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Community and Q&A

Subfloor for Existing Slab on Grade

70sfxr | Posted in General Questions on

I’m having trouble finding advice on first floor slabs, as opposed to finished basements.  I have a 1970’s colonial in the southern part of zone 6a that is mostly slab on grade. We don’t have good head room.  An inch would be the max for subfloor, and even then, I’m not sure it would work with the front door.  We are planning on using Marmoleum click flooring.  Heat will be mini split with pellet stove as needed.  Is the insulation essential to preventing moisture going to the cold floor?  We’re considering Insul-armor because of that.  Building inspector is still trying to get an answer from them for fire code.  I’d like to use something thinner like DryBarrier or DmxAirflow.  Thoughts?

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  1. Jon_R | | #1

    I haven't seen data that shows that an air channel/gap just above a slab floor improves or degrades moisture/mold performance. So I'd add "fully-adhered vapor barrier then flat EPS/GPS/XPS foam" to the options. No idea what PSI foam is needed, but 25 psi appears common. A flat foam "no gap" design is more consistent with:

    Also review the Marmoleum installation instructions carefully. Seems to suggest a 10+ mil plastic sheet as the vapor barrier/retarder. But Joe L says that's a bad idea.

  2. GBA Editor
    Kiley Jacques | | #2

    Here is a related thread with a few resources that might help inform your decisions: Flooring: On-Grade Concrete Slab.

  3. 70sfxr | | #3

    Thanks for the replies. Could anyone clarify, I’ve read about moisture when there is flooring in a warm room on a cold slab. Does moisture from the slab move to the underside of the flooring, or does moisture from the room move to the top of the flooring?

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