Eleven Considerations for Acclimation Success

Acclimation, sometimes called conditioning, is the process of allowing wood to reach its equilibrium moisture content (EMC) within “expected living conditions.” It also is one of the most important steps of hardwood floor installation. Not properly acclimating or conditioning wood flooring may cause excessive expansion, shrinkage, dimensional distortion, or even structural damage.

If the flooring material being installed does not have specific acclimation and conditioning instructions, here are the steps to follow:

  1. Interior environmental conditions vary from region to region and jobsite to jobsite. It is the flooring professional’s responsibility to know what the expected normal living (e.g., as-occupied) conditions are and customize the floor selection around those conditions.
  2. Ensure that the building is enclosed and all wet work has been completed.
  3. Verify that the building is maintained at the expected normal living (e.g., as-occupied) conditions for temperature and humidity. Check the moisture content of exposed wood product on the jobsite for comparison using the EMC chart.
  4. Permanent heating and/or air conditioning systems should be operating a minimum of five days preceding installation to promote proper acclimation/conditioning and should be maintained during and after installation. If it is not possible for the permanent heating and/or air conditioning system to be operating before, during, and after installation, a temporary heating/cooling and/or humidification/dehumidification system that mimics expected normal living (e.g., as-occupied) conditions can enable the installation to proceed until the permanent system is fully operational.
    Note: Use of temporary propane heating systems will introduce moisture to the environment. Large amounts of water are produced by the combustion of propane.
  5. Do not store wood flooring at the jobsite in uncontrolled environmental conditions. Garages and exterior patios, for example, are not acceptable areas to store wood flooring.
  6. Upon delivery, check wood flooring moisture content with a moisture meter to ensure it is compatible with the expected living conditions. Check the moisture content of multiple boards from a variety of bundles. Take MC readings on a minimum of 40 boards for every 1,000 square feet of flooring and average your results, excluding any extreme readings. In general, more readings will result in a more accurate average.
  7. If wood flooring is delivered at the optimal MC for the geographical location and proper relative humidity conditions are maintained, no acclimation may be required.
  8. If the moisture content of the product received is outside of the range of the expected living conditions, acclimating the product to these conditions will be required in order to get the flooring aligned with the facility. This may include introducing moisture to the wood or removing moisture from the wood prior to installation. When doing so, you must take into account the increase/decrease in moisture will likely distort the wood, which may affect the installation.
  9. When flooring needs time to acclimate/condition to the environment, this can be facilitated by separating the flooring into small lots and/or completely opening the packaging. A common practice is to cross‐stack the materials with spacers (¾-inch to 1-inch sticks) between each layer of flooring to allow air circulation on all sides of all boards.
  10. While it takes time to acclimate/condition a product, the ultimate goal is to get the materials to reach a moisture content that is in equilibrium with its expected use. Bringing wood flooring to equilibrium includes conditioning the materials in a stable environment as long as necessary to ensure the materials have reached the proper moisture content based on the temperature and humidity levels. However, all involved parties must understand that when the environmental conditions change, so will the flooring’s moisture content.
  11. Before installation, ensure the facility has been conditioned to receive the wood flooring and that wood flooring is within an acceptable range of moisture content relative to the wood subfloor. For solid strip flooring (less than 3 inches wide), there should be no more than 4 percent moisture content difference between properly acclimated wood flooring and subflooring materials. For wide-width solid flooring (3 inches or wider), there should be no more than 2 percent difference in moisture content between properly acclimated wood flooring and subflooring materials.

Wood is only acclimated or conditioned once it reaches its equilibrium moisture content for the space in which it is expected to perform. Equilibrium moisture content is based on an “unchanging” environment. After a wood floor has been installed, changing conditions within the environment will change the equilibrium moisture content of the wood floor, ultimately resulting in dimensional change.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.

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