Acceptable Moisture Levels in Wood – Knowing the Moisture Content
Understanding Moisture Content in Wood
How Does Moisture Affect Wood?
Everyone who works with wood needs to understand how wood interacts with moisture in the environment. Whether you’re a woodworker making cabinets, a wood flooring professional installing hardwood floors, or if you use wood in construction, wood moisture content (MC) should be always on your mind.
Wood is hygroscopic. It gains or loses water moisture as the relative humidity (RH) of the surrounding air changes.
These varying humidity levels of the surrounding air cause wood to not only gain or lose water moisture but to expand or shrink as well. As the humidity increases, the 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).
According to Dr. Eugene Wengert, professor and specialist in wood processing at the University of Wisconsin-Madison’s Department of Forestry, wood should be dried to a water MC that’s within two percentage points of the EMC where the wood is going to be used:
Before we explain what this means, let’s make sure we have our definitions down.
- MC = the wood’s moisture content
- The EMC (equilibrium moisture content) of the location where the wood is at the moment or the location of where the wood is going to be used = the MC that the wood will eventually attain if it’s placed in that location.
If this is confusing, don’t worry. The chart below will clear things up. Notice that the EMC of the in-use location is the same as the MC:
|Humidity of the in-use |
|EMC of the in-use location||Corresponding MC the wood will attain at this location|
So, using this chart, we know that in an area of the country where the RH inside a home or office is anywhere from 26-32%, both the EMC of the in-use location and the wood MC kept in that location will be 6%.
This means that wood intended for interior use in this location should not only be dried to around 6% but should be kept at this MC both before and during the manufacturing process.
So, the wood must always be allowed to acclimate or come into balance with the RH of the end-use location. Failure to do this will result in warping, cracking, and other problems after the construction of the wood product.
How Do You Remove Moisture From Wood?
Freshly cut wood has a lot of moisture in it. Eventually, this internal moisture will evaporate by itself. However, kiln drying is used to speed up the process. Some of the unfinished wood you see on the market has been kiln-dried to reduce its water MC to around 8% so that it won’t suffer from moisture-related defects like warping and buckling. However, many building materials may have been dried down to about 15% MC.
But, that’s not the end of the story…
Wood MC is always varying. It’s never constant. Wood – freshly cut or kiln-dried – is always interacting with environmental moisture. Therefore, just because the wood is kiln-dried doesn’t mean it has lost the ability to absorb moisture. It will continue to absorb and release moisture until it comes into balance with the surrounding air.
What Level of Moisture Is Acceptable in Wood?
The acceptable MC in wood depends on two factors:
- The wood’s final use.
- The average RH of the environment where the wood will be used.
These two factors make it difficult to say anything specific about acceptable wood MC. It’s more important to understand that the wood is kiln-dried down to a certain bell-shaped range of MCs. There will be statistical outliers on both the low and the high end and you’ll want to catch these by using a quality moisture meter.
How Do I Measure Moisture In Wood?
There are two main ways of measuring wood MC: oven-dry testing and moisture meter testing. Let’s go over the basics of each…
1. Oven Dry Testing
Oven dry testing is the oldest method for measuring the MC of wood. The process is time-consuming but produces accurate results if followed correctly. Here’s how it works…
The wood sample being tested is dried in a special oven or kiln and its weight periodically checked. Once the wood sample’s weight stops changing, its weight is compared to what it was before the drying process began. This weight difference is then used to calculate the wood’s original MC.
While oven-dry testing, if followed correctly, offers accurate results, there are a few drawbacks:
- It takes a long time – We’re talking about hours. The oven drying process must be done slowly or the wood could burn and the test results will be worthless.
- It will render the wood unusable – It often happens that oven drying over dries the wood to the point where it’s unusable.
- It requires a special oven or kiln – Most hobbyists who work with wood don’t have an oven that’s capable of producing accurate results.
These three drawbacks mean that oven-dry testing usually isn’t the option of choice for hobbyists who work with wood.
2. Moisture Meter Testing
The fastest way to test the MC of wood is to use a moisture meter. There are two main types of wood moisture meters, pin-type and pinless.
Pin-type Wood Moisture Meters
Pin-type meters use penetrating electrodes and measure the wood’s MC using electrical resistance. Since water conducts electricity and wood doesn’t, the dryness of the wood can be determined by the amount of resistance to an electrical current. Dryer wood produces more resistance than wetter wood.
Pinless Wood Moisture Meters
Pinless meters are non-penetrating and read the MC via a non-damaging electromagnetic sensor that scans the wood. Because pinless meters scan the wood’s surface and cover a larger area than pin-type meters, they provide a more thorough picture of the wood’s MC.
Pinless meters also don’t leave damaging pinholes on the wood’s surface. This makes pinless moisture meters perfect for measuring the MC of things like expensive hardwood floors.
How to Measure MC in Wood with a Moisture Meter?
Pin-Type Moisture Meters
The general process for using pin-type moisture meters is as follows…
- Insert the pins into the surface of the wood you want to test.
