Under slab sgtrength of EPS Type I rigid foam
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| Posted in Building Code Questions on
I am building a new house lon a slab and I am specifying EPS foam board around the outside and inside of the stem wall as well as under the slab. My builder is questioning the entire process but particularly the under slab insulation. He says ‘code’ requires a compressive strength of 40 psi for a 2″ foam board under the slab and the product I thought would be appropriate – Cellofoam Type I 2″ 4’x8′ sheets does not have enough strength. If there is a code requirement what would it be and can I meet that requirment using EPS foam since I want to avoid XPS? Thanks for your help. Matt
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It seems to me that the requirement depends on the particulars of the house. Our house is on type 1 EPS, and it's a two story with an 8" thick slab on grade. There are denser grades of EPS, so if type 1 doesn't meet the need you don't have to go to XPS.
The thickness of the slab matters, especially if it's a grade-beam type slab holding up the house.
But if it's stemwalls & footings and the slab is only supporting partition walls and the contents of the house, the typical 4" slab does fine even with low density foam.
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Have him cite chapter and verse on that "code" requirement for 40 psi foam under residential slabs.
That 40psi number is fairly standard for industrial building slab applications, but in most homes you won't be driving heavy dump trucks full of rock or operating steel forging equipment on your slab (are you?). Care to calculate your REAL slab loads?
https://www.structuremag.org/?p=1812
Type IX EPS (2lbs per cubic foot nominal density) is typically rated 25-30psi and considered VERY conservative for a residential slab, since that's a higher compressive strength than the soil it's resting on. With an engineer's review it is even used under residential footings that carry the whole weight of the house, or under grade beam type slab on grade.
Type-II EPS (1.5lbs nominal density) runs ~15-20 psi, and fairly conservative for a slab on grade with stemwalls and footings supporting the walls, (rather than slab-integrated grade beams, which are a special case), not necessarily under footings or grade beams. Type II EPS under the slab would get NO push back from "code" enforcers in most locations. (Ask the local code people not the contractor.)
Type VIII EPS (1.25lbs nominal density) runs ~12-18 psi, often used as "walkable" roofing foam on roof decks, and good enough for almost all residential slabs.
Type I EPS (1.0lbs) is ~10 psi, will usually work under slabs, but could get push back if not signed off by an engineer.
So if Cellofoam is your preferred EPS vendor, run the comparative specs of density vs, psi here (which are pretty typical):
https://www.cellofoam.com/PDf/Technical-Data-0816.pdf
See also:
https://www.greenbuildingadvisor.com/article/foam-under-footings
Thank you all. Green Building Advisor has been very helpful so far in our house project and we are only at the slab.
Just for reference/curiosity, I found the specification for the EPS on our structural drawings: minimum 25kPa, which is about 3.6psi. This is for an 8" thick slab with no footings that is supporting the entire weight of a two story house. So 40psi for a slab that is probably thinner than that, and sounds like it's not supporting the house is definitely overkill.