Concrete Moisture Meters vs. Relative Humidity Testing

When it comes to flooring installation on a concrete slab, a critical aspect often overlooked is moisture management. Knowing the precise moisture levels within your concrete is more than just good practice—it’s key to preventing future flooring failures and ensuring the longevity of your installation.

However, the question remains: How should you measure this moisture?

This comprehensive guide will dissect the functionality of two popular moisture-measuring methods: Concrete moisture meters and relative humidity testing. We’ll address some important aspects of this topic, such as:

This article aims to debunk myths, provide insights, and guide building professionals and homeowners to make informed decisions about moisture control in concrete. After all, making the right choice could mean the difference between a durable floor that lasts for years and a disaster waiting to happen.

Keep in mind that when you’re making the final determination as to whether a concrete slab is dry enough for floor installation, concrete moisture meters should only be used if the floor covering manufacturer recognizes their use as a sole testing method to determine the final moisture content.

Whether you’re a building professional or a homeowner, using the right tools and methods to measure and control moisture in concrete is essential for ensuring the longevity and durability of your floors.

Do Concrete Moisture Meters Work?

Yes, concrete moisture meters work, though they only assess the top ½ inch of a slab. If you want a full picture of the concrete’s moisture condition, we recommend using the relative humidity test, which involves drilling into the concrete at 40% depth.

Sometimes, though, testing with a concrete moisture meter is all you have time and budget for.

Though ASTM F2659 doesn’t recommend using one as the final determiner of a slab’s readiness for a floor covering, many floor covering manufacturers allow for it. In these cases, having an accurate concrete moisture meter is key.

Concrete moisture meter readings can be affected by different concrete mixes, concrete densities, aggregate size, and additives, making it more difficult to get accurate readings.

That’s why you’ll want to look for the one with the fewest errors in product testing. We tested our C555 concrete moisture meter on nine commonly used concrete mix designs and found that its readings were nearly 2x as accurate as those of other meters.

Using a Moisture Meter on Concrete

The C555 pinless moisture meter effectively measures moisture in concrete on the top ½ inch of smooth slabs.

Pin-type meters are not practical for measuring moisture in concrete because variable chemical and physical characteristics can cause false moisture readings. This is due to changes in electrical resistance that have nothing to do with moisture.

However, remember that surface moisture meters provide readings only from the uppermost layer of the concrete, which may not be sufficient to assess moisture conditions throughout the slab. For a more accurate understanding of moisture conditions within a concrete slab, it’s best to use RH testing methods.

Measuring Relative Humidity in Concrete

Professionals typically use in-situ probes or sensors to measure the relative humidity (RH) within a concrete slab. The process involves drilling a hole into the concrete slab, often about 40% of the slab’s thickness for an on-grade or above-grade slab.

Once drilled, an in-situ probe is inserted into the hole. This probe measures the humidity deep within the slab and provides a percentage-based RH reading. The percentage indicates how much moisture is in the air inside the concrete compared to the maximum amount of moisture the air could hold at the same temperature.

Relative Humidity and Concrete Moisture Content

While “relative humidity” and “moisture content” may sound similar, it’s important to distinguish between the two. RH refers to the moisture level in the air, specifically inside the concrete slab, and is expressed as a percentage.

On the other hand, moisture content generally refers to the total amount of water in the concrete itself, often calculated by weight.

The Best Way to Test Concrete Moisture

The in-situ relative humidity (RH) testing method is the most reliable way to test concrete moisture. It provides an accurate measurement of the final moisture condition of the slab if it were sealed with a flooring product at that time.

The ASTM F2170 standard, which specifies the RH testing method, is recognized by flooring and adhesive manufacturers as the most accurate way to test concrete moisture. This method inserts RH sensors into the concrete slab, providing a precise understanding of how much moisture is present.

Once a floor covering is placed on a concrete slab, the relative humidity in the concrete slab will equilibrate throughout the thickness of the slab.

Meaning a slab that may have been “dry” (low RH) at the surface (without a floor covering) will see a higher RH at the surface when the floor covering has been installed. This is where the problems occur.

An uncovered concrete slab will have this RH gradient 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 it.

In reality, the surface of the concrete will more closely reflect the RH in the room or building. It doesn’t indicate the potential for a concrete flooring failure due to the high RH deep in the slab. That RH will migrate to the surface when the concrete can no longer emit water vapor due to the non-permeable floor covering.

Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry

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

However, sometimes, concrete moisture meters are the only practical way for installers to measure the moisture of a slab. The good news is that some manufacturers recognize their use. A little more on these tools next.

Concrete Moisture Meter Verdict

Though ASTM F2659 doesn’t allow concrete moisture meters to be used as the sole determiners of concrete slab dryness, some floor covering manufacturers do recognize their use as an acceptable testing method. If this is the case with your particular floor covering and you plan to use a concrete moisture meter, be sure that you’re choosing one with the fewest errors.

Concrete moisture meters can also give relative (qualitative, not quantitative) measurements to indicate the best placement for accurate, quantitative RH sensors within the concrete.

Learn more about RH probes.

Learn more about concrete moisture meters.

Last updated on May 1st, 2024


  1. Robert Donaldson says:

    I recently investigated a slab on grade home built 25 years ago that had a rug with a vapor barrier backing. The rug was installed two years ago and had wet conditions (liquid water) between it and the slab. Is it fair to assume there was no vapor barrier installed under the floor slab allowing a constant flow of water vapor to accumulate and condense under the rug?

    • Jason Spangler says:


      I hate to assume anything. Based on the age there is a strong likelihood you are correct, but the only way to really assure this is to do a core sample of the concrete. Good luck.

  2. Bonnie Tyson says:

    I’m looking for an acceptable way to read moisture in concrete block (CMUs) walls. Our FL condo was hit by the eye of Hurricane Ian. I’m on the 6th floor with a total of seven floors. The roof had more than 100 punctures and the roof is being replaced. We have a dehumidifier and 4 air scrubbers in our 3BR and 3-bath condo (1850 square feet). Two feet of the gypsum board is removed under two windows. The humidity levels in the air are excellent (20-40%), especially for SW Florida.

    My question is…what is the best way to measure moisture in the exposed CMU block wall? What should the percent moisture read with the masonry nails into the CMU?

    The company has used these 3 methods:
    1. The infrared thermography reads no moisture in the block or drywall.
    2. The concrete moisture reader reads all but no moisture in the block (about 1 %).
    3. Drilling stainless screws into the CMU. These are driven in at different depths of the block, creating an inconsistent depth of readings. The moisture reader reads from 8% to 11%. What are the pros and cons of using masonry nails?

    Thank you,

  3. Helene says:

    I’m a 1st floor condo owner in Florida. My exterior walls and ceilings are concrete. I’m noticing stains on ceiling where exterior wall meets my ceiling. I just had house painted. Used Regal primer and Benjamin Moore Waterborne ultra flat ceiling paint. The areas of concern are below the upstairs terraces. I had inspection today and was told no way to test for moisture in concrete ceiling where it meets exterior stucco wall. Doesn’t sound right. Any suggestions?

    • Jason Spangler says:


      If I am understanding you correctly, it would be difficult to test the moisture in the corner where the two plains intersect. That being said, taking readings with a non-invasive concrete moisture meter around the areas that seem to be affected and comparing those to areas that seem to be performing well might shed light on whether elevated moisture may be causing the staining issues. Good luck.

  4. Marie A Maldonado says:

    We are replacing my flooring on my 1st floor. We removed 3 types of flooring, laminate floating floor, ceramic tile and engineered hardwood. The new floor would be all porcelain tile over the concrete slab. The installer used an electrical impedance meter to measure the moisture in the slab. Reading ranged from 3’s to 6’s. He wants to put down a vapor barrier on the concrete slab before installing the porcelain tile. The distributor said we could use a product called Red Guard but he said that is not a vapor barrier. I contacted the company that makes Red Guard and they said it can be used as a vapor barrier when the moisture is not more than 12 pounds per thousand square ft. What are your thoughts?

    • Jason Spangler says:


      Thanks for the question. In order for you to meet the Red Guard specification, you will need to perform a calcium chloride test on the concrete. This is one of only two widely recognized moisture testing methods for concrete, the other being relative humidity testing. Good luck.

  5. 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

    • Jason Spangler says:


      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.

  6. 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!!!

    • Jason Spangler says:


      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.

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

    • Jason Spangler says:

      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. 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!

    • Jason Spangler says:


      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 Being two different testing methodologies, there are no conversions.

  9. 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.

  10. 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.

  11. 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.



  12. 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.



  13. 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.

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