Top 15 Concrete Moisture Testing Myths

In the world of concrete flooring, moisture testing holds a reputation both as a knight in shining armor and a beast of burden. Misunderstandings abound, causing flooring gurus to make decisions that could boost the chances of moisture-induced complications.

Let’s set the record straight and debunk the top 15 concrete moisture testing myths.

Not only will we equip you to dodge moisture-related flooring disasters, but we’ll also reveal how to precisely perform a moisture test for your concrete slabs.

Your Go-to Guide to Moisture Testing for Concrete Slabs

It’s not rocket science, but knowing how to test concrete for moisture can save you from potential flooring blunders. ASTM F2170 lays out the use of in situ probes for determining the moisture state of a concrete slab at the point it will be sealed or overlaid by flooring material.

These probes give you a relative humidity reading at 40% of the slab’s thickness when drying from one side or at 20% for slabs drying from both sides.

In situ measurements aren’t just scientific jargon; they give dependable and precise data to steer your flooring installation decisions. Using RH probes, you’ll need at least 3 tests for the first 1,000 square feet and 1 more test for every subsequent 1,000 square feet.

Want to dive deeper into moisture tests for concrete floors? Check out our in-depth article here.

Concrete Moisture Testing Myths

MYTH #1: If the concrete surface is dry, the slab is dry

Not necessarily. Repeated scientific studies and demonstrations have proven that the surface of the slab cannot serve as an accurate indicator of overall moisture levels. Because there are so many variables that can impact the drying rate of a concrete slab (air movement, ambient temperature and relative humidity (RH), troweling techniques, and more), conditions at the surface do not accurately represent the presence of moisture within the slab. Any method that only tests the surface for moisture should be highly suspect.

In situ RH testing is the only way to accurately determine the moisture condition of a concrete slab. Placing a sensor inside the slab, below the surface, and at the correct depth, gives a clear picture of the final concrete moisture condition if the slab were to be sealed at that point in time.

Myth #2: All floor products have similar moisture tolerances

At one time, flooring adhesives and flooring materials were more consistent in their moisture tolerances because most products had a petroleum-derived base in their compositions. As formulations began to change for different applications it became increasingly important that concrete slabs under the flooring installations were dry in order to prevent moisture-related problems down the road.

Can moisture come up through concrete
The industry has recently put a lot of focus on lowering VOCs (volatile organic compounds) in building products, different adhesives, and flooring products. As a result, the tolerances to prolonged and elevated moisture conditions can vary greatly from one adhesive product to the next.

Therefore, it is advised to always perform accurate RH testing in order to determine the current slab moisture condition, ensure that the specified product is compatible with the slab moisture levels, and choose the correct product to meet the current moisture conditions of the slab.

For a quick link to a multitude of manufacturers that specify relative humidity (RH) tolerances for their products, visit

MYTH #3: The concrete is old so it has to be dry

Even when a concrete slab has been in service for years, it’s still possible that moisture levels within the slab are high for many reasons including recent flooding, leaking pipes, and rising water table. This means that, unfortunately, it’s not unusual for an old flooring system to be removed, only to have the newly-installed flooring start to exhibit signs of moisture-related problems once it has been installed.

Another reality with older concrete and flooring systems is that the products used in decades past were inherently more moisture-resistant than many of the products on the market today. The move to lower environmental impact products (like products with lower VOCs) has resulted in the need for more awareness for moisture tolerances in flooring adhesives, sealants, and flooring products.

So, an older floor that has never shown signs of moisture problems may contain too much moisture for the RH sensitivities of the new flooring product.

An older concrete slab can also be facing moisture intrusion from an unidentified source (a compromised vapor retarder, an appliance or plumbing leak) that has raised the slab’s moisture but not yet reached failure levels for the older flooring.

The best insurance for older concrete slabs is accurate RH testing before installing any new flooring system. Fortunately, the unique design of the Rapid RH® L6 system seals a sensor in each test hole for immediate test results. It’s fast, accurate and affordable testing that saves you valuable time and keeps the ASTM-compliant testing process as simple as possible.

MYTH #4: The slab is not on grade, so there’s no need to do moisture testing

All concrete is affected by ambient RH and temperature conditions. Therefore, all concrete is susceptible to moisture-related complications and should be adequately tested before the next stages of flooring installation can begin.

There are two significant differences for concrete slabs not on grade:

  • Slabs on grade (or on pan decking) have only one surface that moisture can use to exit the slab. Slabs not on grade have two surfaces from which moisture can move out of the slab. This also means that concrete slabs not on grade have two surfaces that can also absorb moisture from a humid environment or external moisture source. The need to do accurate moisture testing has little to do with the format of the pour. The correct test method, however, has everything to do with the on-grade or not-on-grade status of the slab.
  • ASTM F2170 determines that, for accurately testing final moisture conditions within a slab, 40% is the correct test hole depth for slabs poured on grade or in pan decking (or with one surface from which the moisture will evacuate the slab). For slabs with two surfaces exposed to ambient conditions, the correct test hole depth is 20% of the slab’s thickness.

In situ RH testing for concrete is the only way to ensure against moisture-related problems over time. Only testing done at the appropriate depth will provide accurate and actionable results.

MYTH #5: Surface testing concrete gives more accurate readings of the moisture level

How to test moisture in concrete As was discussed above, any test method that reports on surface moisture conditions is at risk of producing a false reading. Several factors can skew the results of a surface test which has given cause for many authorities in concrete moisture testing to evaluate surface testing as unreliable.

