Tile can contribute thermal mass to a passive solar house, and to Christa Campbell it would make a more appealing finish floor than concrete.

Although tile can be placed directly over a concrete slab, products such as Schluter’s Ditra are designed to separate, or “uncouple,” the tile from any potential movement in the substrate and protect the tile and grout from cracking.

The question for Campbell is whether using Ditra offsets some of the thermal mass gains in a passive-solar design.

“I’m wondering if you can lay tile over a concrete subfloor without compromising the thermal mass capacity of the concrete,” she writes in a Q&A post at GreenBuildingAdvisor. “Would an uncoupling system, using something like Ditra, have a negative impact? Should we be looking at using sand instead?”

Campbell’s question is the topic for this week’s Q&A Spotlight.

Lay the tile directly on the concrete

Yes, writes Doug McEvers, Ditra will insulate the concrete slab from the tile and lower the potential for thermal storage. “There is nothing wrong with tile laid directly on concrete providing all precautions are taken regarding moisture proofing,” he says. “Tile and grout work loose if the underlayment flexes. I can’t think of a more durable surface than a concrete slab for holding tile — try removing it sometime. Tile, if installed properly and as long as what’s underneath is solid, will last a long, long time.”

Insulation should be installed beneath the slab, McEvers adds, while the choice of tile also is important: dark tiles will absorb more heat than light-colored tile.

James Morgan agrees with McEvers. Citing Schluter’s own description of Ditra as an uncoupling membrane that permits differential movement between a lightweight substrate (such as a wood subfloor) and tile, Morgan says…