Who has not seen a cupped hardwood floor? Wood movement happens without asking, against everybody’s wishes, leaving unsightly defects such as cupping, warping, shrinking and crowning. When wood moves, the moisture content of the floor planks has changed. Without changes in the moisture content, there is no wood movement. One of the reasons that the wood moisture content changes is the interaction of wood with the surrounding air. 

Corresponding values for stable conditions of relative humidity and wood moisture 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% to 95% at temperatures of 300 F to 1000F. Highlighted are recommended in-house and in-office conditions.

EMC table
EMC Table courtesy of Lignomat.

The all-important pair of tools for the floor installer are a moisture meter pin or pinless and a thermo-hygrometer for relative humidity and temperature with a wide measuring range.

Using the EMC Table

For every wood moisture content percentage you can find the proper relative humidity and temperature, which guarantee stability in wood floors. Example: For 6.2% wood moisture content, a humidity of 30% and a temperature of 70F is required. 

If the relative humidity would climb to 50% for an extended period of time, the wood would gain moisture until about 9% wood moisture has been reached. How much cupping, warping and deformations will occur depends on wood species, wood orientation and exposure time.

On the other hand, for every relative humidity you can find the corresponding moisture content which guarantee stability for the wood floor. Assume the relative humidity in a home is found to be 50% consistently. Wood floors will only be stable if installed at a 9% moisture content.  If the relative humidity would drop to 30%, the wood would lose moisture and shrink, warp and cup. And again the severity of defects depends on species, wood orientation and exposure time. 

Perfect conditions for lasting beauty in wood floors require having the floor installed at 8.5% moisture content in a house with a HVAC, which is kept at 45% relative humidity and 72F. Even while on vacation the HVAC should not be turned off. These conditions are not only comfortable for the wood floor, but also for humans working and living there.

In reality, many homes do not have HVAC’s. Relative humidity in the house is mostly dictated by the weather outside and by heating or air conditioning. As the floor installer, inform your customers about the science behind changes in moisture content. Let them know, that you have installed the floor correctly for the present conditions. However, when the seasons change and the relative humidity inside the home changes, there may be small defects, which come and go with the seasons. 

When installing a wood floor, use a moisture meter to check the moisture in the wood floor and use a Thermo-Hygrometer to check the relative humidity and temperature. Compare the values to the EMC chart. If you notice large discrepancies, stop and check the reasons. Some questions you should ask: 

  • Is the moisture meter and the thermo-hygrometer working properly?
  • Is the wood acclimated correctly?
  • Will longer acclimation to the present conditions help?
  • Is the Thermo-Hygrometer reading representative for the building?

If the relative humidity is unusually high, check for causes, water leaks, etc.

After the installation is completed, it is important to document the present conditions of the floor. Dual-depth moisture meters from Lignomat are the ideal instruments to pin-point the moisture conditions at this time. In every location you can obtain two measurements one at ¼” and one at ¾” deep. If a customer complains later-on about defects in the floor, you can prove that at the time of installation the conditions were right. This will protect you from claims, which are not your responsibility.