The Importance of Relative Humidity Testing in Concrete Slabs





When concrete is the subfloor underneath any moisture sensitive flooring material such as vinyl composition tile (VCT), luxury vinyl tile (LVT), carpet, linoleum, or sheet vinyl, it’s extremely important to know the relative humidity of the slab. There are several reasons why this is true, both for the long-term integrity of the flooring, and for the legal protections the data relative humidity testing provides.

It’s important to note that different tests for moisture in concrete slabs often give significantly different results, and not all moisture testing methods are reliable or accurate. There are numerous factors that can contribute to the moisture content of the concrete slab both at the time of placement and throughout construction. RH testing prior to the installation of flooring provides the information necessary to make the right decision on when and how your floor will be installed.

What is relative humidity in concrete?

Relative humidity is the measurement of water vapor within a concrete slab, and the reason a “relative” reading is necessary is because water vapor can only evaporate from an exposed surface of a concrete slab. Therefore, when a new concrete slab goes through the process of drying, the moisture conditions within that slab gradually change, with the highest amount of moisture in the deepest part of the slab, and the lowest amount at the surface. The evaporation of the surface moisture makes room for more moisture to work up to the surface from the bottom of the slab, so the water vapor level, measured in terms of relative humidity (RH), will be much higher at the bottom or even halfway through the slab than it is at the surface.

Relative humidity testing measures the relative humidity that will exist well after the floor is covered -- the equilibrium moisture level at the top of the slab, which is the area that will interact with adhesives and flooring.

What is the relationship between relative humidity and flooring products?

When a floor covering is placed on top of a slab, it restricts evaporation from the top surface of the slab. At that point, moisture within the slab then distributes itself throughout the slab to achieve an equilibrium, which is affected by the surrounding temperature of the building and chemical interactions from the top to the bottom of the slab.  If the equilibrium moisture level is too high, either a moisture mitigation system is necessary, or if possible, additional drying time. Otherwise the moisture can have an adverse effect on the flooring products and result in a flooring failure.

Relative humidity testing is guided by ASTM F2170 standard.

Regardless of the relative humidity test method, readings obtained from anywhere but the correct depth may not be adhering to the ASTM F2170 standard in every test situation. Testing probes must be placed at a depth equal to 40% of the thickness of the concrete for a slab with one side exposed; 20% if two sides.

According to the ASTM F2170 standards for testing:

  • The site must be at normal operating conditions.
  • Windows and doors have been installed and enclosed
  • HVAC system is up and running.
  • The building temperature and relative humidity are at the expected normal operating conditions for a minimum of 48 hours prior to and during testing.

If the room air and floor are not at service conditions, test results can be misleading. Moisture vapor emission from a concrete surface, and relative humidity within the slab, are strongly dependent on the relative humidity and temperature of the ambient air over the concrete surface.

Proper documentation of test results for all tests includes:

  • Start date and time of testing with ambient temperature and humidity.
  • Finish date and time of testing with ambient temperature and humidity.
  • Date and time hole was drilled
  • Date and time sleeve was placed
  • Date and time probes were placed
  • Date and time probes were read  
  • RH % of the probe  
  • Temperature of the Probe

What should the RH level of a concrete slab be?

Various levels of relative humidity (RH) within a concrete slab can be appropriate depending on how the space will be used, and what kind of flooring will be placed on it. That’s why it’s important to know the manufacturer’s warranty requirements for the specific adhesives and flooring products that will be applied to the concrete. Without knowledge of the moisture tolerances for each product, there is no guarantee that the product’s warranty will be valid.

What is the best method for testing relative humidity in concrete?

The best method for testing relative humidity is one that employs RH probes at various locations across the concrete slab. Because RH probes are less sensitive to short-term fluctuations in ambient air humidity and temperature above the slab, they are one of the most accurate measures of concrete moisture. Once a hole has been drilled into the concrete, it must be allowed to achieve thermal and moisture equilibrium.  RH measurements made shortly after drilling will be inaccurate due to the heat generated during drilling of the hole. ASTM F 2170 requires a minimum of 24 hours from placement of sleeve to final readings to ensure equilibrium has been met. Keep in mind that this is the minimum requirement and it may take longer than 24 hours to reach equilibrium.

Relative humidity testing should be conducted by a certified professional

There are many reasons why concrete moisture testing should be conducted by a third party, certified concrete moisture testing professional. In many cases, contractors and building owners have a preference for the outcomes of testing that could possibly bias their readings and recommendations. Additionally, due the complexity of relative humidity testing and products, professional testing conducted by a certified concrete moisture testing technician is highly recommended. A third party professional offers objective, reliable assessments and recommendations.   

IFTI has a network of certified concrete moisture testing professionals throughout the continental US, as well as in Hawaii, Canada and Puerto Rico. Call IFTI at 800-490-3657, or contact us at to find a certified concrete moisture testing professional near you.

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IFTI Staff

Written by IFTI Staff