Before installing a beautiful new floor, do you know the moisture vapor emission rate of the concrete substrate that you are about to install that floor on? Do you also know the moisture sensitivity of the floor covering and the installation materials? If not, you need to step back for a moment and find answers to these questions. Many decorative floor coverings and their installation materials are sensitive to excessive moisture. Moisture can cause floor coverings to swell or blister and wood flooring may even buckle. The adhesives used to secure the flooring to the concrete substrate can dissolve and allow the floor covering to come loose from the substrate. Continued exposure to moisture allows mold to grow under the flooring material, which can cause discoloration of the flooring and affect the health of the building occupants.

Persistent moisture typically comes from the concrete substrate in the form of vapor. Moisture vapor is the gaseous form of water. It surrounds us every day and is measured as relative humidity in the atmosphere. It is also found in any concrete slab. Water is added to the dry concrete mix to improve mixing, pouring and placement of the concrete. Part of the mix water will bind to the cement particles in the concrete through a chemical process called hydration. The excess water not needed for hydration must leave the concrete as vapor. This is a slow process and can take as much as a month for every inch of thickness in the concrete substrate.

Concrete has a crystalline structure with many capillaries between the cement crystals and aggregate content. These capillaries allow the movement of moisture vapor through the concrete. Based on the laws of physics, the moisture vapor will move to areas of lower concentration or lower relative humidity. The moisture vapor will move to the surface of the concrete through the capillaries where it can accumulate and affect moisture-sensitive floor coverings and installation materials. This occurs in suspended concrete slabs as well as in concrete on ground.

It is not uncommon to also find high moisture vapor emission from older concrete slabs. This can be due to poorly placed, damaged or missing moisture barriers beneath the concrete slab. Ground water can penetrate the concrete slab, move through the capillaries in the concrete and cause damage to the floor covering. This can even be a seasonal issue when the water table rises or there is an increased irrigation of the landscaping. Plumbing leaks in the concrete slab can also be a source of moisture. This can increase the moisture and relative humidity of the slab, making this an issue found in both remodel and new construction projects.

Concrete with a high moisture content does not look any different than concrete with a low content. If the concrete looks wet or is darker than typical concrete, it may be due to water moving through the concrete slab; this is referred to as hydrostatic pressure. Water from beneath the concrete slab is pushing its way through the capillaries and bringing liquid water to the surface. The pressure can be intense and cannot be stopped by treatments to the slab. When this occurs, it is best to have an engineer review the situation and determine a means to divert the water from the concrete slab.

So how can you tell if there is excessive moisture vapor coming from the slab? There are two methods commonly used to measure the moisture vapor emission rate (MVER) from a concrete slab. Both methods can be used with on grade or above grade concrete slabs. ASTM F710 defines the Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. In this standard, the first step is to find out how much moisture vapor is contained or emitted from a concrete slab.

ASTM F1869, the Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, is the older test. It relies on measuring the amount of moisture vapor emitted from the surface of a concrete slab. ASTM F1869 measures MVER in terms of pounds of water per day per 1,000 sq. ft. of floor area. ASTM F710 recommends an emission rate less than 3 pounds per day per 1,000 sq. ft. for the installation of most resilient floor covering. ASTM F1869 measures the moisture in the top ¼” of a concrete slab, but does not measure the moisture deep in the slab that could potentially reach the surface with environmental changes. (Note: this is not an acceptable method for measuring the moisture vapor emission rate in lightweight concrete slabs.)

ASTM F2170 measures the relative humidity of a concrete slab and can be used with lightweight concrete slabs. This is the Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. It measures the moisture present deep within a concrete slab. This is the moisture that can escape and affect floor coverings and bonding materials after installation. ASTM F710 recommends that the relative humidity in a concrete slab be below 75% for installation of most resilient floor covering.

With either test method for MVER, it is important that the room is conditioned to the expected temperature and humidity levels. The standard’s instructions for the number and frequency of individual tests must be followed to achieve accurate readings. In many cases, it is best to run both tests to 1) measure moisture in the slab that is available for movement to the surface, and 2) moisture emission from a concrete slab. Moisture levels in a concrete slab will vary over time and if high MVER is suspected, it may be wise to retest the slab if the project is delayed and you are relying on old information.

While ASTM F710 is meant for resilient flooring, it is also appropriate for laminate, wood and even ceramic tile. Laminate and wood are sensitive to moisture and moisture vapor emission should be kept to a minimum. The manufacturer of the flooring should be asked about acceptable moisture vapor emission rates from the substrate. Ceramic tile and cement mortars are not affected by moisture, but most of today’s ceramic tile installations include some type of waterproofing or crack isolation membranes. These membranes or their adhesives may be sensitive to moisture and should not be used if there is excessive moisture vapor emission from the substrate. As with the floor covering, check with the membrane manufacturer as to its suitability over a concrete slab with a high MVER.

If the MVER is higher than allowed for the floor covering or adhesive to be installed in the project, the concrete slab must first be prepared to accept a moisture vapor barrier. A liquid coating is then applied to the bare concrete subfloor, such as CustomTech® TechMVC Moisture Vapor and Alkalinity Barrier. Most moisture barriers can be applied to concrete with moisture vapor emission rates as high as 25 lbs. of moisture emitted per day per 1,000 sq. ft., or having an internal relative humidity of 100%. Properly installed, the moisture barrier can reduce the moisture emission rate from a concrete slab to below the level most floor covering and adhesive manufacturers find acceptable.

Before applying a moisture mitigation product, ensure that the concrete slab is structurally sound, clean, dry and free from contaminants that would prevent a good bond. The concrete slab should be at least 7 days old and should have an ICRI Concrete Surface Profile of 3. If necessary, the concrete should be abraded or shot blasted to this profile. When installing a moisture vapor barrier, it is important to follow the manufacturer’s directions for mixing the components and applying the barrier to the substrate. Do not assume that every manufacturer has the same procedure for mixing and application. The components should be mixed with a low speed power mixer. Using a higher speed will mix air into the epoxy and cause voids in the barrier.

It is best to spread the material on the floor with a gauged squeegee to the required thickness and then roll it smooth with a lint-free roller. Allow the product to thoroughly cure and then examine the surface for voids, pin holes and air bubbles. Any air bubbles should be shaved off with a sharp blade. A second coat is then applied to fill the craters formed by the bubbles and any voids or pin holes observed. This application should be tight to the surface of the first coat to fill the voids. The newly coated subfloor should be protected from traffic while it cures and until it is covered with any necessary underlayment prior to the decorative floor covering.

Now that the moisture in the slab under control, it’s time to install that beautiful new floor covering. However, remember this is no longer a porous concrete floor. You now have a sealed, non-porous floor that may not accept the bond of the adhesive or mortar you had planned to use to install the flooring. You cannot roughen or abrade the floor, as this will affect its ability to reduce the moisture vapor emissions. It may be necessary to change to a compatible adhesive or use a primer designed to bond to the moisture vapor control membrane.

It may seem like a lot of effort to control something that is not readily apparent, but the consequences of not realizing that there is a moisture issue and not mitigating it, will end in a more expensive replacement of the floor covering when it fails.