What is acclimation? Acclimation is the process that results in flooring adjusting to the surrounding environmental condition.

This is basically the hygroscopic nature of wood to absorb or loose moisture and adjust to an EMC (equilibrium moisture content) in association with the surrounding environment. NOFMA has an EMC chart on its web site (www.nofma.org, Behavior of Wood) that illustrates the EMC associated with typical temperature and relative humidity ranges encountered during the installation of wood flooring. Also included is a map of the US showing average winter (dry season) and summer (humid season). This map was created before most homes were centrally heated and air conditioned so the typical numbers for your area may be quite different. (Table 2 and Map of USA)

We have previously said that acclimation depends on the type of wood flooring, job-site condition, local environment, and any other special environmental condition. (FCI Sept-OCT 2002) Also, manufacturers may direct that some products may not require acclimation or only limited acclimation. So, always follow manufacturer's instructions.

If acclimation is necessary, what is involved in "fine tuning" the acclimation process? First is knowledge of the area environment. A competent flooring contractor should check and record the moisture content of properly performing flooring installations on an ongoing basis. The checks should be made during all the different seasons. Moisture readings should also be taken on the flooring systems to determine what are typical or normal subflooring conditions for the area. This is the basis for establishing the moisture content range of wood flooring for the geographical area surveyed.

For the most part flooring exposed too the first complete heating season will acclimate to a winter base line and subsequent moisture changes will result from normal environmental changes. However, today's building practices can trap unusual moisture in a home that can take more than a single year to normalize. Therefore, to be safe, flooring that has been in place 2 - 3 years and exposed to an established environment will be considered well or "completely" acclimated to the typical area conditions. Readings taken of these installations should be used for the base line.

This average yearly moisture content is the target acclimation for the best performing floors in the area. So you shouldn't acclimate to an excessively dry condition in the winter if your average will be higher during the rest of the year. Neither is acclimating to a wet site or extra acclimation during a very wet summer advised, when you know the house and flooring system will dry out next year.

Next is the knowledge of the flooring product. Check the moisture content of the flooring. About 20 readings per pallet (@750 sq-ft/pallet) are necessary to get a representative assessment. Write them down, mark the range, and average the readings. The readings should be of different bundles located in different layers of the pallet. Since different pallets may be manufactured at different times this will include the associated variation. This will also raise a flag if one pallet is unusually different from the others.

Further, theoverall range should be fairly tight with a 3 percent variation or less; 1 or 2 excessively wet or, "wild boards," does not mean a problem, but triggers an additional check.

We now know the flooring moisture content and have established a target acclimation. If our readings show the average moisture content of the flooring is near the average for the area (within 1%), the flooring is considered acclimated enough for installation. If site conditions are ready, install the flooring. Any further pre-installation acclimation can be counter productive since the site may not be at the target average.

For a site finished floor after installation an additional acclimation period of 1-3 weeks is recommended. All the installed flooring boards are now equally exposed to the environment. The individual boards that do vary from the average have the opportunity to acclimate further and the minor issues related to shrinkage and expansion will be taken care of during finishing.

Factory finished flooring is different from unfinished flooring in that it is coated on one side and normally packaged in a box. These differences delay acclimation and may require extra time to acclimate. Knowing the moisture content of the flooring is critical to accessing acclimation. If the average flooring moisture content is different from the expected average; acclimation will be necessary. Opening the boxes may allow for some acclimation but it may be slow. Racking the flooring may be the quickest way to acclimation as it exposes all the boards.

Finally, if flooring is to be installed during the most extreme season acclimation to the average area MC can be very difficult. In these cases other than normal procedures may be necessary to accomplish acclimation.

What if the average MC of the flooring is low and it is the dry winter season? Even racking the flooring will not allow acclimation to a higher MC and may over dry the flooring. So don't acclimate any further, add some field expansion spacing to accommodate the expected expansion after the winter heating. Also, with a site finished floor don't fill the expansion spaces during finishing.

What if the average MC of the flooring for your area is too high, it is hot and humid and the subflooring is also near the maximum allowed MC? Acclimation to the lower MC will not occur. Cross stacking and covering the flooring with plastic while operating a dehumidifier under the plastic can reduce the MC of the wood. Turning the heat on can also help dry the site and the flooring. It will be important to monitor the MC of the flooring regularly to not over correct.

For optimum flooring performance:

  • Know your area's average and seasonal moisture content.
  • Determine the moisture content of the flooring as delivered.
  • Acclimate as necessary based on these readings.