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Floor over insulated concrete slab, Zone 3

user-1054948 | Posted in Energy Efficiency and Durability on

I am trying to decide what to do with our ground level floors – we have let the new slab with 1 inch XPS underneath dry out adequately (thanks to advice gained here) , and our floor guy is suggesting oak over plywood and 10mm plastic.
We are sensitive to eating up the room height, but are thinking of putting rigid foam under the plywood. Does XPS come in 1/2 inch or 3/4 inch? Is a floating floor with insulation possible?
Might the insulation under the concrete be adequate?
Thanks in advance,

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Does XPS come in 1/2 inch or 3/4 inch?"

    A. Here are some links to suppliers of 3/4-inch XPS:


    Q. "Is a floating floor with insulation possible?"

    A. I would screw down the plywood rather than let it float. Here's an article about plywood over rigid foam in a basement installation:
    The No-Mold Finished Basement

  2. kevin_in_denver | | #2

    The perimeter insulation is more important than the underslab insulation.
    Stained concrete is trendy, bulletproof, and could save $2-$6 per sq. ft.
    If it's a basement, the underslab insulation is even less important.

    Insulation over the slab would also hurt the benefits of the slab's thermal mass. Because of your climate, the home can be cooled at night with outside air (for a good part of the year) and the slab will help keep the home cool through the day.

    So here's a rare case where spending less may get you more.

  3. user-1054948 | | #3

    Thank you for both your responses, Kevin and Martin.
    We are going to go with the 3/4 inch XPS under plywood as Martin suggested.
    Just to clarify, this room is at grade, not below grade and it is in San Francisco, so the problem is always being too cool and never too hot. In fact, my wife wanted to turn on the furnace last night in June as it was 60 degrees inside. This grade level room doesn't get direct sun and has been always too cool, we hope that with new insulation in the walls, rigid foam cladding and rigid foam under the floor we will be at last warm enough to be comfortable. Thanks as always - love this forum.

  4. Lizzieplants | | #4

    We are building a house with a slab on grade in zone 5a (R20 under the slab). Isn't engineered wood flooring recommended on a slab on grade floor?

  5. user-1054948 | | #5

    Hi Elizabeth, I am not a flooring expert - just a homeowner recently educated on more stuff about building than I ever thought I needed to know, but my understanding is that if you want to float the floors (not fix them to a plywood subfloor that is attached to the slab) you would need engineered flooring, but a better option per Martin and Fine Homebuilding article above is to make sure the floor says warm and dry by putting layer of rigid foam between the slab and plywood. Then you can put engineered floor, solid wood, carpet, etc on top (just not something like vinyl which would trap moisture). Best of luck!

  6. user-1034637 | | #6

    I just spoke with a building scientist at BSC (won't mention any names) about this detail on a project in San Francisco and he said to do the floating floor. He said he as seen floors bubble up between the screws. He recommended floating the floor on XPS (1" or 2" even better) and leaving 3/4" at the wall edges. Tape the joints.

  7. user-1054948 | | #7

    Hi Pamela,
    Thanks for your note - it does make me rethink a bit. Can I float an 3/4" engineered floor right on top of XPS or do I need plywood in between.
    Also my floorer says he ususally puts 10mm plastic directly on top of the concrete - any disadvantage to that?
    Martin, since you said you would screw plywood into the concrete slab, what do you see as the downside to floating floors on a slab at grade?

  8. GBA Editor
    Martin Holladay | | #8

    I am going to defer to others who have more experience installing these floors.

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