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 concrete moisture testing 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 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 gives a final 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.

RH Testing Is Accurate and Affordable

When planning to install any type of flooring or finishing product over a concrete floor, only accurate RH 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: 5 More Myths about Concrete Moisture

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Jason Spangler

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!

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