1 inch foam plus 1/2 inch concrete over existing slab
PLIERS
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Can you take poly put it over existing slab, drop down 1 inch foam, and pour about a half inch of self leveling concrete? Same concept of 1inch foam and 1/2 inch wood but concrete can’t rot. Once I have new layer put down some deltafl or dmx one step then lay down lvp.
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What about high compressive eps board. They make the 600 series which is 60 psi and a 1000 series that is 100 psi. How much strength would you need to use under lvp? I don’t know how this compares to wood or 1/2 inch of concrete
The problem isn't really that the foam isn't strong enough--it's that the foam will flex a little tiny bit under pressure, and when that happens, the concrete will crack.
But if you aren't set on using concrete, you could just put LVT directly over foam, with thick enough LVT and dense enough foam. I'm not sure whether you'll find specs for what density is needed--60 psi is more than enough and 25 is probably OK. You might experiment with a 3' x 3' mockup.
If 60 psi is enough then I’m definitely using that setup. Only problem I have now is my floor would be finished before my walls. I’m thinking I could attach foam board to wall later with furring strips so I wouldn’t have to cut the floor for a bottom plate. My other option which probably isn’t safe or recommended is to put down the stronger rigid foam, throw down cheap foam pads for kids and just remove the pads when I’m ready to do walls. I’m thinking I can’t do that because of a fire hazard?
My gut feeling is that a thin layer of concrete like that would have very poor impact resistance, it would crack at a harsh word.
Concrete backer board would work, but it's kind of pricey.
For more information on using concrete backer board as a topping over foam, see Martin's column below, under "Variations on a theme." Also see comments--my thoughts on this are in Comment #20.
Installing Rigid Foam Above a Concrete Slab
You can sandwich rigid foam between an existing concrete slab and new plywood or OSB subflooring
https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab
I read through the article, and I like how easy it was to lay down the durock right over the foam but I do wonder if you can finish a floor over durock like LVP. Leaving the durock over foam and letting it sit would be fine but then if I want to finish the floor would I be left picking up the durock and just tossing it. Why would lvp over a high density eps not be a good solution? You would have the moisture resistance of the eps and lvp on top. I looked at prices for eps with a 60 psi and it isn’t super expensive. I could lay down the high density eps finish any doorways or partition walls I have and then lay down the lvp. When I finish walls I can attach eps to walls with furring strips and not have to rip the floor up. I could even attach rigid foam to walls with some densarmor directly attached to it put all my electrical and plumbing outside of wall and not cover it. There would be literally nothing that could become mold, I would have 0 organic material.
I think you could put the LVP right over the durock. But if the upcharge for 60 psi foam is less than the cost of the durock, that's seems like it's cheaper and easier.