Credit: Peter Craig

Question: I own a storefront built in the early 1950s. It’s being used as a dance studio. There’s a concrete slab covered with vinyl tiles. When it gets hot in the summer, the previous manager said that the brown glue would sometimes squeeze up between the tiles. The current manager installed rubber mats that are perhaps 12x3 feet and they cover 80% of the floor, but she is now complaining that moisture accumulates under the pads. I’ve tried to explain that the moisture is condensation resulting from the dew point and high ambient humidity. Rolling the mats up when the kids aren’t there appears not to be an option for her. Do you have any ideas short of constructing a vented subfloor?

Answer: We’re assuming that this slab is on ground and being this old it most certainly does not have an effective vapor barrier underneath. The “brown glue” is adhesive beneath the tile that liquefies when wet from the high pH water. The water is almost assuredly coming up through the slab from the ground and condensing on the slab surface in the summer when the air is humid. Drier winter air will evaporate more water—not preventing the problem but just keeping it from being so obvious. As a solution, you can try applying a surface moisture barrier system. Several systems are available, including those from Koester, Ditra, Ardex, and Mapei. While we’ve heard of these being used successfully, there are many potential problems and surface preparation is critical. These epoxy-based products bond to the top of the concrete, keeping the moisture inside the slab.