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Q.I'm finishing a basement room for customers who want to put carpet over the radiant slab. I'm concerned the carpet will insulate off some of the heat, but I can't talk them out of it. The slab is 4 to 5 inches thick, and the 1/2-inch PEX tubing is laid out on 12-inch centers, with 11/2 inches of styrofoam insulation under the slab and around its edges. The slab is at least 5 feet below grade; local frost depth is 48 inches. Is there a type of carpet and pad that would allow more heat to radiate into the room? Will the carpet cause heat to be lost into the ground?

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.