Dimensional Changes in Wood Floors

If it would not be for ugly deformations caused by cupping, bowing and gaping, nobody would worry about moisture and humidity. However, failed floors and customer complaints have made installers and clients more aware of moisture problems.

Most materials in the building envelope are hygroscopic. They contain a certain amount of moisture, however if there is too much or too little moisture in the air surrounding them, they will absorb or loose moisture. Problems in wood floors occur when there is an imbalance between humidity in the air and moisture in the wood . Wood is a hygroscopic material, which will absorb and loose moisture.

The finish on a floor plank will slow down moisture movement however it is not 100% tight. A wood floor will follow changes in moisture content and relative humidity. Stability of wood depends on 3 values: moisture content, relative humidity and temperature, with temperature having the least influence. (read more click here)

Corresponding values for stable conditions are listed in the EMC chart, one of the most important tables for any floor installer. The EMC chart shows values for 1.2% up to 24.3% wood moisture content for ambient humidity of 5%-95% at temperatures of 300 to 1000F.

For on-line EMC chart go to www.lignomatusa.com/EMC

If the moisture in wood changes, depending on species, wood orientation and exposure time then the wood shrinks and warps or buckles and cups.

For instance, the EMC chart shows, that wood with a moisture content of 10.1% is stable at 55% relative humidity and 700F. No cupping or shrinking will occur, as long as the relative humidity remains at 55%.

  1. If floor planks measuring a moisture content of 6% are exposed to air at 55% and 700F, the floor will pick up moisture until the relevant EMC value is reached at 10.1%. Wood species, coating and exposure time determine how much moisture is actually absorbed by the wood. However at 55% relative humidity, the floor will never pick up more moisture than 10.1%.
  2. If floor planks were installed at too high a moisture content (not confirmed at the time of installation), the floor planks will dry out and most probably cup during the months following the installation. At the time of the complaint, the floor planks are dry, which can be easily confirmed by a moisture meter. What cannot be verified at that time, is the initial moisture content, the culprit for the cupping. Since the moisture was not noted at the time of the installation, the installer is the most probably party deemed responsible for any repair work. Protect yourself from claims by using a moisture meter and thermo-hygrometer, when installing a floor.
  3. All is well, if a floor has been properly installed at 7% wood moisture and the relative humidity in the house stays at 40%. The floor will not suffer any cupping or gaping, as long as the relative humidity does not change. As time passes winter comes with very low relative humidity. RH inside a home can fall below 15%, unless a HVAC system is used. The drop in relative humidity will cause the wood to dry out, shrink and cup. According to the EMC chart, the wood moisture can drop down to 3.5%. Even though the wood may not reach the lowest possible value, as the wood dries out, little gaps become obvious and the home owner is complaining, trying to blame the installer. To protect themselves from claims, that are really not their fault, the installer should explain at the time the job is finished, that low relative humidity values occurring in the winter time can cause the floor planks to shrink and small gaps will occur. Those gaps will go away again during the summer, because of the increased humidity in the air. The only remedy is keeping the relative humidity between the recommended values of 30-50%.

There are meters available to measure moisture in wood and relative humidity in air with hi-precision relative humidity probes, such as the Ligno-DuoTec BW with Lignomat’s RH BluePeg probe. Wood moisture can be measured with the pinless meter function. As a bonus, the Ligno-DuoTec BW comes with dual-depth capabilities. This allows the installer to pin-point moisture conditions in two levels at ¼” and at ¾” deep. The meter can be equipped with the RH Probe for accurate relative humidity and temperature conditions. A data-logger is also an option as it keeps measurements for months or years of relative humidity and temperature in a home, independent of any climate control in the home.

Photo 1 shows a dual-depth, pinless moisture meter used to measuring an engineered floor. Note the depth has been set for ¼” deep to give reliable measurements of the hardwood top layer.

Photo 2 shows the same meter used to check the relative humidity in a gymnasium.

For questions call 800-227-2105

For on-line EMC chart go to www.lignomatusa.com/EMC

For more info on moisture meters go to www.lignomatusa.com