7 FAQs about Moisture Content in Wood
One of the most common problems sawyers deal with is the moisture content in wood. To help sawyers better understand this issue, here are the answers to 7 frequently asked questions about moisture content in wood.
1. How Much Moisture Does Wood Contain?
Often seen abbreviated as MC, moisture content is the quantity of water in a any kind of material, including wood. The primary factors that affect moisture content in wood include:
Species
Position of wood in the tree
Season of the year
In living trees, the moisture content of wood ranges anywhere from 30 to 200 percent. The reason that moisture in wood can exceed 100 percent is because the moisture content is equal to the weight of water divided by the weight of wood without the water.
Moisture exists as either free water or bound water. As soon as the tree is felled and exposed to the air, it instantly begins to lose moisture. Once wood is dried, the goal is to get the moisture content down to an acceptable amount, which is usually anywhere from 6 to 19 percent, depending on what the wood is going to be used for.
2. What Is Fiber Saturation Point?
The fiber saturation point (FSP) of wood is reached when wood has lost all of its free water but still contains bound water. When wood reaches its FSP, the moisture content is about 30 percent. After the wood’s FSP is reached, it begins to lose moisture in the form of bound water. It’s at this stage that wood has gone from green to drying. It’s also at this point that the change in moisture causes the wood’s properties to change.
This change in the wood’s properties can cause:
Increased weight
Shrinking or swelling
Decreased resistance to decay and insects
Moisture also affects the mechanical and thermal properties of wood, which is especially problematic for woodworkers during finishing process.
To reduce the ability for the wood’s property to change, moisture needs to be removed. The two primary ways to remove moisture from wood include kiln drying and air drying. After the wood is dried, it’s important to know how much moisture the wood contains.
3. How Is Moisture Content Measured?
There are various ways to measure moisture content in wood. The most common and the most accurate ways to measure moisture content is to use the oven-dry method. This method requires the wood to be cut into small pieces and then weighed. After you weight the wood, place it in the oven for 24 hours at 217 to 221 degrees F.
After that, weight the wood again. The difference in weight provides the weight of water lost during the drying process. You can then calculate the moisture content by dividing the weight of water that was lost by the dry weight of the wood.
One of the simplest ways to measure moisture content is to use a moisture meter. This handy tool has probes that get inserted into the wood. Although not as accurate as the oven-dry method, it is the quickest.
To read about other methods, we recommend checking out this article on BuilderBold.com called How to Calculate the Moisture Content of Wood: A Comprehensive Guide.
4. What Is the Allowable Moisture Content in Construction Lumber?
it’s important to know what the wood is being used for in order to determine an acceptable moisture content. The allowable moisture content is different in construction lumber and wood that’s used to make furniture.
According to some sources, the allowable moisture content for construction lumber that has been kiln-dried should have a moisture content of 12 to 14 percent. However, the American Softwood Lumber Standard defines dry as “lumber that is than nominal 5-inch thickness which has been seasoned or dried to a maximum moisture content of 19 percent.”
Building science experts state that moisture content above 19 percent is when mold spores become activated. Other problems that occur when the moisture content exceeds 19 percent include:
Corrosion to metal fasteners
Reduction in overall strength
Loss of joint strength
Compromised resistance to exterior walls
It’s a well-known fact that higher moisture content in construction lumber increases the chances of significant problems. For this reason, it’s crucial to for moisture content to stay below 19 percent.
5. What Is the Allowable Moisture Content in Wood for Furniture?
The allowable moisture content in wood for furniture or indoor applications ranges from 6 to 7 percent for hardwoods and 8 percent for softwoods. This is due to the fact that softwoods machine better when it has a little higher MC, plus they shrink and swell less than hardwoods when the moisture content changes.
When the MC in wood for furniture is too high, it could result in the following kinds of problems:
Sunken joints
Uneven surfaces
End splits
Curved or cupped panels
Splitting and chipped grain during the machining process
Gluing and laminating failure
Cracked finishes
The best way to prevent such problems is to always know the moisture content of the wood before you begin working with it. It’s not enough to just dry the wood, you also need to measure the MC.
6. What Is Equilibrium Moisture Content?
Even after wood is dried, however, moisture still affects it. But where does this moisture from? It comes from humidity in the air. When humidity goes up, the wood expands. When humidity goes down, the wood shrinks.
Wood that doesn’t gain or lose moisture has reached its equilibrium moisture content (EMC). In other words, over time, the wood has achieved equilibrium with the environment. Upon reaching EMC, if the temperature and relative humidity doesn’t change, then neither will the moisture content. For this reason, it’s important that wood gets acclimated to its surroundings before it gets used.
Equilibrium = a state at which opposing forces are balanced.
According to Dr. Eugene Wengert, a professor and extension specialist in wood processing at the University of Wisconsin-Madison, wood should be dried to an MC within two percentage points of the EMC.
7. How Is Equilibrium Moisture Measured?
Along with measuring moisture content, it’s also possible to measure the EMC. This can be done in two ways. The first is to check the moisture content with a moisture meter every few hours until the MC stops changing. When the moisture stops changing, it has reached EMC. The downside of this method is that it can take several days.
Another way to measure EMC is to know how much relative humidity is in the environment, as well as the ambient temperature. More information on this method, as well as the formula to use this method, can be found on the NC State University BioResources website.
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