From the Wood Doctor: Wood & Moisture Training

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

Wood and wood products, even though they have been kiln-dried, are always gaining or losing moisture in an attempt to achieve moisture equilibrium with their environment.

In other words, wood will gain moisture in more humid locations and will lose moisture in drier locations. This gain and loss of moisture can become critical when it is recognized that wood swells when it gains moisture, and shrinks when it loses moisture.

In addition, wood machines and glues poorly if the moisture content is too high or too low.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

The moisture content of a piece of wood and equilibrium with its environment is determined by the relative humidity of the air surrounding the piece of wood. This is true whether the wood is lumber and the manufacturing process is an unfinished component in transit or storage or is finished with the coatings, such as varnish and placed in use.

Temperature does not affect the moisture content appreciatively, and therefore does not make wood shrink or swell appreciatively either. This lack of temperature sensitivity is one advantage of wood when compared to other building materials such as aluminums or plastics.

Before proceeding further, let’s look at the critical relationship between relative humidity and moisture content. For a given average relative humidity in the air, there is an average moisture content that wood would equilibrate to.

For example, if the relative humidity in the air is 30%, wood stored at this condition will eventually achieve 6% moisture content. Hours, days, weeks, or even months may be required for this final equilibrium value to be reached.

The time depends on many factors, including size of the piece of wood, thicker will take longer; the grain orientation, and grain will be shorter; the ambient air temperature, hotter will be shorter; and the coating applied if any, coated wood will take longer.

The moisture in the air is referred to as the Equilibrium Moisture Content or EMC.

For our example of 30% relative humidity, the air is 6% EMC.

There are 4-key values of relative humidity and corresponding EMC that a wood manufacturer or user should be very familiar with, and in fact, would be wise to commit to memory.

The first two, 6% to 9% EMC, correspond to 30% relative humidity to 50% relative humidity, respectively. They are very important, as they are the typical interior values for heated and air-conditioned offices and homes in most of North America. These values apply to lumber, green or kiln-dried, as well as rough parts, manufactured components, cabinets, furniture and millwork.

In cold weather climates, interior conditions may be even drier than 6% EMC. In manufacturing facilities with dust-exhaust systems, and where air is heated for comfort, the in-plant conditions are also often drier than 6% EMC, unless the air is humidified. On the other hand, in humid summer months, especially without air-conditioning, conditions in homes, offices, and manufacturing facilities may sometimes exceed 9% EMC.

For exterior exposures protected from the rain in most of the U.S., including warehouses, unheated sheds, and even unheated homes that are under construction, the air averages 12% EMC equivalent to 65% relative humidity.

For coastal areas, Miami, New Orleans, Seattle, and island climates, Jamaica and Japan, the EMC is higher. Interior locations can be up to 12% EMC, exterior locations up to 16% EMC.

Uncoated wood will change its moisture content faster than well wooded that is coated. But, note that the coating only slows the change in moisture content. Often, wood products with a coating, will change moisture content quite slowly, meaning, that brief extremes of very dry or very humid conditions will not be as pronounced as within uncoated product.

And one other basic concept, kiln-drying does not provide a wood product that will not change moisture. All wood changes its moisture content when the humidity changes. Kiln-drying is just a rapid and quality technique to bring the moisture content of lumber down to a desired value. If kiln-dried lumber is properly dried, but the parts and components made from this lumber are not properly handled and are allowed to regain moisture. Then, subsequent shrinking in a dry manufacturing- plant environment, or in a customer’s dry home or office can be expected.

We will discuss measurement of EMC and moisture content in a moment. But, before we do that, let’s consider some of the reasons for our critical concern about moisture content.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

The moisture content of a piece of wood and equilibrium with its environment is determined by the relative humidity of the air surrounding the piece of wood. This is true whether the wood is lumber and the manufacturing process is an unfinished component in transit or storage or is finished with the coatings, such as varnish and placed in use.

Temperature does not affect the moisture content appreciatively, and therefore does not make wood shrink or swell appreciatively either. This lack of temperature sensitivity is one advantage of wood when compared to other building materials such as aluminums or plastics.

