5 Myths about Concrete Moisture

In a time when a myriad of information is available for the building and flooring industries, it’s always interesting (and a little frustrating) when some common misconceptions about moisture testing concrete will cause building professionals to assume they are not at risk for moisture-related problems with their flooring installations.

Let’s look at a few of them.

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

For most slabs that dry from one side, the surface is the only escape route for the moisture in a drying concrete slab. As moisture evaporates away from the surface, more moisture can move up through the slab. Therefore, it seems reasonable that if the surface is dry, it could be taken as a sign that there is no more excess moisture in the concrete.

However, it has been demonstrated repeatedly 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, troweling techniques, and more–conditions at the surface in no way reflect the presence of moisture within the slab, and leave any test method that only tests the surface to be highly suspect.

The only way to accurately determine when a slab is “dry” is through relative humidity (RH) testing that places a sensor inside the slab away from the surface. RH testing at the correct depth gives a clear picture of the final concrete moisture measurement 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 or some flooring materials were more consistent in their moisture tolerances because most products had a petroleum-derived base in their compositions.

Although moisture-related flooring problems have always plagued the industry, as formulations began to change for specific applications it became increasingly important to be sure that concrete slabs under the flooring installations were dry in order to prevent moisture-related problems down the road.

Floorling AdhesiveWith the recent advent of the industry’s focus on lowering VOCs (volatile organic compounds) in building products, different adhesives and flooring products on the market today can vary greatly in their tolerances to extended elevated moisture conditions.

For the best results, accurate RH testing should always be performed in order to a) determine the current slab moisture condition, and b) be sure the product specified is compatible with the slab moisture levels, or c) choose an alternate product that meets the current moisture conditions of the slab.

For a quick link to a multitude of manufacturers that specify RH tolerances for their products, visit www.rhspec.com.

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.

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.

How is that possible?

Part of the 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.

Hence, an older floor that has never shown signs of moisture problems may still be too high for the RH sensitivities of the new product.

It’s also possible, of course, that an older concrete slab can be facing moisture intrusion from an unidentified source (a compromised vapor barrier, an appliance or plumbing leak) that has raised the slab’s moisture content but not yet reached critical levels for the older flooring.

The best insurance for older concrete slabs is still to do RH testing before installing a new flooring system. The unique design of the Rapid RH® 4.0 EX seals a sensor in each test hole for immediate test results. It’s fast, accurate and affordable testing that saves on your bottom line by allowing ASTM-compliant testing at the touch of the Easy Reader.

MYTH #4: The slab is not on grade so I don’t need to do moisture testing.

All concrete is affected by ambient conditions, and all concrete is susceptible to moisture-related complications if not adequately tested before the next stages of construction begin.

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

1. 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 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.

40 percent depth2. For RH testing, studies have shown 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.

RH testing for concrete is the only way to insure 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 a final accurate moisture level.

As was discussed above, any test method that relies on surface conditions is at risk of producing a false reading.

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

  1. The Calcium Chloride 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.
  2. The Hood Method places a probe or sensor on the slab surface and again seals it under a moisture-resistant, insulated hood in an effort to mimic the conditions that flooring would be exposed to if it were installed.
  3. 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: rebar, certain aggregates and more.

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. 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.

Non Surface Testing Is Accurate and Affordable

When planning to install any type of flooring or finishing product over a concrete floor, only accurate Relative Humidity testing, like with the Rapid RH® 4.0 EX, can provide accurate, affordable and industry-compliant concrete moisture testing. Don’t let some of these common “myths” about moisture testing become a costly mistake for you.

Additional information on Concrete Moisture: Concrete Moisture Test

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.


  1. 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.


  2. 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!

  3. 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.

  4. 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!


  5. 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.


  6. 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.


  7. 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.



  8. 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.


  9. 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.


  10. 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 http://www.nwfa.org. Good luck.


  11. 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.



  12. 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. http://www.dpcalc.org/



  13. 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.

  14. 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.

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