There are two test methods that only provide results at a slab’s surface:

  • The calcium chloride (CaCl) test (or MVER) seals a desiccant material under an impermeable cover and then calculates the MVER rate by weighing the material after a set amount of time. The theory behind this test method is that the desiccant will absorb moisture emitting from the slab. That weight gain will express a ratio of the remaining moisture in the slab. Unfortunately, there is no scientific basis to the test and it has been specifically disallowed for some concrete types.
  • Concrete moisture meters operate by sending an electrical signal into the concrete and measuring its resistance as an indicator of the moisture level in the slab. While they can be a useful survey tool, they typically operate at about ¾” depth and can be susceptible to other elements within the slab such as rebar, certain types of aggregates and more. ASTM F2659 provides guidance to use concrete moisture meters as a means of comparative measurement only.

Ultimately, understanding the distribution of moisture in a drying slab makes it apparent that surface-based test methods are unreliable at best.

Moisture levels in a drying concrete slab tend to be higher at the bottom of the slab but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Therefore, any test method that doesn’t measure below the surface into the slab cannot give an overall picture of what moisture levels will become once the flooring is installed.

MYTH #6: Drying time can be estimated accurately

An industry rule-of-thumb for drying concrete is to allow one month of drying time for each inch of slab depth. However, an understanding of the process of moisture movement in a drying slab will show why any attempt to estimate a slab’s drying time can be a gamble.

In order for concrete to dry, a large percentage of the excess moisture in the slab must be able to reach the surface and evaporate. Moisture within a concrete slab follows a number of small pathways, or capillaries, that form as it combines with the initial mix elements–cement, aggregate, and any other admixtures.

If these pathways are blocked for any reason, or if they are not dispersed well through the slab, the amount of time it takes for the moisture to move out of the slab will be slowed significantly.

An overzealous troweling job, or sometimes even a curing compound applied to the surface, will prevent the moisture from escaping the slab, and that’s only the direct influence on the drying process. Ambient temperature and RH humidity conditions also have a very significant impact on concrete drying times, so any change in RH or air movement and temperature will ultimately affect the final drying schedule.

Only ASTM F2170 in situ RH testing can monitor the actual moisture conditions inside a concrete slab for dependable results.

MYTH #7: All RH tests are the same

Rapid RH L6 Insertion ToolAll RH test methods are the same in that they are based on research that demonstrates how internal concrete moisture measurement (measurement is taken below the slab surface) is proven to be more accurate when testing concrete moisture conditions. However, the way each RH testing product is designed can have significant impacts on testing time, accuracy and performance.

Most RH test methods use removable sensors that must be inserted into the test hole and then allow time to acclimate to the hole for each and every reading. (Calibrations for each sensor must also be checked no more than 30 days before each use.) This type of removable sensor process can make RH testing a time-consuming and tedious process that often involves more waiting than testing. It also leads to “leapfrogging”: moving the sensors from one hole to the next, often with insufficient time allowed to let them equilibrate to each new test hole.

Only the Rapid RH L6 features Smart Sensors that install directly into the slab for a one-time acclimation to the test hole and instantaneous readings taken with the Total Reader or DataGrabber.
Calibration is always certified since each Smart Sensor comes with a NIST-traceable certificate of calibration.

Use the Datamaster L6 app for data gathering.

MYTH #8: Test hole depth is approximate


Click the image for a larger view

The target of in situ RH testing is to be able to determine the moisture condition of a concrete slab once the floor covering is applied. Why? Because, if the moisture condition is too high, it puts the entire floor system at risk. More than a billion dollars is spent on repairing moisture-related flooring failures each year.

It’s important to understand how moisture distributes in a drying concrete slab. Moisture levels inside the slab tend to be higher at the bottom of a slab that is unsealed but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Until the slab is sealed, the depth of the test hole will definitely impact the accuracy of the RH reading.

That fact is why standards like ASTM F2170 are in place–to ensure that testing happens at the correct depth. For slabs drying from one side, like slabs poured on grade or pan decking, 40% has been proven to be the correct depth for RH in situ test holes. If a slab is drying from two sides, testing at 20% of the slab depth is necessary for accurate RH test results.

Only in situ RH test holes drilled to the correct depth will provide results that reflect true moisture conditions in the slab and allow the flooring installer to proceed with accurate information about the moisture conditions that will be in contact with the installed flooring once the job is complete.

MYTH #9: Concrete additives don’t impact drying time

The irony of this concrete myth is that some concrete admixtures are MEANT to impact drying time. Superplasticizers, for example, like lignosulfonate or polymer compounds, are designed to reduce moisture in concrete slabs so (in theory, at least) drying times can be reduced.

Other additives cover a range of intended uses from reducing shrinkage or damp-proofing to extending workability to adding pigments for color applications. Each class of admixture chemically enhances or changes the properties of the concrete batch, and can subsequently impact the actual drying time of the slab.

Different admixtures can also impact certain types of moisture test methods. Only RH testing provides the most accurate moisture testing results for any drying concrete slab.

MYTH #10: Dust on a job site doesn’t impact RH testing

Dust and grit may seem inevitable on a job site involving concrete, but it really can impact the accuracy of RH testing if the test hole is filled with residue from the drilling process. By compromising the air volume around the sensor, excess dust or concrete residue limits the ability of the sensor to provide accurate readings.

