A.John Siegenthaler, a consulting engineer who specializes in hydronic-heating-system design in Holland Patent, N.Y., responds: For an in-floor radiant heating system, engineered-wood flooring can be a reasonable alternative to harder stone and tile, offering better dimensional stability and lower thermal resistance than solid-sawn lumber flooring. I usually specify prefinished 3/8-inch engineered-wood flooring that's surface-glued to the heated slab, with unglued (rather than edge-glued) tongue-and-groove joints on all four edges. This kind of floor provides good heat transfer while allowing for very minor shrinkage without laterally stressing the wood. (See also: "A Radiant Panel Primer," 3/16)
But if the space is designed to be used as a passive solar collector, keep in mind that even a relatively thin engineered-wood floor over the slab will significantly interfere with transfer of solar heat gains to the underlying concrete. A dark-colored tile, stone, or finished-concrete floor would be your best choice here.
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