to it. But some studies have shown little water absorption and no freeze-thaw deterioration from subgrade EPS after many years. I would certainly use a higher-density board if it's available, which would have both a higher compressive strength and less air and water permeability.

    If you're considering using it sub-slab - or any other structural application - then it should be at least 25 psi foam.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Plenty of people are successfully using EPS under slabs. One popular type for this application is Type 9 EPS with a density of 2 pounds per cubic foot and an R-value of 5 per inch.

  3. Tony Olaivar | | #3

    To my knowledge, the Greenhouse Gas impacts of Expanded Polystyrene (EPS) is far more serious than the impacts of Extruded Polystyrene (XPS). Durability shouldn't be a problem if the material is completely buried and good site drainage is provided. These are necesseties with both types of foam. Water intrusion is usually a much more serious issue as it pertains to wall assemblies. Generally, I would reccomend XPS for this application.

  4. Riversong | | #4

    According to Alex Wade, including both the plastic and the blowing agents, XPS has 50 times the lifetime Global Warming Potential of EPS.

  5. J Chesnut | | #5

    Thanks for the quick feedback.

    Tony according to an article Alex Wilson wrote for the June 1, 2010 publication of Environmental Building News titled "Avoiding the Global Warming Impacts of Insulation" XPS and ccSPF stand out as having a greater impact compared to others insulation options because of the blowing agent (HFCs) currently in use.

    This won't help me for this project but I am also wondering if rigid board mineral wool insulation is a good option of sub slab (not sub footing) insulation.

  6. J Chesnut | | #6

    Robert beat me to the draw. I'm referencing the same article.
    Robert thanks for providing the link.

  7. mike eliason | | #7


    type IX EPS has been used on a few PH projects in the NW.

  8. Riversong | | #8

    As far as I'm aware, no semi-rigid mineral wool insulation is rated for sublab installation, but I don't see why it wouldn't work on a floating slab that had no floor drain or other plumbing penetrations that might be disturbed by slight settling (though I expect all settling will occur during the pour and not after).

    It would require a vapor barrier membrane on top of the mineral wool to prevent cement infiltration.

    A 4" slab weighs about 50 psf dead load, and with even 40 psf live load amounts to only 0.625 psi (Roxul drainboard has a compressive strength of 2.3 psi at 10% deformation and an R-value of 4.3/inch).

  9. User avatar
    John Semmelhack | | #9

    Here are the minimum specs for the various types as published by the EPS Molders Association.

  10. John Klingel | | #10

    A gal at Passivhaus USA, told me that the water absorption of EPS "proved to be a myth". One super-insulated builder up here has been using 25 psi EPS under Shallow Frost Protected Foundations (24" down) for several years now, and I THINK 15 psi under the slab, with "no problems". Is not the exposure to freeze-thaw cycles the bane of any sub-grade foam? If you can avoid that, then all should be good, no?

  11. TJ Elder | | #11

    Roxul's roofing insulation has higher compressive strength, 10-12 psi. It also has an optional facing that should keep out cement paste. The green cred may be compromised by embodied energy, because the material density is much higher than other mineral wool panels. R-value is also lower.

  12. Daniel Ernst | | #12


    David Pill used 4" EPS boards underneath his basement slab, although I couldn't find any mention about board density. See the third picture in this link:

  13. JOE MARTIN | | #13

    We've found in the South where we have the occasional termite that the Performguard EPS (Expanded PS) with Borates from ACH is our best bet for under slab and in ground applications. The 1 lb. density is 15 PSI and the 2 lb. weighs in at 25 PSI. My understanding is that moisture retention is not a problem with this product. If I'm reading Alex WIlson's article correctly regarding the global warming potential of the Expanded Polystyrene within reason.

    Martin - What is Type 9 EPS?

