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EPS vs. XPS for Subslab Insulation

graineryarchco | Posted in General Questions on

2″ Rigid Insulation XPS vs EPS

Climate zone 6A
Installing 2″ of rigid foam under basement & crawl slab.
Two products are available in my neck of the woods.
1. Carlisle High Density InsulFoam 2″ EPS notes R8.8 (perhaps slightly less below 75 deg F) 25 PSI rating. $40 per sheet
2. Kingspan 2″ XPS notes R10, 25 PSI rating. $47 per sheet

Does anyone has experience with the InsulFoam? Cost savings and not as bad of blowing agents in production (assumed).
But slight loss of R value for the 2″ depth.

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  1. Expert Member
    Michael Maines | | #1

    XPS eventually ages to about R-4.2/in so the difference is not huge. There is a big difference in climate impact, even with the newer XPS formulation. I always use EPS below slabs with no issues. 25 psi compressive strength is the same either way. Effective R-value goes up as the temperature goes down for every insulation product except polyiso foam.

    The only downside of EPS is that it can be slightly harder to source.

    1. graineryarchco | | #2

      Thank you Michael.
      Spot on - slightly more work to get the EPS.

  2. Expert Member
    BILL WICHERS | | #3

    There is some indication that XPS performs better in below grade applications where it will be subject to very high moisture levels. I prefer XPS below grade for that reason. EPS can work too, but I'd put in some washed stone to help with drainage in that case, to keep the EPS from ever sitting in water.

    I wouldn't worry too much about the slight difference in R value between the two materials. As Michael pointed out, after aging they'll both end up about the same R value anyway.


  3. nrosdal | | #4

    gps seems like a better option than either of those in my opinion. That is what i did with my current build just a hair over 3" gave me r15 under the slab.

  4. jollygreenshortguy | | #5

    Carlisle's Insulfoam EPS Type IX actually increases in R-value as temperature drops. I understand XPS does the same. However polyiso does the reverse.
    I know you're not considering polyiso. I just thought I'd mention the point in case other people reading these comments are considering it.

    1. Expert Member
      BILL WICHERS | | #6

      Not all polyiso behaves exactly the same at low temperatures. It tends to stabilize down around EPS levels too.

      Polyiso isn't suitable for use underground though, so it shouldn't really be considered for this application regardless.


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