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Q.I’ve built a number of homes with staple-up radiant heat tubing under vinyl flooring in the kitchens and baths. Recently a client complained of staining in his vinyl floors. In the bathroom, long dark streaks are bleeding up through the vinyl, and in the kitchen, small dark circles about the size of a quarter are forming. The staining is getting worse with time. The vinyl manufacturers claim that it’s a problem with the radiant heating and the subflooring. I lifted the vinyl and discovered that wherever discoloration occurred, there was a bleeding knot in the plywood below. I have never experienced this problem before. Any ideas?

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 fluid temperature is typically higher, to compensate for having to drive the heat through the plywood subfloor. Since the tubing has no conductive medium to disperse the heat — like the concrete in radiant slabs — the striping effect may be more pronounced with staple-up systems. It’s quite possible that these "hot spots" are causing a reaction between the vinyl flooring and volatile compounds in the plywood or chemicals in the vinyl adhesive.

You might check with your hvac contractor to see if the fluid temperature in the tubing can be reduced. On future jobs, plan to space the tubing closer so the fluid temperatures can be lower.

The second aspect of the problem is the underlayment. According to the APA (American Plywood Association), rated underlayments should not experience problems from the low temperatures associated with radiant heat. By this they mean that no structural damage will ensue, but they make no guarantee that volatile compounds won’t be released. A frustrating part of this issue is that manufacturers of underlayment and vinyl flooring have not worked together to craft a solution, and seem more inclined to point fingers at each other. APA holds vinyl as the villain: It claims that plasticizers and other chemicals found in moderate- to low-cost vinyl react with pitch and volatile compounds naturally found in the wood panels, and that exposure to heat and sunlight speeds the darkening process along. APA’s recommendation is for builders to use better quality vinyl.

On the other hand, makers of resilient flooring, the third piece of this puzzle, claim there is nothing wrong with the vinyl but rather that the underlayment and adhesives used to glue subflooring to the floor joists contaminate the vinyl. Like the APA, vinyl manufacturers typically warrant the use of their products with radiant heat, but limit the temperatures involved. Armstrong, for instance, limits floor surface temperature to 85°F.

As for solving your problem, representatives of the vinyl industry recommend that you strip the discolored vinyl, remove the offending underlayment, then replace it with an underlayment whose manufacturer will guarantee it won’t stain the finish flooring. (Good luck with that one!)

You might want to consider using lauan plywood as an underlayment, since it contains far fewer volatile compounds. However, be advised that using lauan may void the warranty on some flooring products, because underlayment-grade plywood is virtually the only material allowed in warranty specs.

Beyond these suggestions, your options are limited. If you’re using a perimeter-attached vinyl and you can clearly identify the locations of the spotting, peel the flooring back and tape (use masking tape) pieces of aluminum foil over the offending knots to block the diffusion of volatiles and pitch. If you are using a fully-adhered sheet vinyl, use an underlayment that is guaranteed not to react with vinyl — if you can find one. You might also encourage your clients to choose darker vinyl colors.

One encouraging note for the future: Problems like yours are reportedly becoming a thing of the past as vinyl flooring manufacturers, aware of the problem, develop products that resist reactive staining. I would always choose a major manufacturer and a higher-quality product.