XPS Or EPS For Below-grade Use? - Insulation - DIY Chatroom Home Improvement Forum

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Old 05-28-2014, 10:00 AM   #1
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XPS or EPS for below-grade use?

I am preparing to start my footing insulation project but keep going back and forth about XPS vs EPS. A lot of people have told me to use XPS, but many sources seems to claim that EPS is more water-resistant and keeps more of its R-value over time when buried.

For example: http://www.concreteconstruction.net/...ulation_o.aspx

Either way, the foam boards will be touching sand, not dirt, and there will be drainage.


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Old 05-28-2014, 07:04 PM   #2
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Pros and cons in that article.

I have always seen XPS recommended below concrete. If you also check out buildingscience, they always say to use xps.


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Old 05-28-2014, 07:09 PM   #3
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This won't be below concrete; it will be along the side of it. And that article doesn't really discuss the pros or cons of XPS vs EPS for below-grade use. I too have seen XPS discussed for below-grade use over at building science, but haven't found the reason. The study I linked to showed that XPS was more resistant to moisture dumps, but had more difficulty shedding what water it did absorb, while EPS was more permeable for both moisture accumulation and shedding.

Of course the R-Tech EPS panels I'd be using are all poly-faced and the open edges would be covered with more poly sheeting, so maybe it's moot.
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Old 05-28-2014, 07:29 PM   #4
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That article used percentages to make EPS better than XPS. But they were only for idealized tests in lab. Note that REAL numbers were not shown and plastic protective "coating" is not proven. - Looks like a typical article posted in very close proximity to the ad for the product paid for by the prodcer of the "coated" product.

It may be OK for some applications, especially if you are into the "hair-splitting" R-value battle like the pink panther chooses to do and not get involved with the real world actuality.

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Old 05-28-2014, 07:47 PM   #5
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No, it was a real-world study: They investigated real footing insulations that had been in the ground for 15 years. Now, yeah, it's true that the study was done by ACH Foam, an EPS manufacturer. So it's possible this test was flawed, and it's possible that ACH is playing games with the numbers. But I'm inclined not to believe so, especially because [does some quick googling] in 2012, ORNL did basically the same test with XPS and largely replicated the results of moisture-laden foam exhibiting degraded performance, especially in the slab-on-grade foundation with poor drainage. They found that in this application, it got soaked and lost 44% of its R-value until they had dried it out, as it was unable to dry by itself.

I don't have a dog in the XPS vs EPS fight; I just want a cost-effective insulation foam that doesn't perform worse over time. Perhaps the real lesson I should be taking home is the importance of good foundation drainage to render the whole thing moot. In that case, I would be inclined to go with EPS simply because I can afford more of it!
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Old 06-01-2014, 11:51 AM   #6
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I have always understood poly skinned XPS to be best but this doesn't seem to echo that assertion. Probably worth looking into.
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Old 06-01-2014, 09:53 PM   #7
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