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...

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