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Q.I’m building a home for a client who is concerned about toxic offgassing from the OSB in the wood I-joists we’re using for the floor. Is this a real concern?

A.Paul Fisette, director of the Building Materials and Wood Technology program at the University of Massachusetts in Amherst, responds: OSB is usually bonded with phenol formaldehyde in conformance with the ANSI A208 standard (1-1989, Grade 2-M-W). This resin reacts to become insoluble, resulting in negligible or no offgassing. The miniscule amounts of formaldehyde will dissipate in time.

There’s an alternate binder used by some OSB manufacturers that contains no formaldehyde. It’s a type of urethane known as MDI (methane di-isocyanate). OSB made with MDI is more moisture-resistant and dimensionally stable, but it’s also more expensive. It’s mainly used in brand-name T&G subfloor products, where any swelling from moisture might telegraph through to the floor finishes. You probably won’t find a wood I-joist manufacturer who is using OSB made with MDI.

If your client is a "hypersensitive" individual, you may want to use dimensional lumber joists — but you’ll also have to avoid the use of all panel products anywhere in the house, since most plywoods are made with phenol formaldehydes. The offgassing from carpet and furniture upholstery would be a much greater concern than the OSB in floor joists.

But for ordinary individuals, OSB offgassing should not concern you. The Structural Board Association (416/730-9090; www.osbguide.com), can provide you a technical data sheet on the issue.