GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Two layers of poly sheeting?

Morris Farris | Posted in General Questions on

Would it be okay to put two layers of poly under a basement slab? The layers would consist of 4″ of gravel, one layer of poly sheeting, one layer of XPS foam board, another layer of poly sheeting, and then the concrete slab.

Would this be beneficial or would it be overkill? I am building in central KY if that has any implications. Thanks for any input.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Morris,
    It's better to put the polyethylene in just one place. Ideally, the poly should be installed above the rigid foam and directly under the concrete.

    The reason you don't want a second layer of poly under the rigid foam is that, if the water table ever rises, you don't want to trap water near the rigid foam. You want the assembly to be able to drain when the water level drops.

  2. Morris Farris | | #2

    Thanks Mr. Holladay. I asked this in a different forum, and I would like to ask you too. I found a good price on some 10mil poly. It states it is made from virgin resin, but I do not think it is ASTM E 1745 rated. Does that make any difference since it is below the slab?

    I forgot to ask another question. Should the poly also be used for my attached garage slab, or would the gravel be enough? Thanks again.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Morris,
    I'm not sure whether the ASTM E 1745 rating actually matters for residential work. In my experience, polyethylene that is protected from exposure to sunlight doesn't deteriorate, but I welcome the opinion of any GBA readers who know more about ASTM E 1745.

    Q. "Should the poly also be used for my attached garage slab, or would the gravel be enough?"

    A. Polythylene is cheap. You should absolutely include it under your garage slab; you will never be able to go back later and install it where it belongs if you change your mind. And even if you think that a garage slab doesn't have to be as dry as a house slab -- and in my mind, I think that a dry garage floor is a good thing -- remember that garages are sometimes remodeled into living space.

  4. Morris Farris | | #4

    Thanks again. Very good points about the garage. I think I will go with the Grip Rite 10mil poly. A 20X100 roll for $135 seemed like a good deal to me. When I place it under the slab, should the poly come up onto the walls 4" so that it covers the slab edge as well?

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Morris,
    Q. "When I place it under the slab, should the poly come up onto the walls 4" so that it covers the slab edge as well?"

    A. Either way. It's a vapor barrier, not an air barrier -- so as long as it covers at least 95% of the area under the slab, other details don't matter much.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |