PSI of foam for sub-slab insulation
Hi all! We’ve finally started our build, and the footings, foundation walls, and waterproofing are all in place. Exterior foam on the walls goes on next week, and we were planning to do the sub-slab foam as well, but my builder’s come to me with a concern.
His concrete sub had a bad experience doing a pour at a school that used foam beneath the slab, and it’s made him very skittish about projects that use anything other than super-dense foam.
From what I have found here (Dana had a good post a few years back) and elsewhere (a foam salesman telling me the concrete guy had no idea what he was talking about), 25 PSI XPS or EPS should be more than strong enough for a residential pour. The concrete sub wants something like 100 PSI foam, which is insanely expensive.
Does anyone have resources I can show to my builder to help reassure him we don’t need bulletproof foam to keep the slab safe? Thank you!
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Replies
David, there have been many posts here on the topic, and sub-slab insulation is required in northern zones in the IRC--99.99% of the time, that insulation is foam. I can't recall having a project in 20+ years that didn't have foam under a house slab.
If the concrete is reinforced, 15 psi is probably enough, and 25 psi is certainly enough. I have only used 40 psi in garages or under footings with large (~20,000 lb) point loads on them, and several of those I've had licensed engineers approve. I've never seen or heard of 100 psi foam being used in homes.
I don't have a handy reference to share, because it's such a ubiquitous thing that it's not a question where I work.
Thanks, Michael - I actually e-mail some snippets from your previous posts to my builder. Our slab isn't supporting anything, the walls and footings are, so hopefully I can reassure the concrete sub.
My 2 story house, which has stood for 7 years so far, is on 12" of type 1 EPS (10psi). 25psi is above and beyond what's needed.
It would be good to understand what specifically went wrong on his project. If the concrete is poured wet and the vapor retarder is below the foam instead of above the foam, the foam can float up, and/or water can get trapped under the foam and take a long time to dry.
David,
I'm assuming the foam extends under load-bearing parts of the foundation.
The default bearing capacity of soils here is 2000 psf. That's 13psi.
Malcolm Taylor
Hi Malcolm, there's no foam under the footings, if I'm understanding you correctly. There's vertically-installed foam on the exterior of the foundation walls that sit on the footings, and we'll put foam under the slab. Am I mistaken in thinking that the footings and walls carry most of the load of a home in this setup?
David,
Then it's a complete non-issue. The sub's reticence is a bit surprising as foam under slabs is a very standard detail almost everywhere now.
Just for reference, a 100 psi foam is equivalent to a 14,400 psf soil bearing capacity. The default soil bearing that is generally use when no soil report is indicated is 1500-2000 psf.
Snipped this and a piece from a Dow brochure linked in a 2018 post to the builder. Thank you!
Wow, I walk away for 10 minutes and have responses from Michael Maines, Malcolm, and Freyr!
Thank you all for the sanity check; I'll try to figure out exactly what the sub is nervous about since he did a great job with the footings, walls, and garage, and I'd like to stick with him vs. having to find another sub and push out the schedule. .
I would mention to the sub that the purpose of the footing is to distribute the load over a larger area of the soil (compared to the edge of the foundation wall). Allowing for a 12" wide footing, which is the smallest you're likely to see, 25 PSI foam is good for 3,600 pounds -- more than typical soil load numbers as mentioned earlier. If we assume a 40 foot square house footprint (about 1,600 square feet), that gives 156 square feet of total footing area, which would be good for 561,600 pounds using 25 PSI foam. Let's assume a two story house (so no we're up to 3,200 square feet). I'm going to allow 50 pounds per square foot for live and dead load combined for the living space, and 35 pounds per square foot for attid and roof loads. We're now at a combined total of 216,000 pounds of "house weight" resting on that footing which isn't even HALF of the foam's allowable loading! The basement slab doesn't "count" here, since it's resting on the soil directly, and is not directly supported by the footings.
In reality, you have even more safety margin here, because no house is actually going to max out the 40 pounds per square foot code allowable live load over the entirety of the indoor square footage. There is a LOT of safety margin here. Chances are you'll have some central columns in a 40 foot square too, which is going to take even more load off of the footing.
If your sub is worried about localized cracking, put some remesh in the footing, or some rerod. The foam isn't going to be a problem though.
Bill