Plan for Concrete Slab Moisture Testing or Plan for Failure

Excessive moisture in concrete remains a costly problem for flooring installers. Moisture-related flooring failures run an estimated $2.4 billion a year in the United States.

Many of those failures can be avoided or mitigated by properly testing the concrete’s moisture condition before installing the flooring.

• Concrete Slab Moisture Testing Options
• In Situ RH Testing
• Ensuring Project Documentation Specifies the Right Concrete Moisture Test

The protective value of moisture testing is often undermined by indifferent or nonexistent test planning, or by poorly performed testing. Poor planning or specifying can result in a second-tier, unreliable testing method. In situ relative humidity (RH) testing is the method with the strongest scientific validation behind it.

Testing a slab improperly, even with an otherwise accurate method, yields erroneous readings just the same.

RH testing must therefore be done in compliance with ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes), using a reliable RH testing system.

The exact test method may or may not be specified in the project documentation. Either way, failing to specify proper testing leaves everyone who touched the concrete or the flooring exposed if a moisture-related failure occurs.

Naming both the ASTM F2170 standard and the specific RH test in the project documentation is the most effective way to protect yourself and your team.

Concrete Slab Moisture Testing Options

• Concrete moisture meters
• Calcium chloride (CaCl) testing
• In situ RH testing

As noted above, RH testing is the method that provides actionable, accurate readings for deciding when to install flooring or take remediating steps. Still, it’s worth reviewing both common methods to understand how they work and what each one does and doesn’t tell you.

Concrete Moisture Meter and ASTM F2659

A concrete moisture meter is a useful tool for an initial check of the relative moisture condition of areas on a slab. Used that way, it improves the effectiveness of in situ RH testing, for reasons detailed below.

What a moisture meter doesn’t provide is quantitative measurement, which is why it can’t tell you whether a slab is ready for flooring.

A concrete moisture meter takes a qualitative measure of a specific area relative to the slab’s overall condition. It doesn’t measure the slab’s moisture condition, and it certainly doesn’t produce a reading that correlates to what that condition will be once the slab is sealed under flooring.

It also reads only the top half-inch or so of the slab. Concrete’s moisture isn’t consistent throughout, both because of how moisture moves through concrete and because the mix can vary within the same slab. Reading the top layer in one spot can’t establish the slab’s true condition.

The standardized use of concrete moisture meters is governed by ASTM F2659 (Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter).

The F2659 guidelines state:

This guide is not intended to provide quantitative results as a basis for acceptance of a floor for installation of moisture sensitive flooring finishes systems. Test Methods F1869, F2170, or F2420 provide quantitative information for determining if moisture levels are within specific limits.

ASTM F2170 refers to in situ RH testing, and ASTM F1869 to calcium chloride (CaCl) testing. F2420 governed certain uses of RH testing but was withdrawn in 2014. Both F2170 and F1869 are covered further down. The point ASTM makes is clear: electronic moisture meters shouldn’t be used to decide whether a slab is ready for flooring.

Calcium Chloride Test and ASTM F1869

The earliest documented reference to the calcium chloride test comes from an Armstrong installation book on linoleum published in 1941, which called it the “dampness test.” Installers placed covered crystals on the slab and inspected them the next day to see whether they appeared to have absorbed moisture.

In the 1960s, engineers standardized how those measurements were determined rather than relying on visual assessment. The formula used weight differentials in the crystals to calculate the moisture vapor emission rate (MVER) coming off the slab.

Documentation from the 1960s often specified an MVER of two to three pounds; by the 1990s many flooring manufacturers had raised the acceptable rate to five pounds.

The CaCl test was standardized in 1998 with the adoption of ASTM F1869 (Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride).

Under F1869, the crystals are weighed before being placed on the slab and covered, then weighed again 60 to 72 hours later to determine the slab’s MVER. Per the guidelines, “quantity of moisture shall be expressed as the rate of moisture vapor emission, measured in pounds of moisture over a 1000 ft² area…”

Manufacturer instructions that reference F1869 specify the acceptable MVER range for their flooring. Absent those instructions, the industry standard is three pounds, though F1869 is a test method and sets no pass/fail limit of its own. An update to F1869 prohibits the use of the CaCl test on gypsum-based or lightweight aggregate concrete.

CTLGroup conducted decade-long testing on the CaCl test to quantify how accurately MVER reflects the moisture in concrete slabs. According to Howard Kanare, then CTLGroup’s Senior Principal Scientist and the chairman and author of ASTM F2170, the CaCl test can be unreliable, producing both false high and false low results.

In one test, CTLGroup measured four concrete slabs that had stabilized at 50% humidity for years. Following F1869 guidelines, the CaCl tests returned MVER between 2.5 and 4+ pounds, indicating the desiccant was drawing out more moisture than the concrete was emitting, a false positive. That testing is documented in The Construction Specifier.

That was one of numerous laboratory and field tests CTLGroup conducted on the CaCl test and on MVER as an indicator of moisture in concrete. According to Kanare, CTLGroup outlined six reasons why MVER “suffers from serious deficiencies.”

