What Is the Acceptable Moisture Level in Wood?
The acceptable moisture level in wood depends on the final use of the wood, the type and thickness of the wood, and the average relative humidity (RH) in the environment where the wood is to be used. Based on common guidelines or recommendations, the acceptable moisture level for wood objects used indoors is generally 6% to 8% for wood flooring, 6% to 9% furniture, and 9% to 14% for construction.
Moisture Content in Wood
Understanding the basics of how wood interacts with moisture is essential for anyone who works with wood – be it a woodworker, wood flooring professional, or construction professional.
Wood is hygroscopic. This means it gains or loses moisture as the RH of the air surrounding it changes.
As the humidity increases, the moisture content (MC) increases, causing the wood to expand. As the humidity decreases, the MC decreases, causing the wood to shrink. When the wood neither gains nor loses moisture, we say that the wood has reached its equilibrium moisture content (EMC).
Dr. Eugene Wengert, professor and extension specialist in wood processing, Department of Forestry, University of Wisconsin-Madison, says it is critical that the wood to be worked on should be dried down to a MC within 2 percentage points of the EMC of the in-use location.
“The EMC of air is numerically equal to the MC that will eventually be attained by any piece of wood when stored indefinitely at a particular humidity,” notes Wengert.
This is important to know. For instance, if a woodworker brings home kiln-dried wood with a MC of 8% and then stores it in his garage, that wood could absorb another 6% or more of moisture.
Depending on how he plans to use the wood, he must allow the wood to come in balance with the average RH at the location where it will be used. The end-use of a wood product is one of the key factors – along with RH – in determining the acceptable MC.
Failure to allow the wood to acclimate or come in balance with the RH at its end-use location will result in warping, cracking, and other problems after the wood product is constructed.
Wood Moisture from a Woodworker’s Perspective
Because wood shrinks and warps as it dries, woodworkers like long-time furniture maker Lonnie Bird wants it to be pre-shrunk before he uses it. “I don’t want the wood to shrink after I use it because the wood will warp or split.”
Bird, who runs the School of Fine Woodworking outside Knoxville, Tenn., says he knows wood will shrink seasonally, but he wants to minimize that shrinkage and expansion by drying the wood to a MC of about 8%.
To make certain the wood is properly dried, he uses a moisture meter before working on it.
For woodworkers who build cabinets, fine furniture, musical instruments, dishes, toys, and pieces of art, among other things, the acceptable moisture level of wood normally ranges from 6% to 8%.
That range can vary slightly depending on the region of the country where RH can either be very high or very low.
Thus, if an interior location has an average RH of 40% to 52%, the wood placed in that location will average 8% to 9% EMC (based on a chart from Wood Handbook: Wood as an Engineering Material).
A woodworker building a cabinet for that same interior environment would then have to dry his wood to 8% to 9% MC before and during construction to avoid post-construction problems.
The key to ensuring the wood is dried properly, therefore, is to be aware of the RH in which the wood will be kept and to use a moisture meter to accurately determine the wood’s MC.
Wood Moisture from a Flooring Installer’s Perspective
The National Wood Flooring Association (NWFA) has specific installation guidelines for wood flooring and how they relate to MC.
When determining the acceptable moisture level of wood flooring, NWFA states that the flooring professional should establish a baseline for acclimation. Acclimation is the process for conditioning the MC of wood flooring to the environment in which it will be installed.
To establish a baseline for acclimation, the installer will need to calculate the optimal wood MC by dividing the region’s high season and low season EMC. For example, if the expected EMC ranges from a low of 6% to a high of 9%, the baseline MC of the wood would be 7.5%.
The installer should then check the MC of multiple boards and average the results. A high reading in one area indicates a problem that must be corrected.
A good representative sample is typically 40 boards for every 1,000 square feet of flooring. If the MC of the boards is near 7.5%, no acclimation is required.
