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Community and Q&A

Underlayment between concrete slab and high quality LVP

Artemisia | Posted in General Questions on

Hello…I have a new build on a concrete slab with double 2″EPS layers and a 15 ml vapor barrier…all on rock fill with a sand topper and an extensive drainage system around the perimeter, including gutter drainage. I am having a difficult time finding out the definitive answer if I should put a vapor barrier between the concrete slab and the LVP (with attached underlayment). I have been told by a flooring person, that the vapor barrier is unnecessary, and in fact, will trap moisture and create a mold (!!!) problem….and that the LVP needs to breathe to the extent that it can. I have also been told that the LVP (12mm) just does not need an additional underlayment. The LVP has a wood core (WPC), not the stone stuff (SPC), so the feel is pretty comfortable and probably doesn’t need the assistance of more underlayment for give.  
Does anyone have a definite answer, either through knowledge or experience please? After all the trouble I have gone to to discourage wetness in a very wet area (4C) I don’t want to make a bad call now!
Thank you All,

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  1. walta100 | | #1

    What does the manufactures installation instructions say for this product?


  2. Artemisia | | #2

    Thank you for your response...I have poured over the instructions, and of course they (Shaw) want the concrete to be dry and under no hydrostatic conditions, which I think I have supplied. They state that excessive moisture could result in mold.
    They also say an additional underlayment is allowed and can be no more than 3mm and is of no acoustical benefit. This does not address the idea that an underlayment may TRAP moisture. At this point I will probably NOT do the additional layer out of fear.
    Thank you.

  3. Expert Member


    You have done a diligent job on the slab and insulation, meaning you won't get moisture from either below or condensing on the top of the slab. A second vapour-barrier buys you nothing.

  4. Artemisia | | #4

    Bless you Malcolm, so wonderful to get a thumbs up from you. I have tried to bring the info I have learned on this site, over the last couple of years to bear on this final build of mine.
    Thank you!

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