A.John Siegenthaler, a consulting engineer specializing in hydronic heating system design in Holland Patent, N.Y., responds: There are many successful radiant heating installations where carpet is placed over a concrete slab. For best performance, use a low-pile commercial-grade level loop carpet bonded directly to the top of the slab, which will provide low thermal resistance to upward heat flow. If a pad must be used, it should be a low-resistance slab rubber pad approximately 1/4 inch thick, which will add about 0.31 to the upward R-value of the carpet (avoid polyurethane pads because of their higher R-value). Given the tube spacing you have, and the fact that basement heating loads are typically low, adding the slab rubber pad will likely raise the required circuit water temperature about 5¼F.
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