A.Paul Fisette, director of the Building Materials and Wood Technology program at the University of Massachusetts in Amherst, responds: The problem you describe is not unique, though it’s fairly uncommon. What is happening, as you guessed, is that volatile compounds are being released from the plywood and are reacting with the vinyl to cause the staining. White and light-colored vinyls are usually the only ones that experience this discoloration. The problem has three aspects: the radiant heating, the underlayment, and the flooring itself.

The "long dark streaks" sound like they could be associated with the " striping" effect that happens with radiant floors when the tubes are spaced too far apart and the fluid temperature is too high (see "Hardwood Flooring Over Radiant Heat," 9/98, for more on striping). With staple-up systems in particular, the...

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