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Hardwood floor in basement on concrete – new house

MillerConstr | Posted in General Questions on

I’m building a house in CT and I have a question for the forum.

In (under) my basement, I have virgin dirt, 12” of stone, 2-3” of spray foam insulation, then 4” of concrete slab. I am looking to put down ¾” hardwood flooring, and have been given different pieces of advice on what to do.

I was told that I could put down another vapor barrier (6 mil plastic) and then ¾” plywood nailed to the slab, and then the hardwood on top.

I’m worried that if I put the poly on top of the concrete, then I will create a moisture trap around the concrete.

What would you do?

Thanks in advance for the help.

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Replies

  1. John Clark | | #1

    IMO most slabs in the past did not have a vapor barrier between the slab and soil so this is why installers recommend a vapor barrier between your flooring and basement slab. In your instance the ccSPF under your basement slab doubles as a vapor barrier. Something else to think about are spills. Some liquids (ex pet urine) can get absorbed into the concrete depending on the type of wood flooring you install. Good luck getting that smell out of the concrete..

    https://buildingscience.com/documents/insights/bsi082-walking-the-plank

  2. Expert Member
    Dana Dorsett | | #2

    Concrete is extremely moisture tolerant, and functions just fine when completely saturated (as with foundation footings and bridge piers in rivers). Trapping it in a moisture trap is fine.

    Plywood is not moisture tolerant. At 2-3" the closed cell foam is probably vapor retardent enough to protect the plywood, but 6 mil polyethylene is an order of magnitude more vapor tight- think of it as both a slip surface and cheap insurance.

  3. Jon R | | #3

    See below for "The dumb way is to use a plastic sheet."

    https://buildingscience.com/documents/insights/bsi082-walking-the-plank

    I'm curious why you used spray foam vs less expensive rigid foam.

  4. Rob Hunter | | #4

    I did this 30 years ago with one exception: I used PT 1x4's glued to the slab for sleepers under the 3/4" subfloor-rated T&G CDX with the vapor barrier on top of the sleepers. No mold, no finish floor cupping, no "basement" smell. There was one unfortunate flood about 15 yrs ago - water heater supply pipe blew out - but I had put a floor drain in the slab before I knew it was gong to be finished, and after the water all drained out everything dried in a couple of days (dehimidifier helped!) with no permanent damage.

  5. Tom May | | #5

    Rob Hunter has it right, sleepers. But leave air gap on either side to allow ventilation or run radiant tubing between them....

  6. Expert Member
    Dana Dorsett | | #6

    Sleepers are not necessary, and they add (considerable) thermal bridging.

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