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Foam backer board as subfloor?

thegiz | Posted in General Questions on

Ok so correct me if I’m wrong because I’m not a building scientist but isn’t foam backer board basically rigid foam with a membrane coating? So if I were to lay it down on my concrete basement floor I would get insulation, waterproofing, and a base to lay any floating floor I want (carpet tiles, lvp, rubber, etc.) I’m sure it cost more but you wouldn’t need rigid foam and wood for your sub flooring. Plus the floor would be thinner and insulated. I’m probably missing something here because I never heard anyone doing this, is this even possible? If not I would think 1/2 inch rigid with 1/2 inch durock on top would be the same thing but just thicker. I wanted to lay lvp directly on my floor to save ceiling height but I’m not confident enough in my moisture levels. I know I’m annoying with these basement floor questions but it’s for my kids, just can’t take any chance of mold.

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  1. charlie_sullivan | | #1

    Yes. Foam backer board can serve was both insulation and the subfloor for tile or similar. Unfortunately, it's usually XPS, and it's hard to get info on whether it's HFC blown or some alternative without the global warming impact of HFCs.

  2. Expert Member
    Michael Maines | | #2

    Joe, I think this is the fourth thread you've started on the topic of your basement floor, plus a few others that related to your basement renovation? It would be better to keep the discussions to a single thread, for others trying to navigate.

    Instead of trying to find work-arounds, you should do your project right: excavate the thin slab with voids, dig down a foot to get headroom, provide drainage, and put in a new, insulated floor. I certainly would not try to install durock over foam over a 1/2" slab over 8" voids, which sounds like your latest idea.

  3. Deleted | | #3


  4. thegiz | | #4

    Michael excavating the slab would probably be an extensive and expensive job. I will look into it but don't know if I want to rip out my entire slab. I don't know if my entire basement has an 8" void, I would have to look further. I have asked a lot of questions because I do worry about mold. I listened to your advice that putting lvp directly on concrete could be dangerous. It could be fine but I want to be 100% sure I will not have to worry about mold. Maybe my only option here is to just seal my basement floor because right now I can only finish the floors and not my walls. I considered drybarrier because it is inorganic, can support weight, and creates air space between concrete and subfloor. Although based on what I read on here airspace is not really that important in creating a mold free environment. So what happens when rigid foam sits on damp ground in case of moisture? My thought was waterproofing my concrete, using a paint and then 6mm poly with at least a 1/2 inch of eps on top. Again the eps is only to provide security for mold, warm floors not so important. I wanted to use foam tiles over eps so it would be comfortable for kids and removable but not sure if you can even do that because it's all foam. I also thought about getting high compressive foam and laying tiles on top but I don't know of a 1/2 inch eps that comes in a high compressive strength. That's when I thought of foam backer board. Looks like you need to start at least an inch. Same question goes for lvp if it sits on a damp ground. I have seen water tests online that pvc planks (same material as lvp basically) sat in water for weeks and nothing happened to it. Can you use waterproof paint, then 6 mm poly, and then the lvp? Would that be a safer bet or create more problems? Just concerned about what air my kids breath, I'm not trying to be annoying with multiple posts. And yes I'm doing this in a month so I will be using the poly vapor test this weekend.

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