Accurate Relative Humidity Testing vs. Testing with ‘Moisture’ Meters: Know the Facts

Moisture meters should never be used to make the final determination as to whether or not a concrete slab is dry enough for a flooring installation. Here’s why:

No ‘moisture meter’ of any type can give consistently accurate ‘moisture’ readings across the different mixes and densities of concrete. Additionally, other components (metal reinforcing bar, aggregate size, and amount, etc.) can cause false indications of ‘moisture’, especially with non-pin meters. But pin-type ‘moisture’ meters are also not practical for moisture measurement because variable chemical and physical characteristics in concrete can cause false readings due to changes in electrical resistance that have nothing to do with moisture.

Moisture Meter Illustration

Additionally, even IF the so-called ‘concrete moisture meters’ were sufficiently accurate (again, they are not), they only measure ‘moisture’ (not relative humidity) in a very small area near the surface of the concrete slab. This type of surface ‘moisture’ testing tells us nothing about the conditions down in the slab. Once a floor covering is placed on a concrete slab, the relative humidity (RH) within the slab will equilibrate throughout the thickness of the slab. This means that a slab that may have been “dry” (low RH) at the surface (without a floor covering) will see a higher RH (migrated from down in the concrete) at the surface when the floor covering has been installed. This is where the problems occur. The calcium chloride test method (moisture vapor emission), and the UK-based ‘hood’ method (RH) both have the same problem, as they are also surface tests.

Keep in mind that an uncovered concrete slab will indeed have an RH gradient (typically drier at the surface; much wetter at depth) throughout its thickness until a floor covering is put on top. Under normal conditions, the RH at roughly 50% slab depth will be significantly higher than the surface unless the slab has been down for a long time, and a vapor retarder is directly underneath the slab. In reality, the surface of the concrete will more closely reflect the RH in the room or building which gives no indication of the potential for a flooring failure due to the high RH deep in the slab that will migrate to the surface when the concrete can no longer emit water vapor due to the non-permeable floor covering.

Again, most major floor covering and adhesive manufacturers in North America and Europe already have concrete RH specifications in their installation guidelines.

At best, moisture meters (pin-type or surface-type) may have some practicality as relative (qualitative, not quantitative) measurement devices for possibly indicating best placement for accurate, quantitative RH sensors within the concrete. Moisture meters, similar to other non-quantitative test methods, are not final determination tools.

Learn more about RH probes.

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Ron Smith

Ron is a sales manager for Wagner Meters, and has over 30 years of experience in instrumentation and measurement systems in different industries. In previous positions, he has served as a regional sales manager, product and projects manager, and sales manager with manufacturers involved in measurement instrumentation.

3 Comments

  1. Ron,
    I would like to assess the effect of surface moisture on spray foam adhesion. Wood is fairly straight forward, but I am not sure what the right method and device is for the surface of concrete. I recently observed an application where the horizontal ceiling slab delaminated, while the vertical walls were well bonded. Thank you for any advice you can provide on making sure the substrate is ready before the installer begins his work.

    • Ron Smith says:

      Henri, I think it would be best if we could discuss this over the phone. You can call 1-800-634-9961 and ask for me specifically. Our office hours are 7:30am to 4:00pm PT, Monday through Friday. Thanks.

  2. […] The amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. Relative humidity is usually considered on the basis of the weight of the vapor, but for accuracy should be considered on the basis of vapor pressures. Extra Resource: Accurate Relative Humidity Testing Article […]

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