Calcium Chloride Moisture Test vs. Relative Humidity

All types of flooring can be susceptible to failure if moisture conditions are not properly monitored and maintained. But a slab’s moisture condition begins long before the flooring is installed.

If the concrete slab has not been properly dried and cured before the flooring is installed, moisture problems are almost guaranteed.

Moisture-related problems in flooring can also become health issues as mildew, mold, gaps, bumps and unsecured flooring pose additional risks to anyone walking over them.

Obviously, no one wants to have to repair an entire floor system because of moisture problems. The cost of remediating a flooring failure (or, worse yet, a concrete subsurface) can impact business costs, labor, and reputation.

The good news is that some preventative steps can do much to reduce the risks of excess moisture in the concrete slab and in the subsequently applied flooring. But in order to properly understand the ideal conditions for flooring installation, some basic knowledge about concrete is important.

Cement vs Concrete

You may often hear “cement” and “concrete” used interchangeably but they are not the same thing. Cement powder is the (typically) gray powder added to the concrete mix that binds all the components together. Cement is only one of the ingredients mixed together to make concrete.

Sand, water, and rocks (or other aggregates) are mixed with cement powder to form the finished product – concrete.

Types of Cement

There are two basic types of cement. A common cement-like Portland cement is one of the hydraulic cement that harden regardless of surrounding moisture conditions. The chemical reactions that bind these types of cement can even occur underwater!

Anhydrous cement, like gypsum plaster, must be dry to keep their strength. Within these two categories, additives like fly ash, lime, silica fume, blast furnace slag and others give a variety of strengths and colors to the various cement blends and to the final concrete made from these blends.

When the various ingredients – cement, sand, water and aggregate – of a concrete mixture are combined, a chemical reaction takes place that binds the materials together to form concrete. In a 4-inch slab, it takes approximately four weeks for this chemical process to be complete.

This is the process known as “curing.” But a cured slab can still be holding a significant amount (approximately two-thirds) of the moisture from the original concrete mixture – certainly too much to consider applying a flooring product over.

Drying continues after curing is complete through a process that moves moisture to the surface of the slab to then evaporate away and be replaced by more moisture drawn up through the entire slab. If the slab has cured, but not dried, it is certainly not ready for a flooring installation.

And even “dry” may not be dry enough. Because the drying process can be greatly impacted by environmental conditions like temperature and air humidity, the only way to be sure a slab is dry enough to apply a floor covering is through adequate moisture testing.

calcium chloride moisture test

Surface-based moisture tests are not recommended because they are easily affected by ambient conditions and may give misleading results.

Methods for Testing Moisture in Concrete

Accurate moisture testing is critical to understanding the complete moisture levels of any concrete slab. One test on the surface of the slab is obviously inadequate when trying to make a go or no-go decision about installing flooring.

Different areas of a slab may dry unevenly so adequate testing will test a number of different spots on each slab, and will test below the surface of the slab (at service conditions) as well.

ASTM International has provided several standards related to testing moisture conditions with two different test methods before installing flooring over a concrete slab: in-situ probes (ASTM F2170) and calcium chloride testing (ASTM F1869).

Calcium Chloride Test Method

The calcium chloride test method is used to determine the moisture vapor emission rate (MVER) from a concrete slab. Calcium chloride testing involves sealing a small dish of calcium chloride on a clean section of concrete under a plastic dome.

The salt absorbs moisture in that environment (and presumably coming from the concrete slab) and the weight gain after three days is used to calculate the MVER.

While this method is still specified by many flooring manufacturers, architects, and adhesive manufacturers, the calcium chloride test really only tests the surface conditions of the slab.

Learn more about how to conduct moisture tests for concrete floors in our more in-depth article.

(Side note: Calcium chloride testing has also been disallowed as an appropriate method for testing on lightweight concrete.)

Rapid RH L6

RH Testing – The Best Concrete Moisture Test

To test the moisture conditions within the slab, the best indicator of the total moisture picture is relative humidity testing using in situ probes.

A series of test holes are drilled into the slab and a small probe is placed into the hole where it is allowed to equilibrate with the slab before readings are taken.

Research has found that placing the probe internally, at a depth of 40% of the slab’s total thickness, provides the best indicator of the moisture conditions the adhesive and finished flooring product would encounter if they were installed.

Understanding these basic concepts about concrete, and correctly monitoring its moisture as it dries, can significantly reduce the risk of moisture-related flooring problems.

From a concrete specification that suits the time frame available to the flooring contractor that must choose the adhesive best suited to the flooring and slab conditions.