Between readings, use flush-mount protective caps to not only help identify each hole but also rubber seal to prevent site contaminants from getting into the test hole as well.

Myth #11: You can accurately estimate how much time a concrete slab needs to dry

A popular method for calculating how much time a concrete slab needs to dry before the flooring can be installed is to allow one month of drying time per inch of the slab. While this gives a rough indication of what to expect, it’s an approach that overlooks completely the many variables that can impact how much time a given concrete slab needs to dry.

We know that moisture has to move through the concrete to evaporate. How easily it can move through a slab has a significant impact on drying times. The moisture needs to flow through the capillaries (small pathways) in the concrete. The size and number of capillaries present depend on the unique mix of cement, aggregate and other admixtures used for that slab. The amount of water used in the mix also impacts how much moisture needs to evaporate.

These factors account for what’s happening inside the slab. However, outside factors directly impact drying time too. Aggressive troweling or applying certain curing compounds can trap moisture underneath the surface. The ambient conditions around the slab, such as temperature, humidity, and airflow, also affect the drying time.

Considering how all of these variables play into the drying time equation, why would anyone risk guessing, especially when there are cost-effective relative humidity (RH) tests available that accurately measure a slab’s moisture condition?

Myth #12: Old concrete is already dry

This statement may seem to make sense. If a concrete slab has been around for years and has already had flooring installed over it, surely it must be dry enough for new flooring to be installed.

Unfortunately, this is wrong. Many older flooring systems used products that were more moisture-resistant but also had a greater environmental impact, than many of the products used today. This means that an old concrete slab may be holding on to moisture that couldn’t escape the older, more moisture-resistant flooring. That moisture could wreak havoc on the new flooring.

There could also be unidentified sources of external moisture (e.g., a plumbing leak or weak vapor retarder) that have been affecting the slab for years.

For these reasons, never assume an older slab is sufficiently dry for your flooring application.

Myth #13: The surface of the concrete slab is dry, so the flooring will hold

Moisture Gradient SchematicScientific studies have shown that the dryness at the surface of a concrete slab won’t necessarily reflect the slab’s overall moisture condition. Ambient conditions discussed under myth #1 can cause the slab’s surface condition to be very different from what’s going on below, where it matters most.

In addition, as moisture moves through the slab and evaporates at the surface, a moisture gradient invariably forms, such that the moisture condition deeper within can be quite different than at the surface. Because of this moisture gradient, any concrete moisture test that only measures at the surface, such as the widely used calcium chloride test, can’t possibly give reliable results.

The moisture level that matters most is the slab’s point of equilibrium; that is, the moisture level that remains after the flooring installation and once the moisture gradient evens out. That’s the moisture condition that the installed flooring will “see” for the long haul. The only way to get an accurate, reliable assessment of that moisture condition is by taking measurements deeper within the slab, and RH tests are specifically designed to do just that.

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

Myth #14: The exact depth of an RH testing hole doesn’t matter

The depth of the RH testing hole matters a lot, which is why the ASTM F2170 standard for RH testing specifies the exact depth for conducting the test. Why is depth so important? We know that moisture levels within an unsealed slab can vary a lot as moisture moves up and out of the slab during the drying process. Taking measurements at various depths will yield significantly different results.

Rigorous scientific studies have determined the best depth for taking RH measurements, in order to get a true picture of the long-term moisture condition that will exist after the flooring is installed. This is the depth required by ASTM F2170. No guessing or estimating (see myth #1) allowed.

For slabs on grade and drying from one side, the precise RH testing depth is 40 percent of the slab’s thickness; for slabs that aren’t poured on grade and are drying on two sides, the exact depth required is 20 percent.

Myth #15: RH test hole depth is all that matters for an accurate reading

The RH test provides the most reliable way of getting an accurate measurement of a concrete slab’s moisture condition, but it has to be done right. Placing the RH test probe at the correct depth is certainly critical (see myth #4); however, other issues can interfere with your test results if you don’t address them: dust and grit.

When you drill holes for placing the RH probes, you must clean them out properly. RH test kits, such as those from Wagner Meters, come with a wire brush and special vacuum attachment for cleaning out the hole before it’s used.

If you neglect this step, dust and grit can alter the air volume around the RH sensor, which in turn compromises the sensor’s ability to take an accurate RH reading. To ensure particulates don’t contaminate the hole between readings, make sure to plug the top of it with a protective cap.

We now know too much about how moisture moves through concrete and how to test accurately a slab’s moisture condition to remain slaves to myth. A properly administered RH in situ test, as set out in ASTM F2170, relies on the science instead. Myth or Science? Which do you want to rely on?

Bonus Question: Can Moisture Come Up Through Concrete?

Concrete is like a sponge absorbing moisture from under the concrete which causes the moisture to come up through the concrete capillaries. Typically, this will cause moisture-related flooring problems or cause mold and mildew growth.

Additional information on concrete moisture testing: Rapid RH Test

Imagine a photographer taking beautiful photos but not saving them in a file. Pointless.

Why take readings if you’re not going to save them? Those are your insurance. If anything happens to the floor, go back and look at the readings—that will help you figure out your next steps.

And the data could even keep you from a lawsuit by showing that you adhered to the proper procedure for taking moisture measurements.

Ultimately, keeping thorough records of all your data only helps your reputation. Your reputation will speak for itself when future customers see that you operate with integrity.

