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Community and Q&A

insulating above a concrete slab for wood flooring

ritualsunlight | Posted in Energy Efficiency and Durability on

I have a grade level slab which I’m planning to put ¾” hardwood flooring over.
I’d like to put some insulation down before I install the flooring.
I only have 1½” total clearance before the top of the threshold.
I was thinking to put 1/8″ layer of vapor barrier/felt padding. then a layer of 1/2″ rigid foam, or the other way around. 
The wood floor will likely have to be glued down as there is nothing to nail it to.
Any suggestions..?

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  1. Patrick_OSullivan | | #1

    I don't think you're going to find a National Wood Flooring Association (NWFA) spec for glue down of 3/4" hardwood over rigid foam. There's likely too much risk of a point load deforming the foam. If you could buy yourself a little more height, 6 mil poly (vapor barrier), 3/4" rigid foam, 1/2" plywood, and then 3/4" hardwood is probably the bare minimum assembly, but then you're at 2".

  2. Expert Member
    Akos | | #2

    First thing you need to check is to see if you have poly under your slab and make sure ground water is managed around the house. A wet slab, no matter what you put on it is a recipe for troubles.

    If you don't have sub slab poly, I would be careful with including any impermeable layers above the slab as these can sometimes trap moisture and create condensation that can lead to mold.

    Generally if you are missing the sub slab poly, the best is a liquid applied vapor barrier over the concrete. Over this, you can then install rigid plus a layer of 5/8 CDX tapconned down or two layers of CDX floating over the foam. The hardwood can now be nailed to this plywood layer.

  3. AC200 | | #3

    I'm currently researching techniques for installing engineered hardwood on a basement slab and I haven't come across anything that will allow you insulate, moisture barrier and full face glue or glue assist staple install with 3/4" thickness. Even a manufactured panel subfloor like dricore requires tapcon and an extra sheet of OSB or CDX for glue assist staple install. Dricore R+ is already 1" thick.

    You could reduce hardwood to 1/2" thick and tongue and groove glue it as a floating floor, but it will feel like a floating floor when you walk on it.

    I'm interested if anyone has ideas as well.

  4. brooksbend | | #4

    8 years ago I installed this assembly on concrete slab:

    6 mil poly
    2” xps rigid foam
    5/8 CDX plywood just laid flat, not screwed to concrete
    3/4” x6” shiplap white pine boards screwed to plywood with deck screws, countersunk and filled with wood putty.

    Has held up great, solid feel, no apparent moisture issues. Tape seams of foam and plywood would probably be a good idea. I don’t see why you couldn’t change thickness of foam. If your assembly comes up too high for door thresholds, you could make little recessed entry ways where the doors are.

    I think the long narrow boards connected to the sheets of plywood with screws makes everything pretty rigid. Obviously having a way to control moisture would help keep it together.

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