In the hardwood flooring industry the most commonly accepted test for determining moisture levels in concrete slabs is the calcium chloride (CaCl2) test. This test determines pounds of moisture per 1000 Sq Ft in a concrete slab by absorbing water vapor from the slab over a period of 60-72 hours. Once the test is complete, the weight of the calcium chloride sample is entered into a formula and shows how many pounds of water per 1,000 feet of concrete has been absorbed.

While the calcium chloride test has been the industry standard since the 1950s, and there is an ASTM Standard  for this test, there have been some criticisms of the test gaining momentum for the past decade or so.

Opponents say that the calcium chloride test does not reach a proper depth into the concrete slab to accurately gauge the slab’s moisture levels. The test determines the pounds of moisture per 1000 SqFt from a depth of 1/2″ to 3/4″ from the top of the slab. Measuring moisture from the top of the slab could make the readings susceptible to fluctuation in the environment. The length of time per test is also considered a downside to the calcium chloride tests. One test can take upwards of 72 hours from start to finish. Another concern is that the test is cumbersome to set up. An area of the concrete slab must be meticulously prepped before each and every test. After the appropriate time has elapsed, the sample weight is added to a formula that includes other data collected from the test site. Typically confirmation of the results are done by an independent lab, adding to the time frame.

The relative humidity (RH) test has gained popularity because it addresses the concerns raised by the calcium chloride test and offers additional benefits. The RH test is administered by drilling the proper size hole into the slab that is 40% of the depth of the concrete slab to be tested. (If there are two drying sides to the slab the hole should be drilled to 20% depth.) The hole is then sealed up. The mechanic then inserts a sensor into the hole. The reading probe is inserted and  measures the humidity levels of the concrete slab. These readings are considered to be more accurate because the moisture levels inside the slab give a better impression of the how far along the slab is in the drying process. This is contrasted with the calcium chloride test that only measures the very top of the slab.

Most flooring and adhesive manufacturers — as well as architects — are transitioning to the Relative Humidity specification. One example of a relative humidity meter is the Wagner Rapid RH Concrete Meter (check out the new Wagner Rapid RH L6 Starter Kit!).

While each type of test has its supporters and detractors, the main thing to remember is that it is absolutely crucial to test concrete floors for moisture levels repeatedly before attempting to install hardwood flooring over it. Concrete is a porous, sponge-like material. It naturally wants to retain and absorb moisture in the environment. It is of the utmost importance to be sure the moisture in a slab is at the correct level before installing a hardwood floor.