Some meters have a data recording and storage feature that can make saving the readings much easier.

The best meter is…

…the one you’ll use.

Sounds simple, but if you buy a moisture meter that’s confusing to use or one that takes too long to get readings, you won’t use it. Because, after all, the worst thing you could do is not use a meter at all.

So do your research. Find the moisture meter that meets your needs, includes important features (such as species settings), and is in your budget. Then, use it.

And if you’re looking for one that has all the bells and whistles—a true one-stop-shop meter—then consider the Orion 950. Since this meter can do everything you’ll need for nearly every job, it’ll quickly become one of your must-have tools.

Previously published in Tomorrow’s Contract Floors magazine

Last updated on April 19th, 2024


  1. Henry Killingsworth says:

    It stood out to me when you mentioned that a moisture gradient can form in concrete slabs. As far as I know, many roadways around the world are made out of concrete because of its availability. It seems like making roadways moisture-resistant would be the best way to prevent damage from occurring.

  2. Ed says:

    I am remodeling my basement. The contractor put plywood directly on the concrete in some areas for sub flooring and joints in other areas because the floor was not level. Is this a problem? I’m on a hill in San Francisco but if I cover he concrete overnight with plastic, I see some moisture. Thanks!

    • Jason Spangler says:


      Thanks for the questions. I would expect that it would be best to have some form of a vapor barrier between the concrete and the wood plywood and/or dimensional lumber. Your local building codes may specify more directly. Good luck.

  3. Paul says:

    Very interesting info. I’m planning on installing a solid wood floor on first floor level concrete in a closet. I want to glue directly to the concrete and I think I’m aware of all the moisture considerations. My moisture reader device was showing 45% for the concrete yesterday. Today it’s 99.9%. This is when I lay it down on the concrete and use as directed. Can the reading vary that much? The slab has been in place for 20 years and we have no leaks or issues.

    What level of moisture is safe to install the floor on? I’ve ready about some sort of equilibrium between the flooring and the concrete. The flooring as it lies in the same closet is reading less than 10%. They’ll never be ‘equal’ will they?

    Maybe a dumb question but if the glue available contains moisture barrier, why isn’t it sufficient to protect against the moisture coming up through the concrete? If it’s only supposed to help, what is the acceptable moisture content in the concrete whereby the glue is enough protection?

    • Jason Spangler says:

      Thanks for the question. Unfortunately, when you speak of 45% and 99.9%, giving no context of what the % designates, it is hard for me to answer this question. If you are going to glue a solid wood to concrete, the baseline moisture condition should be 75% or less relative humidity when measure with a device installed into the concrete. The wood, prior to installation, should be acclimated and equilibrated to the condition on the air in the environment where it is being installed. This is done by taking a relative humidity and temperature of the air and calculating the moisture content of environment. We have an app called Wood H2O. Once the wood has reached this number and the concrete is below 75%, in most cases you are ok to install. Always check with the manufacturer’s.

      You may be correct with the glue, but you need to check with the various manufacturer’s about acceptable levels because they can be very different. Good luck.

  4. Brenda muriel says:

    Outside of our front door is an alcove which is about the size of 4×6. It has a concrete floor and 4 concrete steps allowing entrance into the house. The concrete is butted by blacktop from our driveway. The is about an inch gap between the concrete and blacktop that is dirt. Our home is over 50 years old. For the past month we have noticed that the concrete is wet, more moisture, like it was wet and now drying, it covers all of the back to the house and a little ways up the foundation, plus on the side of the stairs, about one step up. The strange thin is it does not appear on all the concrete, there is a pattern from the back which runs to TBE driveway, but only up the middle. We are baffled because it dries then it happens again. We have no idea who to call to get this checked. There are no visible lipid leaks in our home. We do have well water. Can you advise?

    • Jason Spangler says:


      Thanks for the questions. I wish I could be more help, but I think I would start with calling out a leak detection company and then if that checks out some type of geotechnical engineer for evaluation. Good luck.

  5. John Milwaukee says:

    Wow this is such great content on concrete! I just had a new slab put in to extend my driveway to the side. It went smoothly with the contractor I hired and they were professional and nice. The guy I talked to told me the “industry standard” of one month before we could drive on it. He even told me that the fall season is the best season to do concrete since the cooler weather allows for more even drying. Not sure if that’s accurate based on your article, but it made me curious. You mentioned that overall temperature and RH do impact drying time, but does concrete harden better in cooler weather?

    • Jason Spangler says:


      Thanks for the question(s). I would agree that his “one month” standard would be accurate, at least for heavier loads. He is talking about the hardening of the concrete, not necessarily the drying. At 30 days, the concrete has reached most of its compressive strength. Regarding the cooler temperatures, when you have higher temperatures, the concrete will tend to dry quicker on the surface, and having that inconsistent heat/temperature isn’t necessarily the best for cracking, curling and other surface conditions of the slab.

