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Community and Q&A

Underlayment for Hardwood Floor Over Slab

maine_tyler | Posted in Green Building Techniques on

Does anyone have advice on what (if any) underlayment to use beneath hardwood flooring (KD Birch) when the subfloor is 2 layers of advantech floating over foam, poly vapor barrier, and concrete.

I admit I don’t totally get why I need an underlayment at all. Information on the web is all over the place, and frankly often doesn’t make sense. The only reason I can see is maybe squeaks, but even then I have a hard time understanding how a squeak is generated between the advantech and the hardwood (seems far more likely that it would be the tongue and grooves of the hardwood itself).

I am concerned that most underlayment’s advertise blocking vapor- as if that’s what I want. It’s most definitely not what I want in my case because then I would have a subfloor vapor-barrier sandwich. Even using a vapor retarder makes me nervous, since I would prefer to maximize drying out the top.

Any thoughts appreciated.

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  1. Expert Member


    The underlay is more to act as a slip sheet to allow seasonal movement of the floor. Beyond the rosin and felt paper, there are quite a few proprietary brands of permeable underlays. Your big box store should stock one or two.

  2. maine_tyler | | #2

    Ok, update after talking with some installers and doing a fair bit of research:

    Using an appropriate glue ('glue assist' method), though expensive, is probably my best option (random width floor 3", 4", and 5" wide). I'm really not convinced an underlayment makes any sense in my situation, and would actually get in the way of glue. Glue was recommended based on the 5" width of the boards. If one wanted to have a VB on top the sub floor (which I don't), they do make roll on products that the glue will stick to.

    I'll be making sure to keep glue out of the tongue and grooves so as not to panelize the floor (making one large board with crazy movement at the edges). And applying a serpentine bead of appropriate adhesive-- flexible to allow movement--polyurethane or modified silane. (Bona 850T sausage gun)

  3. Expert Member
    Michael Maines | | #3

    Tyler, I have never understood the impulse to install a membrane under a wood floor, aside from the rare cases where it is a vapor barrier over a damp basement, which is a band-aid solution at best. Squeaks come from boards rubbing against each other or against fasteners, and a slip sheet doesn't prevent either of those actions. Glue should help but it's a pain to deal with, so it makes sense to limit it to your 5" boards.

  4. Expert Member
    PETER Engle | | #4

    Two layers of Advantech should produce a wonderful subfloor. Having seen some dramatic failures in glue-down systems, I admit to not being much of a fan. Traditional fasteners through the tongue work just fine. The only advantage I see to using glue is that it can (maybe sometimes) help to restrain warping of single boards with moisture changes. This is not a problem if the boards are properly acclimated to the house environment, the house humidity levels are reasonably constant, and the boards are not too squirrely. With quarter sawn wood, there is rarely a problem with warping regardless of humidity changes. Flat sawn boards will warp more than quarter sawn, and 5" boards are wide enough that this could be noticeable/objectionable. Of course, the line between noticeable and objectionable is up to the beholder. I would be comfortable with rosin paper as an underlayment or none at all, and limiting glue use to maybe the 5" boards. Do follow the flooring manufacturer's requirements for acclimatizing the lumber. This usually involves taking all of the planks out of their containers and stacking them loosely in the room where they will be used, with stickers between the boards, and HVAC operating normally for the space, for a couple of weeks. This step is a PIA and therefore, usually skipped, but it does make a difference.

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