How to Choose the Right Concrete Moisture Testing Method for Your Project

A successful flooring installation means taking into account how moisture can affect the finished product.

Concrete naturally continues to release moisture even after it has cured (hardened). Even if everything else is done correctly, flooring installed over a concrete slab before it’s dry enough can cause the floor covering to warp or become damaged.
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That’s why reliable moisture testing is essential.

So which method of moisture measurement is the most accurate? How do you decide which one to use for your projects?

Let’s go over each method to help you make the right decision:

Pros and Cons of the Common Methods of Concrete Moisture Testing

These are the three most common ways to test for moisture in concrete:

  • Calcium chloride test
  • Using a concrete moisture meter
  • Relative humidity test

Calcium Chloride Test

Calcium Chloride testing only reveals how much moisture has evaporated from the testing area during the testing period. It gives no indication of how much moisture remains in the concrete.

For calcium chloride testing, the area must be scraped, ground, blasted, or brushed clean so it’s free of any dust and debris. And before beginning the test, it must have stable ambient conditions for 2 days. Neglecting or rushing through these steps will certainly have an effect on the results.

For decades, the calcium chloride test was the most common method (ASTM F1869) for assessing if concrete could be considered dry or not.

First you would need to grind the floor in a section according to the ASTM F1869 standard.

Next, you’d weigh a dish of calcium chloride and record the weight (though many tests have a set amount of calcium chloride in each test, so weighing the sample is not necessary).

Then you’d place it on a clean section of concrete, seal it under a plastic dome to prevent outside air from affecting the results, and wait.

This test would have to sit undisturbed for 60-72 hours. During this time, the calcium chloride salt absorbs any moisture rising up from the concrete.

Once the time is up, you’d weigh the dish again. Or, you’d send the dish to a lab with highly-precise scales. The resulting weight is then used to calculate the MVER (moisture vapor emission rate).

In theory, this sounds like it would provide the needed information about the amount of moisture left in the concrete. However, this test has some significant problems with accuracy.

Calcium chloride testing only measures the surface moisture. And the surface is the driest part of the concrete slab.

Far more moisture usually lurks deep within the concrete slab, and all that moisture will continue to work its way to the surface long after the calcium chloride test is done and long after the floor covering is installed on top.

So even after quite a while, that floor would be susceptible to moisture damage from the remaining moisture deep within that slab.

Additionally, calcium chloride tests can be influenced by ambient conditions in the dome or if any outside air seeps in.

And readings on lightweight concrete could give different results because of the aggregate used in the mixture.

Due to these problems, the ASTM standard does not recognize calcium chloride as a definitive moisture testing method for lightweight concrete. Its results can imply that the concrete is drier than it truly is.

Using a Concrete Moisture Meter

using a concrete moisture meter

Concrete moisture meters can detect moisture within the top 3/4-inch of a slab. If you opt for a surface meter, you want the most accurate one available. Wagner’s C555 is twice as accurate as its competitors.

Testing with a concrete moisture meter is the quickest test available. In some smaller jobs, it’s the only practical test that can be used. The sensor pad of a concrete moisture meter, when placed flat on the slab, sends a signal into the concrete and can quickly take moisture readings.

Although it’s only measuring about the top 3/4 inch of the concrete, the readings can be taken quickly, allowing you to scan several areas of the slab in a matter of minutes. These readings can help you pinpoint areas of higher moisture that need to be investigated further.

It’s a much faster surface test than the calcium chloride test, which takes up to 3 days to tell you the possible moisture content of a small area of the slab.

Wagner’s C555 concrete moisture meter is the most accurate on the market. It makes for a practical, efficient diagnostic tool to narrow down high moisture sections for further testing. Also, the C555 moisture meter is a good way to determine the moisture content at the surface of the concrete.

Keep in mind, however, that this method of moisture testing alone doesn’t meet the ASTM standard. So if this is all you’re using to assess concrete moisture, you’ll want to check with your floor covering manufacturer to see if they allow a moisture meter to be used as the final determination on whether the concrete is ready for a floor covering.

But in some cases, such as smaller residential flooring projects, a concrete moisture meter may be the only practical method used to make this determination.

Relative Humidity (RH) Test

Relative humidity (RH) in-situ probes measure moisture at 40% depth, giving a more accurate picture of how much excess moisture remains within and will eventually equilibrate throughout.

The Relative Humidity (RH) test provides an in-depth look at the concrete slab with small sensors, inserted at a 40% depth in the concrete. This is the measurement area that will allow a person to determine what the future equalized relative humidity will be once the floor covering has sealed the surface of the concrete.

