PART ONE – General Acclimation / Conditioning Guidelines
ACCLIMATION: The process of adjusting (conditioning) the moisture content of wood flooring to the environment in which it is expected to perform.
EQUILIBRIUM MOISTURE CONTENT: The moisture content of wood when in equilibrium with its environment. When wood is neither gaining nor losing moisture, equilibrium moisture content (EMC) has been reached.
Storage & Conditions
- Do not store wood flooring at the jobsite under uncontrolled environmental conditions. Garages, and exterior patios, for example, are not acceptable areas to store wood flooring.
- Ideal interior environmental conditions vary from region to region and jobsite to jobsite. It is the flooring professional’s responsibility to know what the “ideal” climate conditions are and customize the floor around those conditions.
- Determine what the expected seasonal change of wood moisture content is for your geographical location.
- Upon delivery, check wood flooring moisture content with a moisture meter to establish a baseline for acclimation. Check the moisture content of multiple boards. A good representative sample is typically 40 boards for every 1,000 square feet of flooring. Calculate what the optimal wood moisture content is (baseline) by dividing the high season and low season. Example: If your region has an expected EMC from a low of 6% to a high of 9%, the baseline MC of the wood would be 7.5%. If wood flooring is delivered and recorded to its baseline MC for the geographical location and proper relative humidity conditions are maintained, no acclimation may be required. If the moisture content of the product received is well outside of the range of optimal moisture content, it will be very difficult to acclimate the product properly without substantial dimensional change, distortion, and structural damage. Example: If the moisture content of the delivered wood is 12% and the optimal range is 6%, excessive shrinkage, bowing, cupping and other physical anomalies would be expected during the acclimation process. The wood flooring should not be accepted.
- Optimal wood moisture content represents only a baseline to begin from and does not represent the final EMC required for the interior environment. Acclimation is often required to customize the moisture content of the wood flooring to the interior environment in which it is expected to perform.
NOTE: Some manufacturers do not require acclimation for certain products prior to installation. If the manufacturer recommends that the wood flooring be acclimated before installation, proceed as follows:
- Ensure that the building is enclosed.
- Verify that the building is maintained at normal living conditions for temperature and humidity.
- Where building codes allow, permanent heating and/or air conditioning systems should be operating at least five days preceding installation to promote proper acclimation and should be maintained during and after installation. For radiant heat, see Appendix H.
- 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 and/or dehumidification system that mimics normal living (occupied) conditions can enable the installation to proceed until the permanent heating and/or air conditioning system is fully operational.
- Acclimate the wood flooring as necessary (see Acclimation, Part II below). Note: Not properly acclimating wood flooring may cause excessive expansion, shrinkage, dimensional distortion or structural damage. The worst-case scenario is one in which wood flooring is stored at the jobsite in an uncontrolled environment, then immediately installed. This is especially true when the materials are stored in an area that is subject to excessive moisture and humidity conditions. Acclimation outside of the area in which the wood is to be installed does no good at all; in fact, it is likely harmful to store wood flooring at the jobsite under conditions that don’t reflect expected normal environmental conditions.
- Prior to installation, ensure that wood flooring is within acceptable range of moisture content with the wood subfloor. For solid strip flooring (less than 3″ 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″ or wider), there should be no more than 2 percent difference in moisture content between properly acclimated wood flooring and subflooring materials.
PART TWO – Acclimation
Wood flooring is a hygroscopic material subject to dimensional change as a result of variations in moisture, temperature and humidity within the surrounding environment. Wood flooring simply needs to reach moisture content level in equilibrium with the surrounding environment (EMC) in which it will be installed, at or near normal living conditions. The process of reaching this equilibrium is defined as acclimation, which allows the wood to properly adjust itself to the normal living conditions within the structure; that is, the temperature, humidity conditions and moisture content that will typically be experienced once the structure is occupied.
The Process of Acclimation
If the manufacturer recommends that the wood flooring be acclimated before installation, proceed as follows:
- Acclimation can be facilitated by breaking the floor units into small lots and/or opening the packaging. A common practice is to cross-stack the materials with spacers (¾” to 1″ sticks) between each layer of flooring to allow air circulation on all sides of all boards.
- Most recommendations state that the materials need to acclimate from a minimum of 3 days up to no given maximum. While it takes time to acclimate a product, the most important aspect is that the materials reach a moisture content that is in equilibrium with its expected use. Acclimate the materials as long as necessary to accomplish this task, taking the necessary moisture readings to indicate when the materials have reached the proper moisture content and when no further changes occur.
- For site-finished wood flooring, before installation and before sanding and finishing takes place, allow the flooring to acclimate (settle-in) to the controlled environment, and to stabilize for a period of time. Some flooring professionals suggest 5 to 7 days. Engineered flooring installed using an adhesive application system may require a longer post-installation acclimation period to allow all residual off-gassing to occur prior to application of a finish. Follow adhesive manufacturer’s recommendations.
- Tropical imported species generally require more time in order to properly acclimate the wood flooring. Some tropical species lose moisture or gain moisture at faster or slower rates than domestic species due to higher overall density, oil and resin content and interlocking cell structure. In addition, the resins and oils make accurate MC readings more difficult. Resistance (pin type) meters require multiple readings of multiple boards in order to arrive at a more accurate average MC reading. Pinless meters that include multiple depth level adjustments may offer faster and more-accurate internal readings.
- Engineered and solid factory finished flooring follows specific manufacturer’s recommendations and some may not require acclimation. Follow manufacturer’s guidelines to retain all warranty coverage. Warranty coverage generally requires that jobsite conditions be maintained between 30% to 50% relative humidity and that those conditions must be maintained before, during and after installation for the life of the floor. Failure to comply with these manufacturer’s requirements may result in irreversible structural damage and void related warranties.
Wood’s Comfort Zone
- As a general rule, with geographic exceptions, wood flooring will perform best when the interior environment is controlled to stay within a relative humidity range of 30 to 50 percent and a temperature range of 60° to 80° Fahrenheit. (In some climates, the ideal humidity range might be higher or lower, 25 to 45 percent or 45 to 65 percent, for example.)
- The chart below indicates the moisture content wood will likely have at any given combination of temperature and humidity. Note the equilibrium moisture content in the recommended temperature/humidity range (shaded area) coincides with the 6-to-9 percent range used by most flooring manufacturers during the manufacturing/shipping process. Although some movement can be expected between 6 and 9 percent, wood flooring can shrink or swell more dramatically outside this range. When wood is neither gaining nor losing moisture, equilibrium moisture content (EMC) has been reached.
Equilibrium Moisture Content of Solid Wood Species at Various Temperatures and Relative Humidity Readings Wood flooring has a comfort level too. Wood flooring will perform best when the interior environment is controlled to stay within a relative humidity range of 30% to 50% and a temperature range of 60° to 80° Fahrenheit. Fortunately, that’s about the same comfort range most humans enjoy.
These guidelines have been defined by the National Wood Flooring Association for proper hardwood flooring acclimation.