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Pink vs. blue XPS foam board under slab? …or EPS?

Eric Mikkelsen | Posted in General Questions on

Hi…is there a preference between pink and blue XPS foam board to be used under slab?

…and any drawbacks to using EPS in place of the XPS foam?


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  1. GBA Editor
    Martin Holladay | | #1

    Assuming that you are referring to Owens Corning Foamular when you say "pink foam board" and Dow Styrofoam when you say "blue foam board" -- no, there is no preference. Those are just two different brands of extruded polystyrene.

  2. Eric Mikkelsen | | #2

    Thanks...wasn't sure if one had a better track record for this application.

  3. Expert Member
    Dana Dorsett | | #3

    But "white" is the preferred color to use, since both pink & blue (and green and grey) XPS blown in N.Americal all uses high global-warming potential (GWP) blowing agents, either completely or predominantly HFC134a, at about 1400 x CO2 GWP.

    EPS (the stuff with the macroscopic beads, like disposable coffee cups and cheap coolers) is blown with pentane, with less than 1% the lifecycle GWP of the pink & blue stuff. Type-II (1.5lbs nominal density) has sufficient compressive strength for use under 3" and 4" slabs, but if you want to match the pink & blue stuff on a psi basis you can go with Type -IX (2lbs nominal density). In most cases it will be comparable to or cheaper too.

    And, after 50 years the blowing agents have largely left the pink & blue stuff, reducing it's R-value to about that of EPS of similar density anyway. The half-life of pentane in EPS is measured in days/weeks, so it's labeled R value on day-1 is going to be pretty much how it performs in 50 years, whereas the pink & blue stuff will have lost about 10% of it's total R value by then.

    White is usually right, at least as far as polystyrene insulation goes. It takes about 2.5" to have the same initial R as 2" of the pink & blue, but under a slab that's very easy to accommodate.

  4. Eric Mikkelsen | | #4

    Thanks Dana. Any draw-backs to using the EPS under slab besides increased foam thickness?

  5. Expert Member
    Dana Dorsett | | #5

    Well, EPS pretty gender-neutral, it doesn't have that pink-blue thing to refer to, could get confusing... is that a down side? :-)

    But beyond that, no, if anything EPS has BETTER long term performance in wet environments due to a larger fraction of fully closed cells. When fully submerged it takes on water quickly into the interstitial spaces between the macroscopic beads (which XPS doesn't have) , but it sheds water as quickly. Even when buried in saturated soil for decades and has reached a saturated 5-7% water the hit to thermal performance is in single-digit percentages. XPS takes water on more slowly but also gives it up more slowly. As long as you have reasonable drainage (say 4-6" of gravel) under the foam either will stay pretty dry, but if the water table rises for a few months, EPS will be back to normal within weeks of the tide going out, but it could be months or years for XPS.

    Most PassivHaus / PassiveHouse designs use copious amounts of EPS under slabs, even under the footings for many. It's a tried & true insulation material with a long track record under critical roadways and airport runways too, anywhere frost-heave issues need to be well controlled. It's the right stuff for under slabs- I'm not quite sure how XPS became the standard slab insulation in US construction, but there's no real point to it- EPS does the job as well as anything.

  6. Eric Mikkelsen | | #6

    Thanks Dana...what about foundation perimeter insulation?

  7. Eric Mikkelsen | | #7 the type IX 25 psi foam?

  8. Pete Marthaler - Zone 7 | | #8

    Eric, I suggest if you proceed with using EPS below slab, make sure that you provide a capillary break (ie clean stone) between the subgrade soils and the foam to prevent the foam from absorbing bulk water that it could come in contact with if not for the clean stone and proper footing drains.
    As Dana alludes to in his post, EPS absorbs more water than XPS due to the void space in the cell structure.

  9. Eric Mikkelsen | | #9

    Thanks Pete, yes...i will have 4" of clean washed stone under the foam, also facilitating a passive radon mitigation system. Are there any concerns with using the EPS for the exterior perimeter of the foundation?

  10. Expert Member
    Dana Dorsett | | #10

    Type IX is typically 25psi MIN, and can be specified at somewhat higher PSI when needed (but you'd pay more.) A typical vendors typ-numbers looks like this:

    Almost all insulated concrete forms are made of Type-II EPS, which rather tells you the appropriateness for perimeter insutation. It's the right stuff.

    Pete: EPS doesn't actually absorb more water than XPS, only more quickly during the initial wetting. But it also is rid of it far more quickly. The micro-cell structures have higher fraction of broken cells in XPS- it's the macroscopic bead structures that hold most of the water in saturated EPS. Clean stone is a prerequisite capillary break under ANY slab (even uninsulated slabs).

  11. Eric Mikkelsen | | #11

    Thanks again you know if the borate treated EPS for termites adds much cost? ...or if it is good for carpenter ants as well?

  12. Expert Member
    Dana Dorsett | | #12

    I've never priced the borate loaded goods, can't say what the upcharge would be.

    Borates work for all wood-boring insects, including wasps and carpenter ants. The mechanism for killing the insect is that the borates kill off the gut flora the host insect needs for breaking down the cellulosic fiber of the wood, eventually starving the insects. I'm not sure about termites, but ants & wasps cannibalize their dead & weak, which is a handy vector for distributing the borates to other members of the nest.

  13. Eric Mikkelsen | | #13


  14. Eric Mikkelsen | | #14

    Will simply tunneling through a borate treated foam board kill off that gut flora for carpenter ants?

  15. Eric Mikkelsen | | #15

    Hi Dana, what are your thoughts on this piece of info:

  16. Eric Mikkelsen | | #16

    ...there was supposed to be an attachment...trying again...

  17. Jon R | | #17

    Apparently most of the sources providing data on XPS vs EPS underground are biased and pick conditions that favor their product. For example, one test buried it for 15 years (good) and then let it dry for a month (very deceiving) before testing. Others use short term lab tests (where XPS does better).

    I'd like to think that the 2018 IRC R403.3(1) isn't biased or based on unrealistic conditions. Figures are as shown in the above post:

    Type II EPS horizontal - R2.6
    XPS horizontal - R4.0

    This is a significant difference. It's worth knowing how they will perform in other conditions (basement wall - interior and exterior, basement slab - interior and exterior, etc).

  18. Trevor Lambert | | #18

    I discovered after the fact that the insulated raft under our 8" slab appears to be type 1, based on a sample I weighed and calculated the density. Other than compressing more than 10%, are there other concerns?

    1. Trevor Lambert | | #19

      And on the subject of off gassing, I just finished building a heat exchanger box for my ground source preconditioning loop. I insulated it with 1/2" XPS. Given that the fresh air for the house is coming through this, was this a big mistake? Or will the small surface area make it negligible? Is painting the XPS a good solution? I'd rather not tear it apart and shoehorn 3/4" EPS, but will if I should.

      1. Expert Member
        Zephyr7 | | #20

        I doubt you’ll have any issues from any offgassing. The blowing agents gradually leave the foam very, very slowly, which helps to keep total concentrations in the surrounding air very low. Between the very slow offgassing, and the constant air movement in your space, total concentrations within the structure are probably going to be so low as to be immeasurable.


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