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Floating floor over a new insulated slab

user_736053 | Posted in Green Building Techniques on

We are in the middle of a project where the client would like a “floating” wood floor to be installed in the basement on a concrete slab.  We have the flooring product and the contractor would like to install it this way…

  1. Install some marine grade plywood to help flatten the floor a bit by gluing and fastening it to the slab.
  2. Install a continuous (taped) 2MM closed cell underlayment over the entire floor
  3. Install the “floating” wood floor on top of the underlayment

Does anyone have any comments or see any red flags with this installation?



  • There is R-15 rigid foam insulation and poly under the entire slab.
  • This is new construction
  • The slab will not be heated internally (not radiant)’
  • The basement is conditioned space
  • The house is heated with a GS Heat Pump and has an HRV
  • No signs of any “bulk” water getting into the house for the last 2 springs

Thank you!

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  1. Peter Yost | | #1

    Hi Alan -

    If you are comfortable that bulk water is being managed well enough that the basement can be finished, including the floor, the one load you need to keep in mind is the initial moisture of construction in the basement slab. First six months 1000 sf of 4-inch concrete slab gives off about 1000 pounds of water.

    If the slab is older than 6 months you are good to go; younger than that and you may need to performa a moisture transmission test on the slab (as they do on commercial projects before they put down vapor impermeable finish flooring, like vinyl composition tile (VCT). The best way is ASTM F2170:


    1. user_736053 | | #2

      Thank you Peter. Much appreciated.

      Our slab is over a year old now, and the building has been heated all winter, so I think it is plenty dry. We also haven't seen any water infiltration since construction began, thanks to proper curtain drains, surface grading, etc.

      Do you think I should have any concerns about a vapor barrier directly under the slab [above the insulation] and a vapor barrier (underlayment) above the plywood? Perhaps I need a third layer of vapor barrier above the slab and below the plywood?

      I just don't want to create a situation where the plywood could stay wet and buckle.

      Additional information: This project is a walk out basement in Vermont; Partially above and partially below grade, but enveloped in R-15 XPS.

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