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Community and Q&A

Best subfloor for moisture and insulation under a basement rubber floor?

Kate Wertime | Posted in Energy Efficiency and Durability on

I would like to put a Roppe rubber floor down in my basement; it’s soft and warm on the feet. I have an 80 yr. old concrete slab over which I’d like to add some moisture mitigation and insulation. The installation folks at Roppe suggest a 2 part epoxy paint on the concrete such as Mapei Planiseal, a portland cement for leveling, followed by the Roppe moisture resistant adhesive, and then the 1/8″ rubber floor itself.

Reading the GBA site, I’d like to get 1″ of XPS in there somewhere. The question is – where do I put it among all these layers?

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  1. Kate Wertime | | #1

    More specifically, would I put it above or below the portland cement? Or is it not compatible with this installation, and I should chose something else entirely for insulation?

  2. Charlie Sullivan | | #2

    You can put foam insulation down on top of the vapor barrier on the concrete. You might consider heavy polyethylene sheet as a vapor barrier instead of epoxy, for cost and easy of application without fumes.

    The challenge is that if you just put the rubber flooring on top of the insulation foam, the insulation foam will dent when you walk on it. It need a rigid layer on top of it to distribute the load over a wider area. Plywood or OSB subfloor is the usual material for that.

    XPS made in North America is not a green material--the bubbles are blown with an "HFC" gas that is >1000 X more potent than CO2 as a greenhouse gas. But the good news is that EPS is an alternative that avoids that problem and is cheaper. Some people use the stronger "Type II" EPS for an application like that where compressive strength is needed, although with the plywood that's not strictly needed.

    One other option is called Wedi board, which is made as a tile underlayment system. It consists of European XPS board that is blown with a different system and has negligible global warming impact, with a fiberglass skin that does some of the job of the plywood subfloor on top of the foam. It's mean to be used with tile on top of that--I'm not sure it would be enough to just have the Wedi board, but you could certainly then use thinner plywood on top of it. The US Wedi site has a feature to search for local distributors.

  3. GBA Editor
    Martin Holladay | | #3

    Charlie gave you good advice. The standard way to insulate an existing uninsulated concrete basement slab is to install a continuous layer of rigid foam -- usually XPS or dense EPS -- followed by 3/4-inch plywood. The plywood is fastened through the rigid foam to the concrete with TapCon fasteners.

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