  6. Laura says:


    This is a great wealth of information and I thank you for posting the myths and questions. I am stumped trying to figure out how to help my elderly mother with her condo build on a slab in a planned urban development. Lots of homes are being thrown up in no time flat. I am told that there is a vapor barrier below the slab but not one under the garage. Mom moved into the condo in mid June of ’17, the unit was brand new. It was very cold during a hot summer, that was appealing to her that she wouldn’t have to run up the electric bill with the AC. By mid September ’17, her furniture and belongings exploded into very heavy low lying mold. The builder cleaned the unit but said he did everything correctly and denied any need to test the slab. He said it was impossible to be the cause. I hired engineers to test the slab in 4 location throughout using your RH Rapid Test sensors and pen. Our pen read HI for all locations. It has continued to give those readings throughout the 2 years the sensors are valid. An engineer recorded the results. Today, the sensors still give me readings (we purchased the reader pen).

    2 commercial dehumidication units were installed in the 1600sf unit. Sensors in every room with the exception of the open concept kitchen/living area. 2 sensors could only be installed in this location and it is clustered on one side of an open large area. In the middle of the room, a good distance away from the closest sensor, we saw green’ish mold coming up from a seam in the floating vinyl floor. Since we can’t put a sensor in the middle of the room for appearance purposes, could this slab be so saturated that plastic floors would mold? My Mom’s health is being affected. Her eyes are bothering her again (noticed the eye problem before the mold appeared). Is there any way to dry this floor or is this place a total loss without drastically dry humidity settings on the commerical dehumidification systems?

    We considered a leak but requested water bills for similar buildings with similar occupancy. The water usage is similar enough not to spark further interest in pursuing that lead.

    Do you have any ideas that might help me? I’m prepared for bad news so don’t hold back. I just want to know what our options are at this point. Many thanks Jason!

  7. You made a good point that plumbing leaks can cause some moisture problems for a concrete slab. Perhaps I should first map out the pipes under my garden before decided where to build a new concrete patio. Getting the patio to last will definitely make my house look a lot more attractive and probably raise its value significantly too.

  8. Craig J says:

    Hi Jason,
    I was reviewing a clients basement water issues and ran across your article, just sending a quick thank you for writing it, definitely a big help, thanks and stay safe

  9. Hello!

    Thank you for this very informative article. I was curious what you think about climate and how it effects the cement material through-out seasons ranging from 100 degrees during the summer to below 32 degrees during the summer.


    • Jason Spangler says:


      I will be honest and say I have little experience on this specific topic, but over the years many batch plant facilities and manufacturers have come up with very effective ways of either increasing or decreasing the hydration process based on conditions such as the extremes you outline.

  10. brandon smith says:

    We install epoxy coatings and have run into problems with moisture when a slab has had rain on the slab and we want to coat it. The RH meters we have tried seem to have a hard time reading surface moisture. They usually give us an acceptable reading for the concrete slab even when there is still visible moisture. We will usually fall back to the plastic film test in this scenario however we are trying to come up with a more accurate and more importantly a faster way of measuring this surface moisture. Do you have any suggestions?

    • Jason Spangler says:


      Thanks for the comment. For the long term viability of an interior finished floor, RH is the most accurate concrete testing method out there. In your specific application, you might want to take a look at a topical surface meter like the C555. Just keep in mind that this device, like others of the same type, are not really intended to make installation decisions, but to find suspect areas of surface moisture. With our C555 the concrete needs to also be very smooth to get the best results. Good luck.

  11. Kevin Schreiner says:

    Centennial, Colorado (very dry climate) I have 10,000 Sq. Ft. new construction. The concrete floor was poured in February. Our flooring company is ready to install flooring of 4 different types: Artificial Turf, Rubber, Vinyl, Synthetic Wood. The installer wants to do concrete moisture tests (drill 12 holes in the concrete at $80 per hole. This is new to me. Is this legitimate?

    • Jason Spangler says:


      Thanks for the question. This is more than legitimate (and fairly priced) and you should thank this installer for looking out after your best interests, enabling him to help ensure he can install a product with a warranty. All manufacturers of flooring/adhesive/coating products require moisture testing to ensure a lasting installation. Let me know if you have any additional questions.

  12. Nancy Brooks says:

    I live the Birmingham, Alabama. I have had carpet on a concrete floor for over thirty years and no problems. I recently had manufactured woods installed. The floor started bulking within three weeks. Part of the house is underground. The Company installing the floor did no testing. They are not taking any responsibility. I bought a meter. The humidity levels near the underground area are in the 90 percent range. No water is visible and no orders have ever been detected. The floor is partially removed. Shouldn’t the flooring company have measured the levels of humidity ? What next?

    • Jason Spangler says:


      Thanks for the question and sorry for the issues. You would have hoped that testing would have been done so proper actions could have been taken, if necessary, prior to installing the new floor. With carpet, since it is breathable, you may have had a moisture issue in the concrete the entire time, but it never affected the carpet. Now that something has been installed that is less breathable, the problem is showing up. You could probably bring in a flooring inspector ( ) and try to determine fault and then seek some type of remedy. On the other hand, you could seek out another reputable flooring installer and have them do the job correctly, armed with the new knowledge you have learned. Not sure either is a great option, but I hope this helps.

  13. Judy Ward says:

    We are trying to replace an existing laminate wood type floor on a slab, have been drying out with dehumidifier and fans for 3 weeks and he just checked levels and are still at 4. Can we seal with those levels and then lay new flooring down?

    • Jason Spangler says:


      Thanks for the question. Unfortunately, I am not sure I am familiar with a scale for moisture in concrete that measures 4. I would check with the laminate flooring manufacturer and get their recommendations for your situation. Good luck.

