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 involved 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 loose 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.