What is Kiln Dried Lumber?
Kiln dried lumber is lumber which has been dried in a kiln so that it has a low moisture content. When the drying process is complete, the lumber usually has a moisture content of between six and eight percent, making it significantly dryer than green lumber or air dried lumber. Many hardware stores and lumber companies stock kiln dried lumber for their clients, and this type of lumber is ideal for a wide range of woodworking tasks.
When lumber is first harvested and cut, it is known as “green” because it has not been dried or treated. Green lumber is very moist, and if it is used as-is, it tends to warp as it dries and contracts. Therefore, most lumber companies dry lumber before offering it for sale, so that it will be ready to use. Drying the lumber in a controlled environment is important, as it allows the lumber to contract without warping. The lumber may also be treated with chemicals so that it will resist insects, mold, mildew, and rot; treated lumber tends to be slightly more expensive.
In the case of kiln dried lumber, the lumber is dried in giant kilns with carefully controlled temperature and humidity levels. The goal is to get the lumber to dry quickly and evenly, and to prevent warping of the lumber which could render it unusable. The alternative to kiln drying is air drying, in which lumber is allowed to sit in a breezy area so that it loses moisture and contracts. Air drying tends to take longer, and it brings the moisture content closer to 15% than the desired six to eight percent.
For some applications, kiln dried lumber can be used just as it is. In other instances, it may be necessary for the lumber to acclimate for one to two weeks, and the lumber may pick up some additional moisture during this time. Acclimation is important for crafts like cabinet-making, as changes in moisture content can cause problems with joints and moving parts, as the wood will swell or contract when moisture levels change. Fine woodworkers often allow lumber to acclimate in a corner of a climate-controlled shop for several weeks before they use it.
Some people firmly believe that there are appreciable differences between air dried and kiln dried lumber, preferring one style over another. Others feel that there are no significant differences, and that it is far more practical to simply use the lumber which is available. It is also possible to air dry or kiln dry at home or in a shop, with green lumber purchased from a lumber supplier. For certain applications, supervising the drying process may be preferred, since it allows for absolute control over the moisture content and conditions in which the lumber is dried.
Written by S.E. Smith
