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What’s the best subfloor under nailed-down hardwood? IAQ concerns.

Sabrina Van Engelen | Posted in Green Products and Materials on

Dear Esteemed Experts,

We’re about to remove and replace a hardwood floor that had severe moisture damage. There is no mold. We want to use solid hickory (WD Stang Lund), nailed down over a concrete slab. I want to make sure that we’re using a safe product that’s not going to offgas into our home.

I read in “Prescriptions for a Healthy House” ( that interior grade plywood should never be used as an underlayment. Exterior plywood is recommended instead. I presume that this is because the phenolic formaldehyde is less volatile than the urea formaldehyde used in interior plywood. Columbia Purebond is an interior NAUF plywood, but they specifically advise against using it as a structural material.

So, help! I need to know what we can use as a subfloor that’s not going to leak a bunch of formaldehyde and other VOCs into our airspace.



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  1. GBA Editor
    Martin Holladay | | #1

    What you want is UL plywood. The UL stands for "underlayment."

    Plywood is not a significant source of formaldehyde in the home. By the time a sheet of plywood is delivered to your home, the formaldehyde emissions are essentially zero. More concerning than plywood are other pressed-wood products: particleboard, hardwood plywood paneling (used for decorative wall covering and used in cabinets and furniture), and medium density fiberboard (used for drawer fronts, cabinets, and furniture tops).

    Even in those other products, formaldehyde levels have been drastically reduced by recent regulatory changes.

    If this information doesn't reassure you, it's always possible to use pine or spruce boards as subflooring.

  2. Expert Member
    Michael Maines | | #2

    Advantech Flooring, a high quality oriented strandboard, uses a mix of phenol-formaldehyde resin and MDI (an isocyanate) resin, both very low-VOC, with no added urea formaldehyde, and its formaldehyde emissions are within the stringent California Air Resources Board limits. More info here:

  3. Jim Gramata | | #3

    "Plywood is not a significant source of formaldehyde in the home. By the time a sheet of plywood is delivered to your home, the formaldehyde emissions are essentially zero."

    Martin - can you share the source supporting this statement? I trust you I only wanted to share with my circle and for my own understanding.


    1. John Clark | | #4


      You're really not gong to find one. The amount off-gassing depends on temperature and humidity. Higher temp/humidity the faster the rate. The plywood used to build a home is most certainly exposed to continuous heat/humidity during construction for a couple of weeks. This of course doesn't include the amount of time that plywood has been sitting around at the building supplier prior to delivery to the site.

      The current EPA standard for formaldehyde off gassing in plywood is a max of .05 ppm. This EPA standard came into effect on Jun 1 2018 (after the date of this question). The largest wood-based source in a house is new furniture and cabinets. The federal limit is a little higher for the wood used in these products because they require more resin.

      If one wants to die on the hill of formaldehyde emissions they should look at what they choose to bring into the house.

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