  14. Roxann M says:

    Hi Jason wanted to get your opinion on a situation we have experienced, our house is a couple years old built by hey high end commercial Builder. We live in North Florida and had new flooring installed in January during weather in the 70s. The flooring was manufactured to order it is engineered hardwood. The flooring was installed within a week to two weeks of delivery from manufacturer. prior to installation we had all ceramic tile floors that was removed by the ride on machine method. A dritac moisture barrier glue was also used in the installation process. After a few weeks of the installation we started to notice there was some peeking of the ends of the long part of the planks. They are not really visible in certain lighting although when there is a glare coming from some of the windows and sliders you can see them throughout the floor. Also would like to add that the dining room and office had a cheaper hand-scraped hardwood that was installed when the house was built we did not notice any problems with this although it has a dimmer lighting and also being hand-scraped we were not able to detect if there were any issues with that flooring. Do you have any ideas why this would be happening and could this be an issue with the manufacturing process. Also will a tramex cmex surface moisture meter give us any clues.

    Please help

    And thanks, your expertise is is appreciated

    • Jason Spangler says:


      Thanks for the question. This could be any number of issues, probably all related to moisture in some way. How was the wood acclimated? What were the moisture readings of the wood prior to the install? Based on the installed environment, what was the target moisture % of the wood? What were the moisture readings of the concrete prior to install? Have any conditions in the house changed after install? I would recommend that you look on this website and find a certified inspector to come out and evaluate the issue. Good luck.

  15. Bret Loomis says:

    Just removed vinyl plank without moisture barrier or underlayment product. moisture was present with a few areas of mildew. Immediately treated concrete with a bleach water solution by pump sprayer
    I intend on laying rolls of moisture barrier underlayment that will be taped together, then installing a better vinyl plank product. I am assuming that water may do the same thing under the vapor barrier. I am thinking that I might want to apply a concrete sealer to trap water below the surface before the underlayment. Do I need to worry about this?

    • Jason Spangler says:


      Thanks for the question. This microbial growth issue is one of those issues that, in my opinion, will start to become more prevalent with the increased usage of “loose lay”, products. I believe you should be thinking about this and try to be proactive. Good luck.


  16. Chris says:

    We installed carpet tiles on old concrete floor. We used a sealer glue to install the carpet, but in one spot the water comes back thru the concrete. Can you tell why this keeps happening.

    • Jason Spangler says:


      Thanks for the comment. Based on the very limited information you have provided I would question whether there is an intact vapor retarder under the slab and the center happens to be the path of least resistance for the moisture to escape from the soil. The one way to make this determination is to do a core sample in that center area.



  17. Sushmit says:

    Dear Jason.
    Thanks for your article.. It was very helpful.
    I have a small query.
    If I have to do epoxy floor coating on concrete structures, how much should be the ideal moisture content (%wise) or RH to start the work?
    Means what is the acceptable value of RH for starting the epoxy floor coating.

    • Jason Spangler says:


      Thanks for the question. It depends on the specific coating, but many epoxy coatings I have seen are 75-80%RH. Again, make sure you contact the specific manufacturer to verify.



  18. Jill Reeves says:

    My husband and I purchased a condo in Largo, FL. We had an inspector look at everything, there were 2 minor leaks, one in the shower and one in the laundry room, no big deal, the plumber took care of that. so we went to proceed with the remodel which included new flooring. We took up all the tile and the cement beneath it was wet. So we ran the AC on high and it dried up a bit but the ac went out for a week until we could get it repaired and it was soaked again, you can see where the old grout lines are and it is the ENTIRE floor, not just near plumbing. Of course now we have mold issues being in Florida it grew fast, so we have ripped out the drywall, etc. etc. Not to mention that when we ripped up the tile we discovered all of the lower cabinets were moldy and wet as well, so all of this was going on underneath the tile and nobody knew it (or said it anyway before we purchased). The HOA insists that its not there problem if the cause of the problem is not in the walls or under the cement. I have searched and searched for a company that can test the cement to tell us what the issue is, meaning is it a failed vapor barrier, one guy suggested too much ash in the concrete, at any rate it is clearly coming from the ground. That being said. Are your products user friendly for a non-professional to use and affordable? Also, do they have the potential to tell us if the moisture barrier has failed or if say water is coming up from the ground due to the small pond on site or a large leak from the water company. I know it won’t magically say it, but I’m assuming there is a scale and that you have some kind of guide that would point us in the right direction. Then if we did determine what the issue is, would your company have information on how to best handle it? We really are at our witt’s end and no one knows what to do. the complex was built in the 60’s and obviously it is a first floor unit. The HOA is no help. Reports from neighbors, right next to us she said the flooring people put down a sealer that was pink then turned red (or vice versa, she couldn’t remember) and on the other side (renters) noted that they do feel squishy floor every now and again. Please say you can point us in the right direction, we would be ever so grateful.

    • Jason Spangler says:


      Thanks for the questions. I am narrowing it down to the one main question and that is who. In my opinion, I would want to verify there is a vapor retarder first. Probably your best bet would be a geotechnical engineer to do a core sample of the concrete slab to definitively determine the answer to this question. You might want to start here Good luck.



  19. Tim Hamelink says:

    I live in a ranch style home in the southeast part of wisconsin. My wife and i purchased the home about 5 years ago. I have a problem with
    moisture in the winter months. I think the house is so tightly sealed that just a little difference in the weather that condensation starts to show on my windows. I was thinking that the extra moisture is coming from the basement concrete floor.. The house was built in 2005 and is on a hill. My sump pump crock is always dry and when we get a heavy rain storm the pump seldom runs. My question is would the basement floor and walls be a reason for my high level of humidity in my home. i was trying to find a way to measure the moisture in the floor.