  14. JOE MARTIN | | #14

    My understanding (from a recent conversation with the ACH rep) is that the ASTM C272 test for water retention is a 24 hour test, and therefore misrepresents their expanded foam product. Apparently the extruded foam boards will hold more water over a long period of time with a significant loss in R-value. Expanded polystryrene is more permeable, and drys faster. I like to use the Performguard 2 lb.density / 25 PSI over a vapor barrier under a slab, especially with a radiant slab.

  15. Aj Builder, Upstate NY Zone 6a | | #15

    We use blue billets of extruded for floating docks.

    They do no soak up water.

    We built closed cell spray foam docks and they were fully water logged after 20 years.

    If you use insulation under ground, get your drainage details right. Then use expanded foam with confidence.

    That said I look forward to non fossil based solutions to in ground insulation.

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Type IX EPS (or Type 9) has a minimum density of 1.8 lbs. per cubic foot and a minimum R-value of 4.2 per inch. Many distributors sell Type IX EPS with a higher density and R-value; I'm familiar with 2.0 density EPS with an R-value of 5 per inch.

    More information here:

  17. Riversong | | #17

    Water absorption by EPS is not a "myth", but the ASTM C 272 test involves 24 hour soaking.

    In actual sub-grade conditions with well-drained soils, actual absorption is much less.

    Type IX EPS is 1.8 pcf EPS with 25 psi compressive strength and R-4.2/inch.

    Standard (type II) EPS has 30 times the water absorption of standard (25 psi) XPS.

    Here's a comparison of vapor permeance and water absorption, by standard ASTM tests:

    Foamular 150 (15 psi) XPS
    Water Absorption (% by volume max): 0.10
    Water Vapor Permeance (perm max): 1.1

    Type II EPS (15 psi)
    Water Absorption (% by volume max): 3.0
    Water Vapor Permeance (perm max): 3.5

    Type IX EPS (25 psi)
    Water Absorption (% by volume max): 2.0
    Water Vapor Permeance (perm max): 2.0

  18. Riversong | | #18


    According to the chart you linked to (and any such specifications I've seen) EPS - even at 40 psi - does not offer more than R-4.2/inch.

  19. User avatar GBA Editor
    Martin Holladay | | #19

    Thanks for the correction. You're right.

    I just called Branch River Plastics for a clarification. The rep there, Kelly Smith, had told me that their Type IX EPS has an R-value of 5 per inch. After I called again, she admitted that the quoted R-value only occurs at 25°F -- in other words, the R-value was not tested according to the usual protocol required by the Federal R-value Rule, and is therefore not useful for comparing products.

  20. Aj Builder, Upstate NY Zone 6a | | #20

    At 25 degrees? Lacking way too much information to know what that means.

    Delta t, one side at x and one side at y.

    What is delta t?
    What is x?
    What is y?
    What is 25 degrees f referring to?

  21. Riversong | | #21

    Polystyrene rigid foam boards have the inverse quality of fiberglass (which diminishes in R-value as it gets colder).

    Here is an ASTM chart of EPS and XPS specifications:

  22. User avatar GBA Editor
    Martin Holladay | | #22

    AJ Builder,
    These are ASTM C 518 results. The convention for reporting ASTM C 518 results is to report the mean temperature.

    According to the Federal R-Value Rule, "The tests must be done at a mean temperature of 75 deg. Fahrenheit." The test that Branch River Plastics reported was performed at a mean temperature of 25 degrees.

  23. Anonymous | | #23

    We install only eps for sub grade insul. Whether low density or high density depending on application and have never had complaints... from clients.

    Here's what I found awhile back. Testing was done @ 72*

  24. Riversong | | #24

    Unfortunately, information offered by anonymous posters cannot be considered credible.

    That "study" was based on a single anecdotal incident of a non-identified EPS and XPS that was dug up from an old building. It has been used by EPS manufacturers in the same way that a single Dow test on freeze-thaw resistance was used by them to "prove" the advantages of XPS.

  25. Anonymous | | #25

    Before you jump on your horse sir Robert, might you look throught the site.
    Might I point out that the study was done by a third party. Water expanded or CHEMICALY extruded ; wondering which sounds more earthfriendly or green if you may.

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