Being the only North American ASTM-approved concrete moisture test to break the surface of the slab, it provides a far more accurate look at the concrete’s true conditions than other methods of testing. A reading from deeper into the concrete will almost always report higher moisture than the surface, which gives you a more accurate picture of what the concrete will equalize to after the floor covering is installed.

RH testing is considered to be the new gold standard for concrete moisture testing. This method of testing adheres to the ASTM F2170 standard, which outlines the proper procedure for using and administering the test.

And Wagner Meters provides the quickest, easiest-to-use RH test, the Rapid RH testing kit.

Now that we’ve covered the different moisture testing methods, let’s talk about what to keep in mind when making a choice that best fits your needs.

Factors to Consider When Choosing a Method for Your Project

There are a few factors you’ll want to keep in mind as you’re making your decision. Let’s start with the scope and scale of your project.

Scale and Complexity

Another important thing to consider is the amount of protection your business, and this particular job, needs. What type of test will give you that necessary protection? One important concern to think about is any warranties you’ll want to be eligible for should a problem arise. In this case, they may give you options. If so, you want to choose a test you can trust without fail—one that will protect your installation and indicate whether a floor would be likely to fail.

Keeping both this and cost in mind, RH testing is quite often the best choice. It’s relatively quick, cost-effective, and the most accurate method. And if you also want to do a surface test, the concrete moisture meter is the best option. Combined, these tools make a formidable force against undetected moisture deep within the concrete.

The calcium chloride would not be the best option as a final test or as a surface test.

As a surface test, it’s more expensive than the meter, it takes longer, and it’s more labor-intensive than either the RH test or a concrete moisture meter.

What’s more, there is no science to back up its accuracy!

(However, you may run into the occasional manufacturer that still requires it for warranty purposes.)

Accuracy and Reliability

If you want any project involving concrete to be successful in the long run, accurate moisture testing is critical.

The most precise and reliable test by far is the relative humidity (RH) test, since it measures much more than just the surface. You’ll have readings that tell you how much moisture has yet to be released from within.

This helps you in planning ahead, so you can better determine when it’s safe to start laying flooring on top of the concrete.

Cost (Time and Money)

First, you’ll likely look at the price tag on each testing method.

At first glance, the calcium chloride method looks incredibly budget-friendly. Especially compared to a concrete moisture meter or an RH kit.

But there is much more to consider than just the up-front purchase price.

How much time will each method cost you?

  • A calcium chloride test needs 2-3 days for peak moisture absorbency. And that’s not including prep time and effort, since you have to grind the surface in the testing area and then wait 24 hours to begin the test.
  • A concrete moisture meter gives readings in minutes, so you can quickly get several surface readings around the slab. But if you’re using this as your sole method of concrete moisture measurement, you’ll need to check with the manufacturer to see if they accept this method for the final decision regarding the installation of the floor covering. If not, this could set your schedule back since you’d have to arrange for another testing method.
  • The RH test takes a few minutes of prep work for each test. The test itself takes 24 hours. But after the 24 hours, you can read the results yourself. No need to find super-precise scales or send the test to a lab.

If we look strictly at the time it takes to get moisture readings, the concrete moisture meter wins. But since you’d have to allow for the possibility that a flooring manufacturer may not honor their warranty if a concrete moisture meter was the only testing method used before installing the floor over the slab. Then you’d have to resort to either a long wait with a cheap calcium chloride test that may or may not give you an accurate reading of the whole slab, or you’d have to invest in RH testing and wait another day that you didn’t originally plan on.

So for practical reasons, the RH test is the option that can give you the most accurate moisture readings in the quickest amount of time.

Project-Specific Needs

Not all projects are the same. Some have specific requirements or unique circumstances that make one or more moisture tests impractical or unusable.

For example, consider what flooring will go on top of the concrete. Carpet? Hardwood? Tile? While all types of flooring can be affected by excess moisture, some are more sensitive to it. Hardwood is especially susceptible to too much moisture. If you will be installing a hardwood floor, the concrete must be thoroughly and accurately tested to be sure it’s dry, or the floor has a higher chance of failing. In this case, you would want the accuracy of an RH test.

Environmental factors should also be considered for each project. Is the slab in a basement? Upstairs? In an area where temperature and humidity will fluctuate? In these places, RH testing is also the best option because the environment could affect the drying time.
For smaller residential projects, however, RH testing doesn’t always fit into the budget, timeline, or procedure. But a concrete moisture meter can be an option for making sure the conditions are more favorable for installing flooring.

For larger projects and commercial installations, though, the standard for measuring and managing concrete moisture is RH testing.

