Concrete Moisture Meters vs. Relative Humidity Testing

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.


  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. Stacy Withers says:

    Does this same info apply if the subfloor is 1 1/4″ gypcrete that is being asked to seal when moisture meter reading says 26%
    can you RH test a 1 14/” gypcrete level?

    • Jason Spangler says:


      Thanks for the question. The best way to answer this question is by contacting the gypcrete manufacturer and the flooring and adhesive manufacturer. They should be able to give you specific guidance on this that would cover their specific warranties.



  3. Patrick says:

    AH right? ahwile back i did a laminate install over concrete where carpet and pad was removed……pin type moisture readings were about 3% average which is acceptable……two months later the floor failed due to moisture. we figured the reason was is because the carpet and pad allowed to concrete to breath to a certain exten,t after installing the laminate it covered and sealed to floor so to speak then the moisture deep in the concrete had no where to go and caused the floor to fail. so much for pin type moisture meter (Tramex CME 4) was used.


    • Jason Spangler says:


      Thanks for the comment and very good observations. This is one reason that surface meters are intended for scanning of a concrete floor surface and not intended, per ASTM F2659, to be used to make an installation decision.



  4. Pammy Saini says:

    Hi Jason
    The moisture readings obtained from the meters would effectively be meaningless.
    In water damage situations depending on the amount and head of water the depth to which the moisure has been absorbed will vary greatly.
    When called upon to test the moisture readings it would be advisable to add a disclaimer and advise the flooring supplier should satisfy himself if the concrete moisure content is within acceptable range. The relative humidity test determined by driling holes would be most appropriate. However the down side is that its time consuming and could be expensive, although the expense should be minor consideration compared to the cost for replacement of a failed floor covering.

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