Q. How do you go about using sand instead of concrete for thermal mass in a radiant floor, as mentioned in the Sept. ’98 article "Hardwood Flooring Over Radiant Heat"?

A.Author Doug Mossbrook, president of Eagle Mountain HVAC, responds: Avoid using sand that is too wet. A pile of sand stored outdoors can be quite wet in the middle. I recommend buying play sand (sand sold for use in children’s sandboxes), which comes in a sealed bag. Play sand has a dependable, reasonable moisture content.

To prevent the sand from escaping through the subfloor, you need to be able to seal the cavity well. Either install polyethylene over the subfloor, or (if the plywood has tongue-and-groove joints) you can caulk the edges of the plywood. Install the sleepers over the floor, and then lay in the sand and screed it level with the tops of the sleepers. To make the sand easier to work with, you may want to mix a small amount of Portland cement with the sand — just enough cement to firm it up. Next, turn on the heat to let the sand dry. Once the sand is dry, install a second layer of polyethylene to keep the sand in place. You can install hardwood flooring directly on top of the polyethylene, or you can install plywood underlayment. Remember that plywood is an insulator, and will reduce heat transfer somewhat.