Before proceeding further, let’s look at the critical relationship between relative humidity and moisture content. For a given average relative humidity in the air, there is an average moisture content that wood would equilibrate to.

For example, if the relative humidity in the air is 30%, wood stored at this condition will eventually achieve 6% moisture content. Hours, days, weeks, or even months may be required for this final equilibrium value to be reached.

The time depends on many factors, including size of the piece of wood, thicker will take longer; the grain orientation, and grain will be shorter; the ambient air temperature, hotter will be shorter; and the coating applied if any, coated wood will take longer.

The moisture in the air is referred to as the Equilibrium Moisture Content or EMC.

For our example of 30% relative humidity, the air is 6% EMC.

There are 4-key values of relative humidity and corresponding EMC that a wood manufacturer or user should be very familiar with, and in fact, would be wise to commit to memory.

The first two, 6% to 9% EMC, correspond to 30% relative humidity to 50% relative humidity, respectively. They are very important, as they are the typical interior values for heated and air-conditioned offices and homes in most of North America. These values apply to lumber, green or kiln-dried, as well as rough parts, manufactured components, cabinets, furniture and millwork.

In cold weather climates, interior conditions may be even drier than 6% EMC. In manufacturing facilities with dust-exhaust systems, and where air is heated for comfort, the in-plant conditions are also often drier than 6% EMC, unless the air is humidified. On the other hand, in humid summer months, especially without air-conditioning, conditions in homes, offices, and manufacturing facilities may sometimes exceed 9% EMC.

For exterior exposures protected from the rain in most of the U.S., including warehouses, unheated sheds, and even unheated homes that are under construction, the air averages 12% EMC equivalent to 65% relative humidity.

For coastal areas, Miami, New Orleans, Seattle, and island climates, Jamaica and Japan, the EMC is higher. Interior locations can be up to 12% EMC, exterior locations up to 16% EMC.

Uncoated wood will change its moisture content faster than well wooded that is coated. But, note that the coating only slows the change in moisture content. Often, wood products with a coating, will change moisture content quite slowly, meaning, that brief extremes of very dry or very humid conditions will not be as pronounced as within uncoated product.

And one other basic concept, kiln-drying does not provide a wood product that will not change moisture. All wood changes its moisture content when the humidity changes. Kiln-drying is just a rapid and quality technique to bring the moisture content of lumber down to a desired value. If kiln-dried lumber is properly dried, but the parts and components made from this lumber are not properly handled and are allowed to regain moisture. Then, subsequent shrinking in a dry manufacturing- plant environment, or in a customer’s dry home or office can be expected.

We will discuss measurement of EMC and moisture content in a moment. But, before we do that, let’s consider some of the reasons for our critical concern about moisture content.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

As we’ve already mentioned, but is so critical that it deserves repeating, most offices and homes in North America, have an average interior EMC of 6% to 9%, a relative humidity of 30% RH to 50% RH. Sometimes, the home or office may be drier, such as during in the middle of winter in the Northern U.S. or during the summer in the dry South Western U.S. On the other hand, sometimes, the home or office along the Southern or Western coastal areas maybe wetter.

The correct moisture content for lumber coming in to a manufacturing facility, for wood products during manufacturing, and for the product during storage and installation, depends on the ultimate in use EMC for the product. Because having lumber even a little too wet cause shrinkage, and shrinkage problem are usually more serious than swelling problems, and because the typical home or office is seldom much drier than 6% to 7% EMC, 28% to 38% relative humidity.

The typical desired average moisture content for lumber, parts and components intended for interior products, is 6% to 7% moisture content. Electronic relative humidity indicators are available for under $30 at electronic supply stores. It is therefore easy and relatively inexpensive to determine the precise humidity and associated EMC in any location where wood products are manufactured, stored, installed or being used.

Although the average piece of wood is 6% to 7% moisture content in most interior locations, wood is a variable material with no two pieces being identical, therefore, there will always be some small variation in moisture content under 1% moisture content in individual pieces exposed to the same EMC.

As a rule of thumb, the ideal difference between the moisture content of wood and the EMC of air is 2% moisture content. However, appreciate that the amount of variability that can be accepted, depends on the product being manufactured and its sensitivity to moisture content changes.

