Q: How long does it take a fresh concrete slab to dry out before flooring can be put down?

A: Bill Palmer, editor at large for Concrete Construction magazine, responds: Contractors are correct to be concerned about moisture in a new concrete slab when installing wood flooring or moisture-sensitive impermeable flooring over one. Every year, hundreds of floors suffer damage—including delamination, blistering, warping, staining, and mold growth—from moisture that comes from the concrete underneath. Unfortunately, so many factors affect drying time for concrete, there is no chart to consult. Variables include ambient relative humidity, permeability of the concrete, amount of water in the concrete when placed (water–cement ratio), slab thickness, presence or absence of a vapor barrier in contact with the slab bottom, and the method used to finish the surface (hard troweling can create a barrier to drying). Each one can affect how much time drying might take.

For most impermeable flooring (like vinyl or linoleum), the goal is for the concrete to reach an internal relative humidity of about 85% or surface moisture-vapor emission rate (MVER) of 3 to 5 pounds of moisture per 1,000 square feet over 24 hours. For wood floors, the National Wood Flooring Association recommends 75% internal RH and an MVER of 3 pounds per 1,000 square feet over 24 hours. Under typical drying conditions, with the ambient RH around 50% most of the time, temperatures above 65°F, concrete w/c ratio of 0.5, and a 4-inch slab, drying to an 85% RH level should take three to four months. But remember that the clock starts when the surface is dry. If the slab gets wet (such as being rained on), the drying time can be extended.

The ambient RH can have a huge effect on drying. In Florida in the summer with an ambient RH of 90%, the slab can never dry down to 85%. In many parts of the U.S., getting the RH to 75% may be difficult without closing in the building and operating the HVAC system.

The easiest way to determine the internal RH of the slab is to test. A couple of fairly simple tests are available: the calcium-chloride test to measure surface moisture emission rate (ASTM F 1869), and the internal RH test (ASTM F 2170). I strongly recommend the latter. Don’t be tempted to use electrical-resistance moisture meters (like you would use with lumber). These testers can indicate differences in moisture from one part of a slab to another, but they don’t tell you anything about overall moisture.

One of the most important factors governing the moisture content in a slab is presence of a vapor barrier (or retarder) in the assembly. Years of testing have shown conclusively that the relative humidity of the ground beneath a concrete slab—whether in Arizona or New England or Florida—is 100%. If there’s no vapor barrier, the slab will never dry out. So the best location for the vapor barrier is in direct contact with the bottom of the slab to take ground moisture out of play. Some years back, the consensus was to put the vapor barrier under a sand “blotter” layer, but the sand simply acted as a reservoir for moisture. The best-quality vapor barrier is a poly sheet with a thickness of at least 10 mils, preferably 15 mils. Lap and tape any seams, and seal around any openings.

If insulation is used over the concrete (as in “Insulating Over a Slab,” Jun/18), a four-month drying time should be adequate before installing EPS. With the vapor barrier placed in the right location, the EPS insulation will absorb some moisture, but not a lot (about 0.5% by volume, according to the EPS Industry Alliance). On the project in the article, the double layer of OSB (with its perm rating of 1) will also serve to keep moisture from migrating from the slab to the flooring. Another strategy for projects such as that one is applying a moisture-mitigation coating directly on top of the concrete. A poly barrier could also be used on top of the concrete, but the chances of it being damaged during construction would be high.