    • Jason Spangler says:


      Thanks for the question. Your basement and walls MAY be a reason for the issue. I think the first thing I would check though is whether it is a dew point issue. If you purchase an inexpensive thermos-hygrometer that calculates dew point and an Infrared thermometer you can determine easily if dew point is a potential culprit. The thermo-hygrometer will read the relative humidity and temperature in the air and based on those number, calculate a dew point temperature. With that temperature, you can go around to areas in your house that are showing signs of condensation and use the IR thermometer to measure surface temperature. If the temperature is at or near the calculated dew point, then that is more than likely at least part of the problem. Knowing that many parts of WI are drier in the winter, something as easy as adding humidity to the inside air may help. Here is a calculator that will help you see the various impacts of temperature and humidity on dew point.



  20. Scott Calev says:

    Like your site. I have a wine cellar/ that I place in my home at the start of building. It has all concrete walls 10 feet down, then we build a wood ceiling with 2 x 10 and covered it with plywood and poured the main foundation over it. The basement leaks, we have tried everything. The entrance down is with a ladder through a 2 foot by 4 foot opening. There is never more than 4 inches of water, pressure in,pressure out. Can we just cap over the 2 x 4 entrance with concrete?? Or will the moisture forever come up through the concrete and be an issue if we sell one day. The basement is over the utility room, not a living space.

    • Jason Spangler says:


      Thanks for the question. I would have a structural or soils engineer come out and evaluate the situation. I would imagine you will still need to potentially do some waterproofing on the wall, in general, after a solution has been found.



  21. Gilbert Bryant says:

    Two years ago, my 40 year old house on a concrete slab flooded (6 inches of water for a few hours). Following a 6 month drying out period, new wood flooring was installed where carpet once existed. Now, in selected areas, moisture from the slab has stained the wood and required the removal of a very minor portion of the flooring. RH testing of the slab in those areas is very high despite no cracks or deteriation of the slab. What is your recommended remediation before the wood flooring is replaced?

    • Jason Spangler says:


      Thanks for the question. Trying to selectively mitigate areas like this may be difficult and tricky. I would work with a certified wood flooring installer in your area to remedy these issue. You can find one by looking at Good luck.


  22. Pat says:

    I have pulled up carpet due to plumbing leak in my 1992-built basic slab Houston area track home. While letting the concrete dry, I’m debating whether to paint the floor instead of laying LVT planks. Wondering if the slab needs to “breathe” and if painting latex Porch Floor paint would allow it to breathe?

    • Jason Spangler says:


      Thanks for the question. MOST paint is quite breathable. In your case though, it is also latex and may be more susceptible to moisture, especially is the surface of the concrete is wet. The slab may need to “breathe”, but we can’t really answer that question because we don’t really know if or how wet it is. You would typically perform some method of moisture testing to quantify if there is a problem. Good luck.


  23. Jan Blue says:

    My house is on a concrete slab and I had a leak in the bathroom wall that sent water to the adjoining room. Didn’t know for a week until discovered the carpet wet in the room. Pulled up the padding and rug to let the concrete dry. After two weeks my moisture meter still show 100% in the area that was wet. Other areas of the room not effected by the leak show 30 to 50%. How long will it take to dry that I can put carpet back? How do I accelerate the drying? Thanks for any advice you can provide.

    • Jason Spangler says:


      Thanks for the question. Its hard to say how long it will take, but air movement across the surface area of the affected area might help expedite the process. Also, depending on the size of the area, it may help to use some type of dehumidifier. Good luck.


  24. JoAnn Long says:

    Jason ,hope you can help. I have parquet flooring throughout my house and it has buckled to the point of no return. Insurance co say’s they believe it’s a
    busted water pipe under the slab., call a plumber. Plumber say’s no busted pipe. he say’s I need to put a drainage pipe around the back and side of my
    house and do waterproofing also. I’m at a loss, I don’t know what to do or who to call. I need to put down knew flooring but I need to find the root of the problem first. I keep doing research but that’s only confusing me more. The parquet was glued down on the bare slab and now I can pull it up with my bare hands. I don’t want parquet again nor do I want carpet. I was thinking about vinyl that clicks together but I’m also concerned about the same issue.
    Any thoughts on the matter would be greatly appreciated.

    • Jason Spangler says:


      Thanks for the questions. You really have too many variables you have thrown out here and we would go back and forth with various questions and answers for me to even begin to formulate a reasonable response. One thing you haven’t told me is what the actual moisture levels are in the slab. This is information you need to determine in order to start formulating a solution. Beyond that, it would be more efficient for us to have a phone discussion. Please feel free to contact me at 800-634-9961.



  25. Peter Giri says:

    A fascinating read indeed. I have a query. Can you please answer this question?

    If a dry concrete is placed in an environment with high relative humidity, can the humidity inside the concrete structure be higher than the surrounding environment because of the moisture absorption by concrete structure? Say, the relative humidity of surrounding is 90%, can the relative humidity inside the concrete be more than that?

    Thank you.

