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.
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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