Interior walls set on top of rigid foam
I’m building a living space (750sq ft) into a building in zone 6a. The slab has a 6 mil vapor barrier and 2″ foam under slab and 2″ foam on slab edge. The space is completely empty now. (no interior walls). I cannot tolerate concrete floor to walk around on because of hardness and cold feeling. I would like to lay down another vapor barrier then 2″ foam then 2 layers of 1/2 ” plywood glued and then finished floor materials. The building sits very high and has crushed rock under slab with 2 ft overhangs and proper landscape grading. I doubt there will be any moisture issues.
My question is could I then build the interior walls on top of this foam and plywood floor. It seems like it would be so much easier to build the interior walls after the foam and plywood is already laid down than before.
The walls would mostly be non load bearing as the trusses above will carry the plywood with drywall service cavity ceiling assembly. the trusses have a 10psi bottom cord dead load and the trusses manufacturer says I’m fine with the ceiling weight.
Is the foam able to carry the 2×4 walls without shifting and compressing and can I anchor these 2×4 walls by just drilling thru the plywood foam into the concrete and then fastening with concrete fasteners? What spacing of fasteners would be appropriate ?
Thanks for any advice
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I've done that in the past with 60 psi rigid foam boards on the floor, but that needs to be verified by an engineer since there are many variables to the solution, like loads, bearings, soil conditions, etc. You can also get 20, 40, 100 psi foam, so make sure you get what you need fo your application.
The concrete slab was originally designed for the load of tractors heavy equipment. It's 4000psi 6" thick with rebar every 2ft. I'm more concerned about the foam itself than the concrete. Thanks for your input
Without any structural loads, with a double floating subfloor, I think 15 psi foam could work and have used 25 psi foam in a similar application (with 2x4 sleepers and a single layer of floor sheathing). But Armando is right; it would be best to talk with an engineer.
Smokey, the psi rating of foam is based on how much load it can take, while limiting deflection to 10% of the thickness.
Floor loads are generally in the 40-50 psf range, or 3.3 to 4.2 psi. The deflection curve is pretty linear, so if you take 25 psi foam and load it with 5 psi, it will deflect roughly 20% of the 10% it is rated for, or 2% of the total thickness. 2" foam, then, would deflect roughly 0.004".
It gets more complicated when the loads are sustained, though, because long-term creep sneaks into the equation; one company I know of says to use a 30% safety factor for creep. For our example, that means plan on 0.012" deflection.
Where it really gets tricky is if you have point loads or larger loads in some areas, which could lead to differential settling.
For what it's worth, geofoam spec sheets often give the loads for 1% deflection as well as the 10% deflection specs Michael Maines is talking about. Here's an example: https://www.insulfoam.com/wp-content/uploads/2014/04/10000_Insulfoam_GF.pdf
I'm not sure how available geofoam is in small quantities or how helpful that tidbit is to you, but perhaps it helps!
so even If i put the walls directly on the concrete and floated the rooms separately I would still have to be concerned about the floor loading deflecting the foam if it's psi rating is to low. If building a wood floor over the concrete would 2x4 be sufficient and the foam in between . What spacing? Of course how much thermal bridging will this cause. I guess the psi would have to be around 75
Smokey, if your bearing walls are going directly on concrete then you should be fine to float the rest of the floor. Where do you anticipate you might have thermal bridging? You wouldn't have any at the slab. You might have some at the roof truss, depending on the design, but it would be minimal--no more than the rest of the ceiling.
Table R602.3(1), items 14 and 15 provides some guidance on fastening bottom plates: https://codes.iccsafe.org/public/document/IRC2015/chapter-6-wall-construction.
Your assembly is unconventional enough, and structural concerns are finicky enough, that I would again recommend you run your design by a licensed structural engineer. They are usually pretty affordable for the service they provide. Although I have a degree in engineering and engineer most of my own projects, I also work with licensed engineers regularly when the situation is outside of my comfort zone.
https://buildingscience.com/documents/insights/bsi-059-slab-happy
Look a figure 3 & 4.
Thanks for all the help. I'm not against talking or paying an engineer if I can find one where I'm at. I've read so many posts on here of people putting foam on their basement floors with and without sleepers I hadn't heard much about there being any foam deflection issues so ill have to think it thru somemore. I realize my original question was concerning putting walls on top of the foam but I just assumed since they weren't bearing any structural loads there wouldn't be a problem. Again ill have to look closer.
The link john clark posted is understandable and I could build it like that but in those diagrams the floor between the walls is still just foam( with no psi rating listed) with t&g over and subject to the floor live loads and any associated deflection of the foam. I guess it's not really floating though. It also details the plate is under load bearing walls only. Thanks again for all the help guys
@Smokey
Contact foam manufacturer (eg DOW) since they are responsible for the performance of their foam.
https://www.dow.com/en-us/building/construction-applications/flooring
Thanks for the links john clark . ill contact them see what they say. If I do decided to put the walls on the foam instead of the concrete ill run it by someone smarter than me. Ie an engineer
Given that your walls aren't load bearing, my recommendation would be 25 psi foam with two layers of OSB subfloor on top, overlapped so the seams don't line up, and screwed together. And then the walls built on that. But it seems that the consensus is that you should get an engineer to review it, so that's just one thing for the engineer to consider.
If the wall is holding up joists for a second floor there's a load, even if it's not holding up the roof. If not, it's like any other partition wall, and the compressive specs on the foam don't much matter.
Thanks for your added advice Dana and Charlie. There is no second floor just the ceiling load that is carried by the trusses bottom cord dead load which the manufacturer already said was well within the specs. The walls only would carry the sheet rock load on them and maybe some sound deading insulation and whatever else normally goes in an interior wall. Thanks again for everyone's help
Michael, I think your PSI math is in error. Wouldn’t a 50# psd floor loading be less than .5 psi? 50/144 or am I missing something? If that’s the cas, 15psi foam should have a huge safety factor.
Rick, you are totally right, of course; rookie-level mistake on my part--I divided by 12 instead of 144.
Thanks Rick for that clarification. I haven't been to school in 30 years but I can see after you pointed it out that .34 psi (50/144)is a lot less load than 4.1psi (50/144). It definitely covers the 30 % safety factor that was referenced. I probably will still put the walls down on the concrete according to the detail that John Clark referenced from the building science page. It doesn't seem any more difficult really than the wall completely on the foam. Thanks again to everyone