GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

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?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  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

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

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |