What is Relative Humidity in Concrete Moisture Testing?


Flooring failures are a costly and avoidable setback. To prevent them, you need concrete moisture testing that goes well below the surface.


One of the most common culprits behind de-bonding, peaking, warping, microbial growth, and other symptoms of flooring failure, is excess moisture in the concrete slab.

Moisture tests performed prior to installation by an independent concrete moisture testing service, are the most proactive and cost-effective approach to preventing these kinds of failures. Relative Humidity testing, which accurately measures the moisture below the surface in your slab foundation, is among the vital tests that may be included as part of a comprehensive approach.

Read more about Independent Concrete Moisture Testing.

What is Relative Humidity?

Relative Humidity (RH) is a measurement providing the percentage of moisture content in the air as compared to the amount of moisture that the air can hold. This is the same measurement used in weather forecasts to provide the “feels like” temperature.

The ASTM (American Society for Testing and Materials) F2170 defines it as:

"(The) ratio of the amount of water vapor actually in the air compared to the amount of water vapor required for saturation at that particular temperature and pressure, expressed as a percentage."

The measurements of relative humidity in concrete assess the percentage of RH in the concrete as compared to the amount of RH that it can hold.


Relative Humidity in Concrete Moisture Testing

There are myriad approaches to concrete moisture testing, but some are just scratching the surface when it comes to getting an accurate measurement of the moisture that may be lurking beneath. That’s why comprehensive independent testing services are integral to any new construction, remodel, or failed flooring remediation.

Why is it important to test for relative humidity?

After concrete is poured and cured, the excess moisture needs time to evaporate before it is sealed. Most concrete slabs are poured atop a vapor barrier or retarder to minimize the ground moisture that will permeate a foundation.The vapor retarder also prevents the excess moisture escaping through the base, so it seeks to escape through the surface, instead.

Once the surface of a concrete slab is sealed, it will cease drying. When the moisture can no longer be released through the surface or the base, it will evenly distribute throughout the slab and reach a state of equilibrium. If it is sealed too early, however, the RH% when it reaches equilibrium will be too high, which can trigger a costly domino effect :

  • Excess moisture is trapped in the slab and is distributed throughout since it can no longer escape.
  • The pH levels of the concrete rise due to the high RH%, and may negatively impact adhesives, stains, coatings, and flooring materials.
  • The integrity of the concrete is also affected, as an elevated RH% leads to decreased durability.
  • Flooring may show signs of blistering, curling, bubbling, cracking, or shrinking.
  • The increased moisture may also lead to mold, mildew, and bacterial growth which can further damage the strength of the concrete and lead to health problems for those living or working in the building.

Surface vs. Internal Testing

Concrete moisture levels are not fixed, and moisture levels at the bottom of the slab are higher than those near the surface. Surface moisture tests that assess moisture at or close to the surface level of the slab do not provide a full account of the moisture below the surface, nor are they capable of predicting the future moisture levels that will exist after the slab is sealed.

Relative Humidity testing in the US dates back to the 1980s, and the ASTM recognized it in 2002. It is the only test that measures moisture in the middle of the slab, using in situ probes to provide more accurate data about moisture levels at the time of testing as well as projecting the outcome if it is sealed under the current conditions. This reading helps to drive decisions about when to begin flooring installation.

Every major flooring material manufacturer requires an evaluation of the concrete subfloor prior to installation. A thorough evaluation may include a combination of surface and internal tests to assess the slab’s moisture and pH levels. Involving a flooring testing and inspection expert at the earliest stages of your project will allow them to develop a testing protocol and schedule that incorporates your site needs and meets the specifications required by the flooring manufacturer.  

Read more about relative humidity testing in concrete slabs.

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Case Studies  

ASTM F2170 standards require readings to be taken at a depth of 20% or 40% depending on the number of exposed sides. It also dictates the number of probes, and various other factors which may be influenced by site conditions to achieve an accurate reading. To ensure all of the standards are met, and test results are appropriately documented, these tests should be performed by a certified concrete moisture testing professional who will provide assessments that you can trust for their objectivity as well as their reliability.

Skipping this step, or hiring the wrong professional, can lead to costly repairs.

New Construction

When a newly constructed fitness center began to experience flooring problems only a month after installation, they contacted IFTI to complete an analysis and determine the cause. The project had included an architect, general contractor, and flooring installer, but this was the first time they'd consulted with an independent concrete moisture testing service. IFTI's experts completed a thorough analysis of the project documentation and found that improper recommendations had been made regarding both the moisture testing and the adhesive. After conducting comprehensive moisture testing, excess moisture was revealed in the slab and and a report containing specific guidance was given to remediate the problem.

Commercial Remodel

Moisture mitigation can be costly, especially when it's needed at more than one site. When a major retail chain received change orders from their flooring contractors for moisture mitigation at 80% of their locations, they contacted IFTI to validate the contractors' moisture testing results. IFTI's experts completed assessments utilizing our Risk Management System and uncovered that mitigation was needed at only 20% of the locations, saving the customer more than $1.5 million in upfront mitigation costs.

Independent Concrete Moisture Testing

Relative Humidity testing should always be conducted by certified concrete moisture testing professionals whose experience and expertise have been proven to deliver accurate and unbiased results. Call IFTI at 800-490-3657, or contact us at info@ifti.com to connect with a certified concrete moisture testing professional near you and set up a complimentary consultation.

IFTI Staff

Written by IFTI Staff