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What is the best way to install floating laminate over basement slab?

Carfar96 | Posted in Green Building Techniques on

We are installing laminate flooring (and one room of tile) over an unsealed basement slab. The basement has exterior waterproofing, drain tiles, and sump pump. We live in the midwest – so lots of temp changes and humidity. What is the best way to install floating laminate flooring over basement slab? We do not have rigid foam installed under the slab unfortunately, so I am thinking we do need some under the flooring. I want to do everything we can to prevent condensation and moisture/mold/mildew problems. The basement is a walkout on the entire 4th wall. Thank you!!!!

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  1. peaceonearth | | #1


    I have a few thoughts, which will be corrected/adjusted by the experts if needed. First you could lay out 6 mil polyethylene on the floor, typically done below the slab these days (along with rigid foam also under the slab). If you are using rigid foam above, as you ask about, that is also a moisture barrier, so the poly may be redundant (but probably wouldn't hurt). I would assume you should have at least 2" of rigid foam with a good compression value so as to make the floor feel firm. You must know you have adequate head room to lose a few inches of height. It may be that the flooring could be laid right down on the rigid foam, which would simplify things. All pressure is downward, with both foam and floor pinned in by the walls (or footing), so it isn't going anywhere. If poly is not needed below the foam sheets it might be nice to use foam adhesive to bind the sheets to the slab floor, if only to keep the sheets firm so that there is no feeling of give or bounce. If wood is deemed necessary to fasten the laminate to, others will suggest how this is best done.

  2. Carfar96 | | #2

    Thank you Howard! We do have 6 mil polyethylene under the slab (and pea stone under that) as we installed a passive radon reduction vent system. I am not sure if we should install another layer or not. We have good headroom (9 foot concrete walls), but I am a bit worried about the sliding doors, as they are already framed.

  3. peaceonearth | | #3

    At least with a sliding door you don't have to worry about a slightly raised floor level blocking an inward swinging door. And, you might not like this scenario, but if a raised floor arrangement was SLIGHTLY above the door frame, like an inch or so, and all else about it was workable, it would just amount to a tiny step down and out the door. There is probably a small step down beyond the door frame anyway. So, may not be a show stopper.

  4. LucyF | | #4

    I am not a builder but an interested layperson. I found a few articles for you. Some of the best articles are from Fine Homebuiding which may require membership or subscription. If you are building a home I would highly recommend that just as I would recommend a paid subscription to GBA.

    The best advice in my opinion is very similar to Howard's but I don't see that he recommended a plywood layer. If you only want to read a couple of articles, I would read references 5,6, and 7.

    Standard advice on the internet:
    1. Advice from a laminate floor company - Concrete slab has cured for 90 days, test for moisture, ALWAYS a vapor barrier.

    2. Today's homeowner has similar advice - slab cured for at least 30 days, test for moisture, vapor barrier -

    Great advice from Fine homebuilding - usually involves a foam layer with overlying plywood (especially for engineered flooring):

    Martin Holladay answers a question similar to yours

    4. 2005 article in Fine Homebuilding "The Stay Dry, No-Mold Finished Basement". Did not use a vapor barrier, but taped foam with 2 1/2 layers of plywood

    5. 2009 article in Fine Homebuilding "A Wood Floor That Can Survive Anywhere"- did not use foam, but included a 15 ml vapor retarder with 2 layers of 1/2 plywood

    Great advice from GBA:

    6. Nov. 2015 QA - "Hardwood over a slab: Is 6ml plastic or roofing felt best?"

    7. Dec. 2011 QA - "Wood Floor, Concrete Slab, and Moisture"

    P.S. It is easier to reframe the sliding doors and do the flooring right, than hope that you got it right and redo the floor in 5 years.

    Hope that helps. Lucy

  5. Carfar96 | | #5

    Thank you! So, one other issue is that the exterior walls (concrete) are studded for insulation. How does this work since there is a treated 2x4 glued to the concrete around the perimeter. Thank you!

  6. dickrussell | | #6

    Carolyn, regarding that studding for wall insulation, from this collection of blogs:

    you will find this one:

    You'll find that you shouldn't put porous insulation (eg. fiberglass batt) up against the concrete wall. You want some vapor retardent insulation, typically rigid foam board, up against the concrete, over which you could have a stud wall with batt insulation for extra insulation.

  7. Carfar96 | | #7

    Thank you! I can't seem to find anything about just installing the foam insulation around the wood 2x4's on the perimeter. Our builder says it is fine, but I just wanted more opinions. Also, I have no idea how we would insulated the walls with the studs already on them.

  8. Carfar96 | | #8

    Hello all! We had our builder demo the existing studs and are starting fresh. We plan to put 2" XPS on the walls and 1" XPS on the slab. The resources I looked at showed installing the slab foam, then plywood, then wall foam, then studs. However, if I do it this way, it doesn't seem like there will be a continuous vapor barrier (plywood layer between the foam at the joints and in contact with the concrete). Could I install the floor foam, then install the wall foam, and then tape and seal everything. Then attach the plywood to the floor and put the studs/walls on that? Thank you!

  9. GBA Editor
    Martin Holladay | | #9

    Q. "Could I install the floor foam, then install the wall foam, and then tape and seal everything. Then attach the plywood to the floor and put the studs/walls on that?"

    A. Yes.

  10. Carfar96 | | #10

    Great, thank you Martin! It just seemed like a better moisture barrier this way, but I just wanted to make sure it was structurally sound. Thank you!

  11. Carfar96 | | #11

    One more you have any suggestions for an underlayment between the plywood and the 12mm floating laminate floor? I am not sure if I even need one. If I do, I am not sure if a waterproof underlayment is a good idea. I would prefer a nontoxic, low voc option. Thank you!

  12. GBA Editor
    Martin Holladay | | #12

    When it comes to your flooring underlayment, you need to follow the flooring manufacturer's installation instructions. If you don't have a paper copy of these instructions, they are usually available online.

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