EPS foam density for use under concrete slabs and around stem wall
Hi everyone,
I’m right at the point where I need to purchase some EPS foam for our slab foundation / stem wall on a residential net zero project. I’ve read and posted on the project in the past and there seems to be a consensus that Type II EPS foam should be fine from a compressive strength perspective in both applications.
The question is that the manufacturer is recommending Type IX due it’s more impressive compressive strength attributes, does his suggestion have merit over Type II for residential foundation work? I figured I would verify his recommendation by asking folks who have used EPS in this sort of application in the real world who don’t have sales bias 🙂
Our foundation is a 4″ slab with 6″ thick stem wall, 3″ inches of rigid EPS under the slab and another 3 inches on the outside of the exterior of the stem wall. Example: http://foundationhandbook.ornl.gov/handbook/images/Chapter%204/4-11_no-cap.png
The Type I EPS is rated at a compressive strength of 15-21 PSI, the Type IX is rated at 25-33 PSI. The manufuacturer I’m looking at is Insulfoam, data sheet is here: http://insulfoam.com/wp-content/uploads/2014/04/13001-Below-Grade-EPS-Overview-TDS-WEB-8-16.pdf
Any advice is much appreciated.
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Replies
Joshua, for most situations, type 2 if fine for sub-slab use. Its compressive strength is similar to sandy soil. A 4" slab contributes about 50 psf of load; add a 40lb live load and you're still below 1 pound per square inch. The compression curve for foam plastic is usually pretty straight, so insulation rated at 15 psi would compress somewhere in the neighborhood of 0.015 of its thickness. For 3" foam, that's less than 1/16". Not enough to worry about.
Type 9 is better for areas that support roof upper floor loads, such as below footings.
Michael,
Thanks for your very helpful comment. I've decided to edit my article, “Choosing Rigid Foam,” to include your advice.
-- Martin Holladay
Joshua: Remember to look for reclaimed foam.
For almost all residential slab applications Type-II EPS is fine. But under footings or load bearing posts it may need to be something denser. EPS insulated concrete forms (ICF) are all Type-II, and it is sometimes quicker / easier / cheaper to pour the stemwalls in ICFs, unless you find a source for used roofing EPS, which is almost always Type-Il, to enable a "walkable" membrane roof. Type-I is too easy damager or to leave permanent boot tracks in, even though it's still strong enough once the slab is installed. With Type-II you can usually walk on the foam prior to the pour without screwing it up (unless you are a 300lb guy carrying 150lbs of rebar on one shoulder.)
Reclaimed EPS from commercial building re-roofing/demolition is fairly ubiquitous and cheap, typically 1/4-1/3 the cost of virgin stock goods from salvage outfits and foam reclaimers. It's out there, you just have to search a little. Try searching your local craigslist for: rigid insulation, eg:
https://boston.craigslist.org/search/sss?query=rigid+insulation
https://denver.craigslist.org/search/sss?query=rigid+insulation
https://sacramento.craigslist.org/search/sss?query=rigid+insulation
Dana, you mention using reclaimed foam. How about factory seconds where the boards are not of uniform density? Some areas are very firm while others are spongey. Will those areas compress immediately as the slab is poured or will it result in later cracking from uneven settling?
For higher loads, see here:
http://www2.owenscorning.com/worldwide/admin/tempupload/pdf.077_HighDensity_E_1.pdf
Thank you for all the helpful information. I've looked around for reclaimed Type II and haven't found any in the Washington State area. Anyone have a good source?
Washington State covers a lot of area- can you be a bit more specific? (If you live in Olympia you don't exactly want to be shipping it from Spokane, but Portand or Bellingham wouldn't be out of the question.)
Nationwide Foam ships anywhere (for a price based on distance from the depot), and has a depot within a day's drive of any WA location.
http://nationwidefoam.com/
We are building in Yakima WA.