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

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.



Asked by Sabrina van Engelen
Posted Apr 29, 2018 4:43 PM ET


2 Answers

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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.

Answered by Martin Holladay
Posted Apr 30, 2018 6:13 AM ET


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:

Answered by Michael Maines
Posted Apr 30, 2018 10:14 AM ET

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