With the right moisture condition information, it should be possible for every professional on a building site to prevent flooring failures.

Free Download – Which Rapid RH Sensor is Right For You?

Last updated on March 15th, 2024


  1. Keith Papulski says:

    Ambient conditions affect both relative humidity testing and calcium chloride testing according to ASTM 2170 and ASTM 1869.

  2. Reena Patel says:

    Hi Jason, no clue if you still respond here, but curious…I have garage moisture that neither my builder, builder’s plumber, concrete vendor, engineer or anyone else can seem to figure out why it’s happening. My inspector seems to think that if there is a higher percentage of calcium chloride in the concrete (more than 2%) than it may attract moisture in humid situations. Is that accurate in your opinion?

    • Jason Spangler says:

      Hi Reena,

      Thanks for the phone call and email. As we discussed, I would look to see if a dehumidifier helps you in the house and if it does, maybe have an HVAC contractor come out and evaluate the efficiency of your unit. Good luck.

  3. Brandon says:

    What moisture level in a calcium chloride test would be considered problematic for most flooring adhesives?

  4. Elaine Cote says:

    Hello Jason,
    I own a 2100 sq ft bungalow, built in 1957, in Eastern Ontario, Canada. The house has a full 8-9’basement and experiences the extremes of 4 full seasons. Temps reach 30deg. C (90+F) in the summer with high humidity and 35degC below zero (-25 deg F) in the winter with high humidity. The basement floor has no actual water issues but does seem to have moisture issues. The house has the original poured concrete floor which has been painted over at least 2x before we purchased the home 10 yrs ago. The paint throughout most of the floor is bubbling and peeling with a lot of salt crystals forming in most of these areas.

    At some point I would like to scrape off as much paint as possible and lay a floor.
    I would imagine that the large areas of bubbling/peeling paint and crystals means moisture of some kind, but I am really not sure as to what is the best way to approach this problem.

    Jason, could you please give me any information you may have and suggestions as to how it is best to handle this floor/situation. Thank you,

    • Jason Spangler says:


      Thanks for the question and it sounds like you are probably correct about the moisture issue. The white “crystals” you are seeing are more that likely internal salts from the concrete that are being transported to the surface by high internal RH% levels. Based on this limited information and the age of the house I would recommend a VERY breathable coating or flooring product. When I speak of age, in this situation I am skeptical as to whether there is a vapor barrier under the slab so moisture will potentially be a constant issue. The other issue is the extremes of the environment. I would consult with a qualified flooring installer in your area to see if they have suggestions. My gut feeling tells me that a durable coating/paint might be your best option. Good luck.

  5. Neil says:

    House was built in 1987 and has finished basement with carpet glued directly to concrete floor. Am ripping up carpet to lay either LVP or laminate over an underlayment. Is the moisture test and moisture barrier necessary in this scenario? There was no barrier under the carpet and there was never an issue.

    • Jason Spangler says:


      Thanks for the question. The easiest way to answer the question is for you to review the necessary installation procedures. I would be EXTREMELY surprised if they didn’t require some type of concrete moisture testing. With carpet, the material is breathable so in most cases there typically isn’t a problem even if there is high moisture in the concrete. The breathability allows the moisture vapor to transfer through the material, into the air, and not build up under the floor.


  6. angel says:

    If the floor has been installed, but the moisture level throughout the house seem to be high according to the moisture reader bought at Lowe’s. Is there any way that I can figure out the problem? Or even fix it in a cost efficient way?

    • Jason Spangler says:


      Thanks for the question. You state the floor has already been installed and now you are checking moisture levels. Why? What has happened to the floor to prompt you to check moisture levels at this point? Also, please keep in mind that all moisture meters are not created equal and you may not be getting an accurate picture of the situation with the meter you are utilizing. If there is a problem with the installed floor it may be best to find a certified flooring inspector to help identify the potential issue. Try this group: Good luck.


  7. Ray says:

    What is wrong with the chloride calcium chloride test.and why is it no longer acceptable under AS 1884-2012? Thank.

  8. Jason Spangler says:


    A warmer, drier ambient environment is conducive for moisture to move from the slab into the air. At some point, the air may become increasingly saturated with the moisture moving from the concrete, so emissions will diminish The opposite can be stated for cooler, wetter ambient conditions. The whole key here is that by not having “service conditions” in the area being tested, getting accurate results is impossible. Here is a link to some short video’s that may help more



  9. bill hendricks says:

    when doing the calcium chloride test why does the room have to have the HVAC on a minimum of 48 hours before testing. How does the air temperature and humidity affect the test results

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