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Community and Q&A

XPS vs EPS — Below-grade slab insulation

Peter L | Posted in Green Products and Materials on

Home Depot now sells pink XPS from Owens Corning. A 10 foot sheet by 2 feet wide was only $10 per sheet. The R-Value was R-5 per inch of foam.

I like the way XPS is easy to score and break without having EPS snow everywhere. The XPS also carries a higher R-Value per inch.

Which is better to use below grade under a slab? Which can carry the higher concrete loads of a slab?

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  1. User avatar
    Stephen Sheehy | | #1

    Both are fine. Buy recycled.

  2. User avatar Expert Member
    Dana Dorsett | | #2

    The R5/inch eventually drops to R4.2/inch for 1.5lb density XPS (Foamular 150), as it's HFC blowing agents leak out to do their climate damage. At 20 years it can't be counted on for even R4.5/inch despite the "lifetime warranty" of always retaining at least 90% of the labeled R. (It's a warranty that would rarely be taken, since the cost of sampling and re-testing, and the cost of re-installing would make collecting on it prohibitive.)

    At 1.5lbs density "Type-II" EPS, is also R4.2, and does not rely on blowing agents for it's performance- it 's performance on day 20,000 will be pretty much what is was on day 1.

    Cost-wise Type-II EPS is typically 20-25% cheaper per labeled R than XPS (and it's a stable R over time.)

    XPS has a somewhat higher compression rating than EPS, but they are both several times higher than necessary for supporting a slab. (But neither are good enough for going under a footing that is supporting the whole house unless the footing has been engineered for it.)

    Type-I EPS (1lb per cubic foot nominal density) is fine for below grade walls, but can take on enough moisture under slabs to take a performance hit under super-wet conditions. It's usually labeled ~R3.9/inch.

    Cutting EPS with a heated 4" putty knife with (with the side sharpened) makes quick work of up to 1.5" material (R6-ish). Some people rig up heated wire for doing artsy-craftsy complex shape cutting of polystyrene, but for straight cuts heating a putty knife blade works fine.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Stephen and Dana gave you good advice. For more information, see this article: Choosing Rigid Foam.

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