A major change is taking place in the way that the floor covering industry measures moisture in concrete floors. This change affects concrete floor contractors, general contractors, construction managers, owners, architects, and floor covering installers. Just as the concrete floor construction industry made the shift in flatness tolerances from straight-edge to the more appropriate statistically based FF/FL system, now the floor covering industry faces a major shift from measuring moisture vapor emission rate (MVER) to measuring relative humidity (RH).

This sheet vinyl shows typical rutting due to excessive moisture in the concrete slab.
This sheet vinyl shows typical rutting due to excessive moisture in the concrete slab.

The anhydrous calcium chloride test for moisture emission rate was developed in the 1940s as a qualitative evaluation of floor moisture condition. Without any documented scientific basis, it became a quantitative test in the 1960s. Now, nearly a half million MVER tests are performed each year in the United States. In the past decade, we have learned that the test can be unreliable; capable of producing both false high and low results; and dependent on ambient temperature and humidity, water-cement ratio, use of lightweight aggregate, the presence of curing compound, how hard a floor was troweled, and how the test site is prepared.

Over the past 10 years, CTLGroup has investigated the performance of the MVER test method in the field and in the lab, and we have found that it suffers from several serious deficiencies:

  • The test has no pedigree: There are no published or existing data from the 1940s to the 1960s used to establish the test kit dimensions, time of exposure, choice and mass of desiccant, or basis for calculations. There are no flooring performance data supporting the widely used 3-pound limit publicized in the 1960s (actually 2 pounds in the earliest printed versions), or the 5-pound limit for some products published by flooring manufacturers in the late 1990s.
  • There is no practical way to calibrate MVER test kits. There are no standard reference concretes available with controlled MVER levels. Kit dimensions have been arbitrarily standardized to provide reproducibility between brands, but they are not absolutely “accurate.”
  • The test determines a portion of the free moisture near the surface of a slab, generally the upper 12 to 20 mm (or ½ to ¾ inch), providing no information about moisture conditions deeper in the slab.
  • The test does not accurately determine the true MVER of concrete; it overestimates low moisture-emission levels and underestimates high emission levels.
  • Ambient conditions interfere with test results—warmer, more humid room air can yield higher MVER results even if the internal concrete moisture condition is unchanged. Floor surface preparation for testing, such as gentle grinding, can significantly change measured MVER.
  • Limits set for MVER based on product type—one level for sheet vinyl, carpet, or rubber and a different level for VCT and felt-back resilient flooring—neglect the fundamental fact that adhesives play a major role in flooring performance.

RH for floor moisture measurement is not new—it was first used to measure moisture in concrete floors as part of PCA applied research programs in our laboratories beginning in the 1950s. RH instruments can be independently calibrated directly traceable to national standards. There are a variety of commercially available RH instruments specifically for measuring moisture in concrete. Although RH instruments cost more than a calcium chloride MVER kit, large savings in testing time and labor are making RH the method of choice. More importantly, RH testing gives a much more useful picture of the actual moisture condition within the concrete regardless of mix, aggregate types, floor thickness, or surface conditions. Properly conducted RH testing can prevent premature flooring installation that can lead to costly repairs and litigation.

RH has been the preferred method for assessing concrete floor moisture conditions in a number of countries for many years. Limits for installation range from 60% RH for direct glue-down wood parquet to 90% RH for some vinyl tile products. Flooring manufacturers in the United States must establish realistic RH limits for acceptable performance of their products, through rigorous scientific testing, taking into account the various components of their systems, such as patching/leveling compounds, primer, adhesive, and floor coverings. I anticipate that manufacturers eventually will develop “tiered systems” that will allow design professionals and contractors to select flooring products for various moisture levels, to produce enduring, successful outcomes.

— Howard Kanare is a senior principal scientist at Construction Technology Laboratories (CTL) Group, Skokie, Ill., 847-522-2285 or hkanare@ctlgroup.com.