Relative humidity (RH), as a general definition, is a ratio measurement of water molecules expressed as a percentage of the saturation point at a given temperature. If the amount of water molecules remains constant, cooler temperatures increase relative humidity while warmer temperatures decrease relative humidity. In the flooring industry, we are concerned about (1) the relative humidity of concrete, (2) the relative humidity of wood, and (3) the relative humidity of air.
RH of Concrete
The relative humidity of the concrete is important because it verifies that the concrete is sufficiently dry to install flooring. The relative humidity of concrete is measured by the in situ probe test following the guidelines of ASTM F2170. Targets established by flooring manufacturers commonly range from 75% to 80%. NuBroadlok™ Premium Multi-Purpose Adhesive for Mohawk's broadloom carpets requires: "In-Situ relative humidity reading (ASTM F2170) must not exceed 80%...."
The alternative to testing relative humidity is testing for the moisture vapor emission rate (MVER). The RH test gives a reading of the middle portion of the concrete slab (at about 40% depth). By contrast, the MVER test gives a reading of the top portion of the concrete slab. The RH test is the more accurate method, but some manufacturers require that both the RH and the MVER testing methods be used. Armstrong explains: "The use of both testing methods performed concurrently in a building offers the greatest depth of data and confidence in decisions to install flooring. A combination of tests is the smarter choice." Both old and new concrete slabs can present moisture risks. New slabs start with 100% relative moisture and present a strong risk if the installation of flooring is rushed. Generally, new concrete should cure for 90 to 120 days before the installation of new flooring. "The problems range from cupping, buckling, blistering and adhesive failure to discoloration and mold growth," according to Armstrong.
RH of Wood
The relative humidity of wood is important because it determines when hardwood can be installed in comparison to the relative humidity of the substrate. The relative humidity of wood is measured by a pin or pinless moisture meter. Rising relative humidity in wood will cause the wood to swell, cup, crown, or delaminate.
RH of Air
The relative humidity of air is important because it alters the results of a calcium chloride moisture test or an in situ probe test. The relative humidity of air is measured by hygrometer. High relative humidity of air will slow the curing process of flooring adhesives, mortars, and grouts. Furthermore, the high relative humidity of air against the low surface temperature of a substrate can cause condensation or sweating slab syndrome. Finally, the high relative humidity of air can cause flooring products to swell. In addition to harmonizing temperature, we acclimate flooring materials in order to achieve the equilibrium moisture content (EMC) of the flooring product compared to the surrounding environment.