As part of a remodel of his San Francisco area home, Torsten Budesheim is converting the 700-square-foot lower level into living space. An existing slab has been removed, and Budesheim has removed a few inches of material to increase the finished ceiling height. Now, he’s nearly read to place a new 5-inch-thick slab that will include tubing for radiant heat.
Budesheim’s architect has supplied a drawing for the concrete slab and its base layers, and the details are making Budesheim nervous. In particular, he wonders about the call for a 2-inch-thick layer of sand immediately below a 6-mil waterproofing membrane and an inch of rigid insulation.
What has the gears turning are two articles Budesheim has come across, one by GBA senior editor Martin Holladay and another by the Building Science Corporation (BSC). In each, Budesheim finds a warning that a layer of what’s known as “blotter sand” beneath the concrete will lead to problems.
“Is the above the right layer structure or do we need to revise?” Bludesheim asks at GBA’s Q&A forum. “One thing we do not want to change is the concrete slab and radiant heat in it. So I’m basically interested in what should go below the slab at this point. We really don’t want to have to worry about ever touching that slab again!”
Is the architect’s recommendation to include a layer of sand a good idea? And what other issues should Budesheim consider before the new slab goes in? Those are the topics for this Q&A Spotlight.
Get the sand out of the picture
Ask your architect to read the same articles you have, Holladay suggests, and notes, “The sand layer shouldn’t be there.”
Although the sand layer is unnecessary, at least…