- 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"?
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