Beyond the Build


Why Floors Sweat (and Other Unpleasant Moisture Issues)

I can think of two projects, one in Miami and one on the Gulf Coast, were we found ourselves with unusual quantities of moisture sitting on the slab, within in the top portions. The folks on both of these jobs were not familiar with atmospheric conditions inside buildings and were hell bent that it was moisture coming through the slab. At the time of the Miami job, I was a freshman in the business and I drank the Kool-Aid.  In reality, the atmosphere, not the slab, was the source of the moisture.

The solution in Miami was installing humidistats with supply and exhaust fans. This means that the interior ambient air maintains the same temperatures and humidity as the exterior. Enough air changes prevent moisture from condensing on the slab overnight. The situation on the Gulf Coast was similar but different. Our project superintendent had all of the RTUs going to pump the moisture out of the inside air, but we were leaving the doors open just enough to in effect recycle moisture from the atmosphere back into the building, and then back to the atmosphere.

In regards to floor coverings, the adhesives used to be solvent-based, but are now water-based, like about everything else. Also, the synthetic backing in most flooring materials today doesn’t allow moisture to permeate as they did in the past, which sets up for failure. This is not fun to address in a finished environment.

A few thoughts relating to floor materials over concrete.

  1. Don’t put meeting the schedule ahead of good work. The concrete slab will cure in days, but dry time but takes weeks or even months. Provide sufficient time for concrete to dry naturally. Curing, sealing and bond-breaking compounds may save time, but they can inhibit the release of free moisture from within the slab.
  2. Ensure the below-slab vapor barrier system is installed to specs. This should be watertight and free from tears or openings.
  3. Pay attention to your materials and installation standards. Short cuts not allowed.
Merrill Stewart Jr.

Merrill Stewart is Founder and CEO of The Stewart/Perry Company, a commercial building contractor based in Birmingham.