I have used both RH probes and calcium chloride tests on projects for years and agree that moisture testing is extremely important. Case in point, a 16,000 sf rubber tile installation at an elementary school, 8,000 sf of which was below grade. Before agreeing to perform that installation, I used both types of tests on the slab. However, there are sources of error for both tests.

Calcium chloride tests rely on the air contained by the hood as the medium to transmit moisture escaping from the concrete to the crystals in the Petri dish. If one affixed a calcium chloride test kit to the surface of the melamine-topped desk (through which no moisture is being transmitted) at which I’m now sitting and waited a couple days, the air contained by the hood will be less humid than the air outside the hood, as much of the moisture in the air contained by the hood will be absorbed by the crystals.

Moisture will evaporate into less humid air more easily than into more humid air. Calcium chloride tests tend to draw moisture from the concrete immediately adjacent to the concrete directly under the hood. Thus, calcium chloride test results will often be a bit on the high side.

While RH probes are increasing in popularity, using an RH probe can be tricky. The main source of error is related to accurately determining the thickness of the slab. In new construction, one is more likely to have a good idea of expected slab thickness than in a remodel.

Take the elementary school I mentioned above. It was built in the 1960s. No one could tell me for sure how thick the slab was supposed to have been. The best answer I could get was “probably four inches” thick. So, how deep was I supposed to drill for the probes? I used “probably four inches” as a guide and hoped for the best, figuring that the calcium chloride tests would be the ones I’d pay the most attention to, given the uncertainty in accurately ascertaining the slab thickness at that jobsite.

Note: I used the phrase “supposed to have been.” The ground under most slabs is not of uniform density, nor is it graded tabletop smooth before the pour. Even if the slab was slated to be poured to a thickness of four inches, as the heavy concrete is poured onto the ground, there is sometimes some settling in the less dense areas and a four-inch slab can be five inches thick in spots, or thicker. It can also be a bit thinner here and there.

Another source of error in RH probes is the assumption that the moisture content number at a certain depth on an uncovered slab will be the same at the surface after a slab is covered. The laboratory tests used to determine this differs dramatically from some real-world situations.

While the shape of the containers was more tray-like, essentially the tests were done with metal bottles containing water with concrete corks above. The moisture content of covered concrete corks at the surface was roughly the same as the results from 40% of the way down into the uncovered concrete corks. When one is installing a flooring impervious to water continuously across a large area of concrete, that real-world situation is likely quite similar to the laboratory.

However, if one is installing a mix of flooring – for example, runways of impervious flooring surrounding carpeted areas (such as the spa and Jacuzzi tub showroom I did recently) – the assumption that the moisture content of the uncovered slab 40% of the way down will be the same at the surface where a four-foot-wide strip of vinyl planking is glued between two carpeted areas is false. At the surface near the edge of the strip, where the vinyl transitions to the carpet, the moisture reading will be lower than at the surface at the center of the strip.

Regardless of the potential sources of error, moisture tests are absolutely essential. When I test, I use both types of tests. I have noticed that while some manufacturers are still only referencing the ASTM F1869 test, others are now referencing the F2170 test only. I think that manufacturers should always reference both. - Kent Korzon, Manager, Sav-On Flooring & Cabinets, Anchorage, Alaska


MC: Kent, thanks for sharing your insight into this issue. I agree that both tests have their advantages and disadvantages. As you pointed out, there is no one-stop, quick-fix response to a problem as complicated as moisture – and that includes the tests themselves. Whenever possible, it is a good idea to compare the results of readings from both types of tests to grasp the scope of the situation.



If you have insights into this or any flooring installation issue, I encourage you to write to me at ChmieleckiM@bnpmedia.com.