- Make sure they’re aligned with the grain and not across it.
- Turn on the meter. The electrical current will then move from pin to pin and measure the resistance encountered.
Pinless Moisture Meters
Pinless moisture meters are even easier to use. Just press the scanning plate against the wood’s surface, turn on the meter, and receive the reading.
How Accurate are Wood Moisture Meters?
The ASTM D4442 standard determines the accuracy of wood moisture meters. This method uses the oven-dry method and then compares the results to those obtained using a moisture meter. The difference is the moisture meter’s measurement error.
For more information, see our article comparing Pinless Moisture Meters vs. Pin Meters.
MC of Wood from a Woodworker’s Perspective
Because wood shrinks and warps as it dries, woodworkers want it to be pre-shrunk before they use it. Furniture maker Lonnie Bird weighs in,
“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, Tennessee, says he knows wood shrinks seasonally but wants to minimize the shrinkage and expansion by drying the wood to an MC of about 8%.
To make sure the wood is properly dried, he always uses a moisture meter before working with it.
The MC of freshly cut wood is typically somewhere between 40-200%. If you’re wondering how wood can have a 200% MC, here’s how that works…
Because the MC of wood is equal to the weight of the water in the wood divided by the weight of the wood without the water, it’s possible for the MC to exceed 100%. In other words, the water weighs more than the wood fibers.
The normal MC of wood (or EMC) varies from 7%-19% depending on the RH in the air.
For woodworkers who build cabinets, fine furniture, musical instruments, dishes, toys, decorative art, boat restoration, or various other wood products, the acceptable wood MC normally ranges from 6% to 8%.
However, this range will vary slightly according to the geographic region because of varying RH levels.
If an interior location has an average RH of 40-52%, wood placed there will have an average EMC of 8-9%. This is based on a chart in Wood Handbook: Wood as an Engineering Material.
Therefore, in order to avoid post-construction problems, a woodworker building a cabinet for this particular interior environment would need to dry his wood to an MC of 8- 9% beforehand and then keep it that dry during the construction process.
The best way to do this is to use an accurate moisture meter.
MC of Wood 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 levels in wood flooring before installation, the 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 where it will be installed.
To establish a baseline for wood flooring acclimation, the installer will need to calculate the optimal moisture level of the wood 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.
We really can’t overstress the importance of taking plenty of moisture readings. When you do this, you not only ensure that the entire batch is okay on average, but you’re also far likelier to catch boards that are statistical outliers and could cause problems.
If the MC of the product is well outside the range of optimal MC, the wood flooring should not be accepted because 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 an 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. For example, from 25% to 45% or from 45% to 65%.
The National Wood Flooring Association (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 and 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 the moisture in wood subfloors and concrete slabs as they can also affect 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.
The National Association of Home Builders’ Green Home Building Guidelines for solid strip flooring and wide-width flooring are as follows:
- For solid strip flooring (less than 3” wide) there should be no more than a 4% difference in MC between properly acclimated wood flooring and subflooring materials.
- For wide-width solid flooring (3” or wider), there should be no more than a 2% difference in MC between properly acclimated wood flooring and subflooring materials.
MC of Wood and Lumber from a Builder’s Perspective
For most areas of the United States, acceptable moisture levels of wood and lumber can be in the range of 9% to 14% for exterior wood or for building envelope components within constructed assemblies. An MC in this range, therefore, is considered sufficiently dry for exterior in-service wood.
Using wood with an MC above 14% isn’t recommended because it may have detrimental long-term effects on the construction.
In fact, 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.
When it comes to the MC of plywood or dimensional lumber, an MC of 17% to 19% reduces the overall strength of plywood, and an MC of 20% or more reduces the strength of dimensional lumber (i.e., lumber cut to certain predefined 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 a 40% loss in joint strength and concluded that a 20% MC may significantly compromise shear resistance of exterior walls.
When exposed to a constant RH, the MC of wood or lumber will come to equilibrium with its environment, resulting in an EMC for that species of a wood-based composite.
The EMC of wood or lumber exposed to an outdoor atmosphere varies across the U.S. For instance, in the coastal city of Seattle, the EMC of wood or lumber 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 or lumber 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 or lumber ranges from 4.0% to 8.5%.
Acceptable Moisture Levels for Wood In a Nutshell
Based on common guidelines or recommendations, the acceptable moisture levels for wood are as follows:
- Wood objects used indoors: 6-8%
- Wood flooring: 6-9%
- Construction: 9-14%
Keep in mind that a wood’s acceptable moisture level will depend primarily on how it will be finally used and the average RH where it will be finally used. However, the wood species and the thickness or size of the wood may also factor in.
In all cases, determining the acceptable moisture level of wood requires the use of an extremely accurate moisture meter.
Failure to allow the wood to acclimate or come into balance with the RH at its end-use location could result in any number of moisture-related problems after the wood product is constructed. These include warping, cracking, buckling, diminished wood strength, corrosion of fasteners, and even fungal growth.
Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.