Wood Floor for Radiant Heat
Choosing the right material can make all the difference
My husband and I are building a two-story home in Massachusetts, with in-floor hydronic heat. Although we love the look of flatsawn 3-inch-wide solid-oak flooring, I've read that 2 ¼-inch quartersawn oak is less likely to cup in this application. But the local flooring and heating contractors both say that if the wood is properly dried and installed, 3-inch flatsawn would be fine. What do you think?
— Kathleen, Dracut, Massachusetts
Norm Abram says: What you've read is correct, but that doesn't make your contractors wrong. Cupping is least likely with solid wood strips 2 ¼ inches wide (or less) or with engineered flooring, which is made of wood veneers. The floor most likely to cup would be 6 inches wide (or more), flatsawn, and made of solid wood. In other words, when it comes to cupping resistance, narrow is better than wide, engineered is better than solid, and quartersawn is better than flatsawn. What you like is somewhere in the middle of these extremes, so it should be fine as long as its moisture content doesn't exceed 6 percent and it's installed properly. In my house, I added a little extra insurance against cupping by priming the back side of the flooring with shellac, which helps seal out moisture. You can also minimize problems by keeping the temperature and humidity inside your house as even as you can — hydronic floor systems shouldn't use daily temperature setbacks anyway — and by holding the floor's surface temperature below 85°F. I once saw a floor where the homeowners let the hydronic system get too hot, and the wood was baked so badly it shrank and never recovered. One more thing: If you decide to use engineered flooring, check the warranty — some products are not suited for use over heated floors.