    • Jason Spangler says:


      Thanks for the comment/question. You will usually find the two environments, concrete and the ambient conditions will attempt to reach equilibrium with each other. Depending on the variation in RH% between the two, the process will start fast and then slow as they get closer to each other. Unless there is another source of moisture, say the soil, then the ability of a “dry” concrete to be higher RH% than a consistent 90% RH ambient environment is negated.


  26. Steve says:

    Hey – can you help?
    My builder has just put down a sand/cement concrete screed floor at 67mm thick (under floor heating beneath) and wants to lay engineered wood flooring on top of it in 2 weeks.
    Everything i am reading suggests waiting maybe 2 months! What would be your advice and the best cost effective gadget to measure this?
    Would it help in 4 weeks putting down a DPM – paint liquid version all over the floor?


    • Jason Spangler says:


      Thanks for the question. If it was my project, I would be asking the installer how they were going to determine the screed was sufficiently dry for flooring installation in that two week period? I would also get the installation documents for the engineered flooring and the adhesive to be used. In these documents, there will be specific requirements for moisture testing and thresholds of acceptance. If the method the builder is prescribing to moisture test and the manufactures methods aren’t the same, then you are better informed during your conversations with the builder. DPM’s are very effective, but they are not all created equal, so again, get the installation documents for products you may use and compare what their maximum moisture tolerance is and what their warranties include and exclude. Good luck.


  27. Vincent Pezzino says:

    I am stumped !! I am a concrete restoration contractor in Florida. I have a project that has elevated walkways. ( 2 ways for vapor to exit ) The assembly is as follows. The topping cementitious coating is the top layer. Underneath is a cementitious waterproofing membrane. Under that is a cementitious flash patching material. The flash patching or third tier down is crumbling apart after 2 years of being installed. As you can guess, the 3 tiers are now coming up. Before I read your facts on moisture meters I did check the elevated slab. High readings on the prog meter I used. What I find interesting is the underside of the catwalk has been painted……over and over again for years. The paint is a breathable product but after a preliminary ancient test of taping cellophane to the ceiling I get zero moisture exiting 1 of the 2 exit points. Using the same exercise on top, moisture evident via water droplets. My Theron is the water vapo is taking the path of least resistance and exiting entirely at the catwalk surface breaking down the flash patch material.
    My next step is to remove an area of paint from the stucco and also in another area remove paint and stucco from the underside of the catwalk. My expectations is I will now see moisture. Would that concur with my aformentioned theory that the high amount of water vapor exiting the top portion of the catwalk is chemically not conducive to the material and breaking it down ?

    I can use some input if you have time.

    • Jason Spangler says:


      Thanks for the comment. Sounds like you have an interesting puzzle you are trying to put together. Your theory may be correct, but I give other things to think about.

      1) The plastic sheet test really isn’t a good test, especially in this situation. Many times, the presence of moisture under a plastic sheet can be caused by condensation related to dew point considerations. Given that surface temperatures of the concrete, even in an open environment, could be different because of direct vs indirect contact to sunlight and varying ambient conditions, the dew point threshold would vary between surfaces, potentially causing the “moisture” to appear on the plastic. This being said though, this would have little to do with the moisture coming out of the concrete.
      2) You talk about moisture moving out of the bottom, but what about moisture moving into the bottom due to high relative humidity in the air and higher temperature on the top side of the slab? Although you theoretically have two directions for moisture to move, having varying conditions, both temperature and relative humidity, exposed to both surfaces, may affect the direction of moisture travel. Concrete being hygroscopic, it can expel and absorb moisture depending on the environment.

      Unfortunately, I don’t have a solution, but I hope this information helps you in finding a solution. Good luck!


  28. Tony Branon says:

    Raleigh, NC . I am building a restaurant in the ground level of a parking deck. This means PC concrete walls and roof, like a bunker. some people contends that if I don’t spray-foam insulate the place, with the air conditioning and people coming in, at some point it will begin to “rain” inside. I really like the industrial looks of the place, but I am really concerned this may be a real possibility. what do you think?

    • Jason Spangler says:


      Thank you for the comment. With the addition of heat from the restaurant equipment and the in and out traffic, you may have an issue. The only way I know to be sure is by finding an engineer that can input the different variables into a computer model, which includes exterior vs interior conditions, concrete thickness, etc. I hope this helps.

  29. Jacob says:

    Hello Jason,
    We live in North Texas and found out that there is no plastic moisture barrier in our concrete slab. House built in 2004, but purchased 2 years ago by us. We put down a vinyl plank with no moisture barrier and it started buckling and bowing after only a fe months. I finally took it all up and found that there was moisture underneath (Not enough to pool), and the smell of mildew. What product should I put down? Moisture barrier under new product to trap any moisture, or something that might allow it to breathe a little such as a ceramic tile? Any ideas would be appreciated.

    Thanks in advance!

  30. Brian Benford says:

    I installed a vinyl tile on a concrete slab in February.The installation was fine until the following summer when the flooring started to lift.The installation site was in Northern Ontario Canada where the outside temperature is normally way below 0 degrees Celsius ,from late October to mid March.Average temperatures would be minus 10 for those months.
    Is there more moisture in a on grade concrete slab in the spring and summer than in the cold winter months??

    • Jason Spangler says:


      Thanks for the comment. If there is no vapor barrier below the slab, then moisture IN the slab will fluctuate based on the amount moisture in the soil below the slab. Hope this helps.


Leave a Reply

Your email address will not be published. Required fields are marked *