Why RH Is THE Accurate Slab Moisture Test



Narrator: Whether it’s monitoring the concrete drying process of a project, or predicting the outcome of a flooring installation, the Rapid RH<sup>®</sup> helps you manage your risks against flooring failures. Knowing the signs behind relative humidity testing will help you understand the importance of this proven quantitative testing method.

Howard Kanare: We’re all familiar with relative humidity in the air. It’s simply the percentage of moisture in the air compared to what the air could hold if it were saturated.

Relative humidity –or RH—has been used for over 50 years to measure moisture in concrete floors. There’s a clear and valid relationship between relative humidity levels in concrete and floor system performance. RH has a solid scientific basis. RH instruments can be calibrated for absolute accuracy. That’s something you cannot do with a calcium chloride moisture emission kit.

Water vapor behaves differently in concrete at lower RH than it does at higher RH. Most coatings and adhered floor covering systems perform very well when relative humidity is below about 80%. At that level, water molecules are strongly absorbed in the porous structure of the concrete.

Now the performance of a product depends on the moisture sensitivity of that particular product. The manufacturers of flooring products determine the acceptable level of RH for each product, so accurate RH measurement is really critical. That way, users can be sure that moisture conditions are acceptable for installation.

Narrator: The Rapid RH is a revolutionary, state of the art, relative humidity probe that significantly decreases the time it takes to measure moisture in concrete slabs. It eliminates guesswork, messy testing methods, is disposable, and can be left in the testing area.

And each disposable smart sensor comes with a NIST traceable factory calibration certificate, providing reliable accuracy and completely alleviating the need for periodic calibrations typically required by other sensing technologies.

Peter Craig: When I was first introduced to the Rapid RH concept, I immediately saw it as a tool that could significantly improve the ability to monitor the drying progress of the concrete slab without adding substantial cost to the project. The self-contained sensor units allow one to take periodic readings over time, such that there is no need to conduct a full battery of moisture tests until there’s good reason to believe that an acceptable level of dryness has been reached. In addition, because the sensors are installed below the surface of the slab, there’s far less risk of damage to the units or interference in normal construction activity.

In my opinion, the Wagner Rapid RH Sensor is a valued asset to the overall moisture testing program on any project where moisture sensitive finishes ought to be applied.

Narrator: Market data suggests that there are more than five hundred million dollars spent each year on mitigating flooring failures.

Ray Thompson: Moisture in concrete is a major issue in the flooring industry. Owners, general contractors, flooring contractors, and installers need to know whether a slab is dry enough to receive a moisture sensitive flooring. With all of the testing methods today we need to be sure that the test methods are accurate and we can determine the drying rate of the concrete slab. Owners and general contractors need to know how the drying of the concrete slab is progressing. The flooring contractors need to know for their scheduling purposes, and the installer needs to know for the success of his installation.

The Wagner RH Probe provides accuracy without worrying about calibration. Plus, the results are available to anyone who needs to know as often as they need to know it.

Narrator: As more flooring manufacturers recognize the importance and accuracy of relative humidity testing and specify RH percent limits in their installation and warranty requirements, risks for flooring failures will be minimized. You can control your losses and manage your risks with Rapid RH.


  1. What kind of reduction in RH can be expected in a recently poured concrete slab? We have a project with slabs that were poured 1-2 months ago and the RH readings are too high to install flooring. Is there a “normal” rate of reduction that can be anticipated per week or month (e.g., loss of 2% per month)?

    • Jason Spangler says:

      The rate of reduction is really based on the ambient conditions to which a slab is exposed. A “typical” rule of thumb is, it will take approximately 30 days of drying time for every 1 inch of slab thickness, for a slab to reach somewhere in the 85-90% RH range. Keep in mind, the clock starts once the building is enclosed and the ambient conditions are conducive for drying. There are many variables that can affect this, but this at least gives you an idea.

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