Concrete Moisture Meters vs. Relative Humidity Testing

Concrete 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.

concrete moisture meter measurement

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.

16 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 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. Ron Smith 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?

    • Ron Smith Jason Spangler says:

      Stacy:

      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.

      Thanks,

      Jason

  3. Ron Smith 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.

    Patrick

    • Ron Smith Jason Spangler says:

      Patrick:

      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.

      Thanks,

      Jason

  4. Ron Smith 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.
    Pammy

  5. Ron Smith PCTE says:

    I loved this blog, and this blog provides all the information that is required to know about concrete moisture. All my queries about this equipment got solved after reading this. Keep it up.

  6. Ron Smith Nong says:

    Hi Jason,
    I am work at applicator of flooring paint. In the technical data sheet of two suppliers differ in concrete moisture content :
    + Substrate moisture content is between 4% to 6%.
    + The other is Substrate moisture content within 75% RH
    Can you show me the relationship of these two quantities? And can I convert them together?
    Thank you!

    • Ron Smith Jason Spangler says:

      Nong:

      Thanks for the question. The 4-6% is usually measured with a surface moisture meter and the 75% RH is usually measured with a device such as http://www.rapidrh.com. Being two different testing methodologies, there are no conversions.

  7. is 75% a typical RH level for a coatable concrete floor?

    • Ron Smith Jason Spangler says:

      Mike,
      Thanks for the question. Many of the coatings manufacturer’s I see have a 75% maximum listed. Some of the more cutting-edge companies may have a higher number because they have done testing to confirm they can go higher. In any event, ALWAYS check with the individual manufacturer for their specific information.

  8. Ron Smith Carlos says:

    What is a concrete moisture content on meters?
    On a range from 0-6 what is an acceptable reading to install? Im confused on that. Help please!!!

    • Ron Smith Jason Spangler says:

      Carlos,

      The meters you are referencing aren’t really intended to make absolute decisions on installations, they are intended to identify hot spots on a given floor where you should do accepted tests like calcium chloride and relative humidity tests. That being said, in the wood flooring installation world, there are some adhesive companies that do recognize this type of testing, but you would have to consult them for acceptable numbers. Good luck.

  9. Ron Smith Clay Bailey says:

    We moved into a new build home on Feb 28, 2020 and have noticed the vinyl plank flooring starting to chip and break. The flooring company came out and did a pin meter moisture test is a couple of areas beneath the vinyl and carpeting in different parts of the home. Under the vinyl the meter was maxed and under the carpet it was mid 80% reading, I purchased a meter at Lowes and checked it and got a 100% vinyl reading and 82-87% reading under the carpet. Since we closed and moved in Feb the slab is less than a year old, we do you recommend? We are still under warranty and do you think we may have serious foundation issues.

    Thank you, Clay

    • Ron Smith Jason Spangler says:

      Clay:

      Thank you for the questions. First for the meter readings you describe. A pin meter isn’t the industry standard for testing moisture in concrete for the purposes of installing flooring, so I can’t really judge the numbers you have provided. I can state though that I have never heard of moisture in concrete causing chipping or breaking of vinyl plank. Secondly, based on any information you have provided here, I see no reason to think there is an issue with your foundation. Good luck.

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