Moisture loss of lumber is dependent on temperature, relative humidity, and air velocity. In a kiln, the operator has control over these variables. This control results in faster drying times, reduces drying degrade, produces phytosanitary lumber, and has a more consistent moisture content compared to air dried lumber.
Why Kiln Dried?
People kiln dry lumber to produce dry lumber fast and efficiently. Air dried lumber takes years to equalize to the proper moisture content. In a kiln, depending on the species and thickness, it normally doesn’t take more than a month or two. This doesn’t mean that the lumber has more drying degrade just because it was dried quicker. In fact, kiln dried lumber, if done properly, will have very little drying degrade. The tight controls allow the kiln operator to produce high quality lumber but still drive the moisture out of the wood at an accelerated rate. This is done by tracking the moisture loss.
Most companies also use a combination of air drying and kiln drying. They use air drying to remove most of the liquid water (free water) in the wood and then put the lumber in a kiln to pull the saturated water (bound water) out of the wood and bring it to the proper moisture content.
Moisture Content
Kiln dried lumber has a very consistent moisture content. As most wood industry people know, if the lumber you start with isn’t the proper moisture content, it causes the lumber to move (shrink) much more in the future. This causes pieces to warp, cup, twist, etc. Mother Nature has control over how fast air dried lumber is going to dry. Some examples of this are one side of a pack might be exposed to more light/heat or air movement. This will cause that side to dry faster, therefore, causing it to be a lower moisture content compared to other parts of the pack.
Pests and Bacteria
The kiln also kills any pests or bacteria that might be in the lumber. The pests and bacteria are killed from the elevated temperatures. Unfortunately, we do have diseases in our forests. And sometimes these diseases target specific species, like dutch elm disease or the emerald ash borer. Phytosanitary documents are needed for trading lumber internationally, over most state lines, and even over some county lines. This is to slow down and isolate these diseases from spreading.
The only downside to kiln dried lumber is the kiln drying process induces drying stresses in the wood. But don’t worry; kiln operators add an extra step to the process to relieve this stress. This step is called conditioning. The stress is relieved by introducing live steam into the kiln. The steam then condenses on the surface of the wood and relieves the stress.
It’s not much steam. The wood only absorbs about 1% moisture content in this stage. Most air dried lumber will have very little drying stress. This is for a few reasons. The first being the lumber is dried much slower. The second is because any drying stress that is introduced is relived at dew point (if the lumber is outside).
Kretz Lumber Difference
Kretz Lumber sells kiln dried lumber. It is the only way to guarantee a consistent moisture content, highest quality lumber, and phytosanitary lumber. We dry our lumber to 6-8% moisture content and make sure it is properly conditioned. We want to make sure that your products will stand the test of time. If you have any questions about our kiln drying process, please contact us at 1-800-350-1438 or email kretz@kretzlumber.com
