The determination of moisture content (MC) is an essential part of quality control within the flooring installation process. Installers must know the moisture content of not only the wood flooring but of the substrate as well. Moisture meters are a vital part of this process, and the installer who continues to work without one is directly jeopardizing his or her jobs.
Moisture meters can measure the moisture content and provide information to help the installer accomplish the following:
- Determine if the wood flooring is properly acclimated and conditioned for installation to proceed.
- Determine if the wood subfloor is ready for floor installations.
- Determine when the second coat of finish can be applied.
- Assess water damage.
Each moisture meter is unique, and it is important that you know your meter to get the most out of it. There are two main types of moisture meters: pinless, which are also called dielectric, and pin-type, which are also called electrical resistance.
Dielectric Moisture Meters
Pinless, dielectric moisture meters, which are also referred to as noninvasive, nondestructive, scanning or surface meters, do not leave any holes in the flooring. Signal penetration for these types of meters typically ranges up to 1”. The meter can be moved across the surface to identify elevated areas of moisture. It is relatively unaffected by temperature. Rough surfaces have very little impact on the reading. Measurements can also be taken through coatings, varnishes, or paints without damage to the surface. Grain orientation does not affect readings.
There are three basic types of dielectric moisture meters: capacitance, power-loss, and capacitive-admittance. A capacitance dielectric moisture meter operates on the relationship between moisture content and the dielectric constant of the wood cells. An alternating field of electricity is transmitted into the flooring sample and picked up by the receiving electrode. The meter converts this signal to a moisture content reading. The reading is typically accurate between 5-30 percent, dependent upon the meter manufacturer.
A power-loss dielectric moisture meter operates by radiating an electrical field into the wood sample. The energy absorbed by the wood sample reduces the amplitude of the signal. This power-loss is dependent on the moisture within the wood sample. The meter converts this to a moisture content reading.
A capacitive-admittance moisture meter is a combination of a capacitance and power-loss moisture meter.
When using these meters on engineered flooring made up of multiple species (wear layer, core layer, and backing), check with the meter manufacturer for testing protocol, accuracy, and appropriate species correction values.
The pin-type meters measure the electrical resistance across opposing sets of pins, which are pushed into the wood. Most meter manufacturers recommend placing the pins parallel within the grain direction of the flooring. With this type of meter, an electrical current is forced through the wood sample between the probes. Because wood is a poor conductor of electricity and water is a good conductor of electricity, the meter measures the voltage that develops and calculates the resistance. The higher the moisture content, the lower the resistance. The results are displayed as moisture content instead of ohms.
These pins are available as insulated or non-insulated. Non-insulated pins will read as deep as they are inserted and will average out the resistance through the entire depth of the pins which can vary in length from 1/4” to 1”. Insulated pins are typically available in many different lengths (from 1” to more than 3”), and are used with a slide hammer extension. Insulated pins only measure the resistance at the tips of the pins, allowing for multi-depth readings through the sample, in measuring moisture gradients through the sample of flooring or a flooring system.
Additional considerations for moisture meters:
The temperature of the wood will significantly influence the readings of a pin-type meter. Be sure to check with your meter manufacturer for temperature correction values. Temperature correction is the adjustment that is made to the moisture meter reading to compensate for the phenomena that the electric conductance of wood increases as the temperature increases, and vice-versa. This adjustment, whether manual or automatic, allows for accurate measurements of moisture content even at extreme temperatures (for example, less than 50 °F and greater than 90 °F).
Most meter manufacturers calibrate their meters to Douglas fir/pine/hemlock. Verify with the manufacturer the correction to use for specific meters and species. Some meters have a species correction adjustment built in. Species correction is a meter manufacturer specified user-adjusted setting that is made to the moisture meter to compensate for either varying electrical properties (for pin type meters) or densities (for pinless meters) of the species under test, as compared to the species of the reference calibration. Making these adjustments allows for a more accurate assessment of the moisture within the wood being tested.
Meter drift is the decrease (or increase) in true moisture content over a specified elapsed time. To ensure accuracy, be sure to record readings from meters within the first 2-3 seconds.
Calibration ensures the meter is giving accurate readings. All meters must be calibrated from time to time. Some meters can be checked for calibration internally or by use of a calibration block supplied by the manufacturer. Check with the meter manufacturer to determine when and how to get your meter properly calibrated. Most meter manufacturers will provide a calibration certificate, which verifies the equipment being tested is operating properly.
Fresh batteries will ensure your meter is operating properly.
Be sure to follow manufacturer recommendations with regard to pressure applied to pinless meters.
Wood subfloors are very easy to check for moisture content. The most important element of testing wood subfloors is ensuring the wood subfloor has had adequate time to become conditioned to the environment in which the flooring will be installed. It is critical to check with the manufacturer for proper settings or correction values when testing wood subfloor materials.
Due to the variability in wood subfloor materials and the non-wood resins often used within them, it can sometimes be difficult to get an accurate moisture reading of this material. When in doubt, use a moisture meter to check the MC of other conditioned wood materials (2x4s, newel posts, wood beams, etc.) within the structure in order to get an idea of where the EMC is in comparison to where it should be, and use this value as a baseline for testing the subfloor.
After adjusting your meter to the subfloor material being tested, test for moisture a minimum of 20 areas per 1,000 square feet and average the results. Pay special attention to exterior walls and plumbing. Any excessively high readings should not be included in the average and the source for the elevated readings must be identified. Installation should not proceed until the origin of the moisture is identified and remedied. Be sure to document all of your readings.
Owning a moisture meter is the first step in being a responsible flooring professional. Knowing how to use it can minimize job failures, and increase your value within our industry and to your customers.
Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at email@example.com.
* Editor’s Note: We realized that we accidentally included a photo of an ohms voltage meter in the print version of this article in the August/September issue of Hardwood Floors Magazine. We strive for accuracy and regret the error.