Why Relative Humidity Testing is the Best All-around Method for Flooring Installers

As a flooring installer, nothing is more important than the accuracy of the moisture test readings. Other than gathering preliminary information, what would be the point of moisture testing if you can’t fully trust the results?

The most accurate test on the market today is the RH test because of its advanced, direct method of detecting moisture. Here’s why:

It Follows the ASTM Standard

The RH test follows the ASTM F2170 standard, which outlines the proper procedure for measuring relative humidity and determining the concrete’s moisture content. It has greatly raised the bar for moisture testing because it allows readings to be taken from within a concrete slab, instead of just the surface.

It Yields Additional Benefits

Because it’s a more accurate and thorough method of assessing concrete moisture, RH testing provides you, as a flooring installer, with its related benefits:

  • Prevention of costly flooring failures. You definitely don’t want to be called back to a job a few months after installation due to warping, adhesive breakdown, or mold growing in the floor. The accurate readings from RH testing will arm you with the information you need to be able to prevent those moisture-related issues from happening.
  • Avoiding additional costs. Going back to fix a moisture-damaged floor costs you time, materials, and money. And that unplanned expense will usually cost much more than the price of a reliable RH test.
  • Client satisfaction. When the flooring you installed continues to look beautiful years after, free from any moisture damage or warping, your clients will be pleased. They’re more likely to return to you again and they may recommend you to their friends for your outstanding work.

As far as moisture-related flooring issues go, the readings of an RH test will give you a more accurate picture of how much moisture is still in a concrete slab.

Mistakes to Avoid in Concrete Moisture Testing

No matter which method of concrete moisture measurement you choose, you want to make sure you’re getting the best, most useful readings possible.

To help you do just that, here are some common pitfalls to avoid, and what to do about them.

Inadequate or rushed preparation for testing

All methods of concrete moisture testing require specific steps of preparation. And when you’re trying to stay on schedule, it can be tempting to search for things that could be rushed or skipped. But inadequate preparation can affect the accuracy of the test results. And that can affect the outcome of your installed floor.

No matter what testing method you use, don’t neglect or rush the process for establishing the planned ambient conditions the floor will “live” in. Conditions should be stable for at least 48 hours before obtaining moisture readings.

Additionally, there are some specific situations to avoid based on the testing method you’re using.

For example, if you’re doing a calcium chloride test and you don’t properly clear and scrub the testing area, the results will be skewed. Not taking extra care to maintain stable ambient conditions could also affect the test.

When using a concrete moisture meter, make sure that it’s calibrated and ready to use. Otherwise, you could get inaccurate or inconsistent readings. While Wagner’s moisture meters are pre-calibrated and also come with a calibration block for verification, other meters may require additional steps or methods of checking the calibration.

For RH tests, remember that reusable probes require calibration verification within 30 days of each use.

Misinterpretation of moisture readings

First of all, remember that when using a concrete moisture meter, the reading it gives is not the total percentage of moisture in a slab, as it can only read down to 1 inch below the surface. For a more precise or comprehensive measurement, you’d need to use RH in-situ probes.

For readings from RH probes, according to the ASTM F2170 standard, the RH reading should be under 75 percent, or whatever the manufacturer of the floor covering specifies. Most floor covering manufacturers specify a higher percentage than is outlined in the ASTM standard. It’s always important to check with the manufacturer of each floor covering to determine what their particular RH needs to be.

Also make sure to read the manual thoroughly beforehand, so you know exactly what steps are involved and how to prepare for them.

Skipping the test

If your project is rushed or behind schedule, it could be tempting to skip the moisture-testing step and just proceed with the flooring install. Especially if the concrete slab looks and feels dry already. But if you think about the project from start to finish, moisture testing actually saves the process from unexpected delays because it will help prevent future flooring failures and costly repairs.

Keep in mind that RH testing only requires one extra day after the probes are placed. And you’ll be able to more confidently determine if you need to hold off the floor covering installation a bit longer or if the concrete is ready for flooring to be installed over it.

It’s All About Quality, Proper Usage, and Being Thorough

Whichever method of testing you choose, while they can vary in time, cost, and degree of accuracy, what’s most important is that you thoroughly understand the instructions, the related standards, and what is needed to prepare for moisture testing.

Depending on the size and scope of your project, however, accuracy is not something you want to take chances with. And generally speaking, the higher quality the testing product, the higher degree of accuracy you can expect. Always read through product reviews and warranty information before deciding what to purchase.

Looking for reliable concrete moisture testing products? Check out Wagner Meters’ high-quality moisture meters and Rapid RH testing kits.

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