Achieving better moisture content uniformity requires more effort during kiln-drying and better dry lumber storage in handling, which both add to the price of the lumber and components. However, reducing the risk of rejects will be well worth the extra cost and care.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

The key to successful control of moisture content begins with the kiln operation. Then once the lumber is kiln-dried, the lumber must be stored at the correct EMC to prevent undesired changes and moisture content. Most commercial drying operations dry to the correct moisture content and store lumber at the correct EMC or relative humidity.

The second step in controlling moisture content and this second step is even more critical than the first is at the manufacturing plant. Kiln-dried lumber in process parts and manufactured wood components when they first arrived must be stored at the correct EMC to prevent undesired changes in moisture content. Because components are much smaller than lumber they have more surface area per volume and more exposed and grain than lumber.

Therefore components will change moisture content much faster than lumber if the EMC of the air isn’t equal to the moisture content of the component. Parts and components are also much more valuable than rough lumber so proper storage of parts and components is economically more critical than storage of lumber.

A critical element in controlling moisture content in wood components is controlling the EMC in the manufacturing facility. The EMC in the facility must be very close to the EMC in the customer’s home or office. This may involve the addition of moisture to the air in the manufacturing facility. But even moisture control in the plant will not correct moisture problems if the components or parts are not stored properly when first received.

Another critical concern is the storage of finish goods at the customer’s home or office. Good should never be stored outside or in an unheated building or shed. The EMC is much too high. It is also possible that in a building under construction the EMC may temporarily be much higher than when the building is occupied. If so, stored wood products will regain moisture at first and then lose moisture quickly after the building has been occupied developing warp, cracks, and other moisture defects.

A target EMC in a room can be easily maintained.The easiest way to lower the EMC and relative humidity of the air, that is “dry the air”, is by adding heat to the room. A rule of thumb is that a room heated about 25 degrees Fahrenheit above the morning’s low temperature will develop 7% EMC, which is considered to be a good target EMC value.

The EMC in a small lumber storage area can also be controlled by enclosing the storage area and controlling the relative humidity with a small home sized dehumidification unit. This will work only when the lumber is already at the correct moisture content. There is not enough power in the unit to dry the lumber further.

Similarly there should be no substantial outside air leaks in the room. This D H controlled storage area could also be used in humid environment to store work-in-process or finished goods to prevent unwanted regain of moisture. Although such storage involves a little effort, the pay back is a better quality product with no customer complaints.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

Moisture content of wood components should be monitored to assure that unnecessary gain or loss of moisture hasn’t occurred. Once the moisture content changes appreciatively, it will take a long time to restore the wood to its correct moisture. In fact, it’s a big waste of time and money to process wood that’s at a wrong moisture content.

Proper handling throughout the manufacturing process will assure this doesn’t happen. Wood components, parts, or even final products that are too dry, under 5% moisture content, are difficult to restore to their previous quality.

The best procedure is to put the wood in a room that’s perhaps 1 percentage point above the intended EMC. The room should also have several fans to stir the air and pass it across the wood surfaces, then wait for as long as it takes for the moisture content to adjust; several weeks or longer.

Wood parts or components that are too wet can often be re-dried successfully. However, it’s difficult to successfully re-dry a finished product. If the high moisture content is a result of poor storage conditions, the wood can usually be placed into a warm room that has an EMC 1 or 2 percentage points drier than the required moisture content. A little heat will speed up the process of drying. The wood needs to stay in this room until the core has achieved the correct moisture content and not just for the time required for the outer layers to achieve the correct moisture content.

By Dr. Eugene Wengert – Copyrighted material.
Used with permission by Wagner Meters

We have discussed the moisture content of wood, but have not yet indicated the definition of moisture content.

The strict definition of wood moisture content is the weight of water in a piece of wood compared to these pieces oven-dry weight. Moisture content is always expressed in percent (%), so we multiply the answer by 100.

We measure the amount of water by weighing the piece of wood, then oven-drying the piece at 215 degrees Fahrenheit for 12 to 36 hours, until the wood is dry. And then, reweighing the piece after oven-drying.

The difference between the two weights is the amount of water that was in the wood before drying. The final weight is the oven-dry weight.