Dow Gray Board XPS for Sub-Slab Insulation
Michael817
| Posted in Green Products and Materials on
I am trying to find if anyone knows if this used Dow Gray Board XPS insulation is suitable for sub slab installation, or possibly what year it was in production. My engineer says I need 3000psf slab, for sandy soils in CZ6, so I am concerned if this is only 1500psi it may have a high deformation rate.
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1500 PSI = 216,000 PSF > 3,000 PSF.
3000 PSF = 21 psi.
I can't read any numbers in that photo. Maybe a photo zoomed in on those details would give someone enough information to figure out what the rating on this is.
Charlie, there isn't any reference to compressive strength on the board anywhere. It is old and used. I used the wayback machine to try to find any articles, and I've asked a few contractors with no success. I was hoping someone would remember what year it was last in production, and possibly where it was previously specd.
I'm not suggesting that the compressive strength would printed on it. I'm just saying that we'd have a better chance of figuring something out about it with the ability to read what *is* printed on it.
Are you sure it's XPS, and not polyisocyanurate? The stuff in the picture looks like a type of fiberglass reinforced facer found on 2lbs density roofing polyiso. Dow XPS has the trademark "Styrofoam" printed on it somewhere, usually in bold letters as large or larger as the printed "Dow". I don't see anything that might be "Styrofoam" in the image.
Compressive strength ratings rarely matter under residential slabs, since it's only supporting the slab and contents of the home, not the structure of the house itself. Type-II XPS (1.5lbs per cubic foot nominal density) is usually rated at 25 psi (= 3600 psf, >3000 psf) but that's usually overkill even for sandy soils, unless the slab is <3" thick &/or not reinforced. Most 2lbs roofing polyiso is rated for 20psi ( 2880 psf) which is pretty close to the 3000 psf specified by the engineer.
But WATER ABSORPTION matters (quite a bit!) in sub-slab applications. Polyisocyanurate is NOT suitable under slabs due to it's hygroscopic characteristics.
It is listed as XPS by the seller here https://www.repurposedmaterialsinc.com/Used-EPS-Insulation. Dana, if you say it is ok for sub slab, I would accept that.