But if the MC of the product is well outside the range of optimal MC, the wood flooring should not be accepted as it will lead to shrinkage, bowing, cupping, and other physical problems.
For example, if the MC of the delivered wood is 12% and the optimal MC is 6%, then physical problems will occur during the acclimation process.
To avoid this problem, wood flooring should never be stored where environmental conditions are uncontrolled, such as garages and exterior patios.
As a general rule, with geographic exceptions, wood flooring performs best when the interior environment is controlled to stay within a RH range of 30% to 50% and a temperature range of 60 to 80 degrees Fahrenheit. However, the ideal humidity range in some climates may be higher or lower, such as 25% to 45% or 45% to 65%, for example.
The NWFA has a chart that indicates the MC of wood at any given combination of temperature and humidity. The EMC in the recommended temperature/humidity range coincides with the 6% to 9% range used by most flooring manufacturers during the manufacturing/shipping process. Although some movement can be expected between 6% and 9%, wood flooring can shrink or swell more dramatically outside this range.
Installers should also measure moisture in wood subfloors and concrete slabs as they, too, can affect the wood flooring. The maximum subfloor moisture level for solid strip flooring or wide-width solid flooring is either 12% or 13%, depending on the manufacturer.
According to the National Association of Home Builders’ Green Home Building Guidelines, solid strip flooring (less than 3” wide) should be no more than 4% MC difference between properly acclimated wood flooring and subflooring materials. For wide-width solid flooring (3” or wider), there should be no more than 2% difference in MC between properly acclimated wood flooring and subflooring materials.
Wood Moisture from a Builder’s Perspective
For most areas of the U.S., MC in the range of 9% to 14% is the acceptable or recommended MC for exterior wood or building envelope components within constructed assemblies. Wood MC in this range, therefore, is considered sufficiently dry for exterior in-service wood.
Using wood with a MC above 14% MC may have deleterious long-term effects on the construction and is not recommended.
According to M. Steven Doggett, Ph.D. LEED AP, the founder of Built Environments, Inc., wood MC as high as 15% can cause corrosion of metal fasteners and at 16% may lead to fungal growth.
Wood MC from 17% to 19% reduces the strength of plywood, while a MC of 20% or greater reduces the strength of dimensional lumber (lumber cut to certain pre-defined sizes, such as 2x4s).
A study by Imamura and Kiguchi (1999) showed that wood MC in excess of 20% can cause a 5% loss of nail shank diameter in four years and a projected 25% loss in 30 years. The same study showed 40% loss in joint strength and concluded that 20% MC may significantly compromise shear resistance of exterior walls.
When exposed to a constant RH, the MC of wood will come to equilibrium with its environment, resulting in an EMC for that species of wood-based composite. The EMC of wood exposed to an outdoor atmosphere varies across the U.S.
For instance, in the coastal city of Seattle, the EMC of wood is higher than the EMC of cities inland or in the Southwest.
Seattle’s EMC ranges from 12.2% to 16.5%.
In the Midwest, the EMC of wood in Des Moines, Iowa, ranges from 12.4% to 14.9%.
In contrast, Las Vegas in the drier Southwest has much lower EMC percentages than most other U.S. cities. The Las Vegas EMC of wood ranges from 4.0% to 8.5%.
In a Nutshell
Based on common guidelines or recommendations, the acceptable moisture level for wood objects used indoors is generally 6% to 8%; for wood flooring, it’s 6% to 9%; and for construction, it’s 9% to 14%.
The acceptable moisture level in wood depends primarily on the final use of the wood and the average RH at the place where the wood is to be used. Other factors may include the wood species and the thickness or size of the wood.
In all cases, determining the acceptable moisture level of wood requires the use of an accurate moisture meter.
Failure to allow the wood to acclimate or come in balance with the RH at its end-use location will result in any number of moisture related problems in the wood – including warping, cracking, buckling, diminished wood strength, corrosion of fasteners, and even fungal growth after the wood product is constructed.
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