1. No scientific basis existed for the standards when they were created, so MVER as a measure of moisture has no quantitative foundation.
2. MVER kits can’t be calibrated, making it impossible to determine their accuracy.
3. The test measures moisture only at the surface of the slab and provides no information about moisture below it.
4. CaCl testing doesn’t measure MVER accurately; both false positive and false negative readings are common.
5. Ambient conditions interfere with the results. The crystals attract humidity out of the air, which then gets attributed to moisture within the concrete.
6. Certain limitations on MVER don’t account for the impact adhesives have on the flooring’s long-term performance.

The third deficiency, that a CaCl test measures only surface moisture, matters because moisture moves through concrete and doesn’t level out until the slab is sealed.

Measuring the surface, even accurately, tells you little about the moisture below it. And it’s the excess moisture below the surface that later moves up into floor coverings installed too soon.

The related problem is that surface moisture correlates poorly with the moisture that remains in the slab after flooring goes down. Once the floor is installed, no more moisture evaporates, and what’s left equilibrates throughout the slab.

That remaining moisture is what determines whether the floor suffers damage later. It rises without being able to evaporate, carrying moisture vapor and chemicals that get trapped between the slab surface and the flooring.

A reading that captures only the surface at one moment in time says little about the slab’s condition once it’s sealed.

The CaCl test continues to be used despite these documented shortcomings, partly on the belief that it costs less than RH testing. The kit does cost less.

CaCl testing is far more labor-intensive, though, which gives it higher direct costs than RH testing and higher indirect costs through inefficient use of time and labor.

The CaCl test also has a longer field history than the RH test, which some may mistake for greater credibility. Many flooring manufacturers still specify an acceptable MVER for warranty coverage, which sustains that impression.

In Situ RH Test and ASTM F2170

Work at Lund University in Sweden in the 1990s was central to developing the in situ RH test method used today. Those researchers examined RH levels within the slab and how they relate to the slab’s equilibrium moisture content (EMC) after flooring is installed, and determined the depths at which an RH sensor reads the percentage that reflects the slab’s EMC once sealed.

For slabs poured on grade, that depth is 40% of the slab’s thickness; for slabs drying from both sides, it’s 20%. The first industry associations to issue standards for in situ RH testing were in Sweden and Finland.

Those standards, colloquially called the “Nordtest,” were published in 1995. ASTM used the Nordtest as the basis for F2170, first approved in 2002.

The scope of scientific validation is a notable distinction between the histories of F1869 and F2170. The CaCl test and its standardization came from anecdotal experience, with controlled testing later revealing its weaknesses. In situ RH testing was developed through scientific testing, and the field use standards followed.

Continued testing has improved our understanding of the method, and led to an update to F2170. The original standard required waiting 72 hours for the air in the test hole to equilibrate before an ASTM-compliant reading could be taken.

A precision and bias study commissioned by ASTM and conducted by an independent laboratory tested the efficacy of that 72-hour period, taking readings at multiple intervals beforehand to track the difference against the required 72-hour reading.

Readings taken at 24 hours proved statistically equivalent to those taken at 72. Occasional deviations were consistently small enough to have no statistical impact, which made the 72-hour requirement moot.

ASTM updated F2170 accordingly, allowing compliant readings 24 hours after the sensor is inserted. That makes in situ RH the fastest of these methods, since F1869 users must still wait at least 60 hours for a compliant reading.

The most significant distinction between the two methods is what they measure. The CaCl test measures surface moisture, while the condition below the surface is what predicts performance.

An in situ RH sensor measures RH and temperature within the slab, which is what makes it able to tell you about the slab’s moisture condition once the flooring is installed.

Validated Reliability of RH Test Kits

The science behind in situ RH testing also means RH test kits can be calibrated to traceable national standards. The inability to calibrate MVER equipment was one of the CaCl test’s main weaknesses in the CTLGroup research. Without calibration, there’s no way to verify that the equipment is returning an accurate reading.

In Situ RH Testing: Accurate and Fast

The most reliable option here is also the fastest, which isn’t always how it works out.

The revised F2170 requires only a 24-hour waiting period, where F1869 still requires a minimum of 60 hours before you can take an actionable reading.

In situ RH kits can also simplify installation and data collection, which compresses the moisture measurement timeline further.

Wagner Meters’ Rapid RH^® L6 sensors install in minutes: drill the hole, clean it out, insert the sensor. CaCl tests, by contrast, require careful setup to seal the crystals under the cover.

The Rapid RH L6 system also includes optional accessories and free mobile apps that streamline or automate data collection and F2170 reporting. Collected data also supports trend analysis, which shows how a specific slab is drying rather than only where it stands today.

F2170 stipulates the number of RH sensors based on the square footage of the slab, including where certain sensors must be placed. A concrete moisture meter helps target trouble spots within a test location so they get the attention they deserve.

Look for a meter that reads below the surface, like Wagner Meters’ C555 pinless concrete moisture meter, which reads 0.5 inches into the slab. It returns the spot’s comparative moisture condition, which shows where the slab is holding the most moisture.

Ensuring Project Documentation Specifies the Right Concrete Moisture Test

In situ RH testing is the more reliable and faster method, and the CaCl test still has its adherents. General contractors and flooring installers tend to default to whatever test they’re most comfortable with, and nobody wants to spend time after the pour arguing about which test to use.

The time to insist on an F2170-compliant in situ RH test is when the project is being specified. Naming the standard and the testing system in the project documents settles the question before it can become a dispute.

Download the Rapid RH ASTM F2170 Checklist.

Last updated on September 17th, 2026