What is Kiln Dried Lumber?

Mary McMahon
Mary McMahon

Kiln dried lumber is lumber that has been heated 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 6% and 8%, making it significantly dryer than green lumber or air dried lumber. Many hardware stores and lumber companies stock such lumber for their clients, and this type of wood is ideal for a wide range of woodworking tasks.

Kilns in a lumber yard.
Kilns in a lumber yard.

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 their wood before offering it for sale, so that it will be ready to use. Drying it in a controlled environment is important, as it allows the wood 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.

Stacks of lumber.
Stacks of lumber.

In the case of kiln dried lumber, the wood is dried in giant kilns with carefully controlled temperature and humidity levels. The goal is to get the boards to dry quickly and evenly, and to prevent warping that could render the wood unusable. The alternative 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 6% to 8%.

For some applications, lumber that has been kiln dried 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 wood 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 that is available. It is also possible to air dry or kiln dry at home or in a shop, with green lumber purchased from a 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 wood is dried.

Kiln dried lumber is lumber that has been heated in a kiln so that it has a low moisture content.
Kiln dried lumber is lumber that has been heated in a kiln so that it has a low moisture content.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


Kiln dried timber will always gain some more moisture after. Wood is always balancing its humidity with the atmosphere.


@healthnwell- There are different ways to kiln dry wood, and, as @write79 said, there are different kilns available for the process. Some of these kilns use a system of heat being introduced through steam that is conveyed through tubes. Then the humidity is ventilated out of the kiln. Some kilns heat the wood with solar radiation, while others use microwave radiation. While the wood kilns used do not burn the wood, some of them may have negative effects on the environment.


The whole process of wood drying, no matter how you go about it, seems to be a very time consuming process. Even when kiln drying lumber, it may need to be acclimated to the environment it will be used in. The fine woodworkers that are willing to wait weeks for their wood to be acclimated, must have a lot of patience.


I thought I knew a lot about the different types of woods available before I read this, but I didn't realize that wood could be kiln dried. This is so interesting, because I thought the wood would just burn. Can anyone explain how this does not happen?


If you are looking to purchase a wood kiln to do your own wood drying, you should be aware that there are five different types that use different technologies. These are conventional wood drying ovens, solar kilns, vacuum wood kilns, dehumidification wood kilns, and radio frequency wood kilns. Make sure you pick the wood kiln that best fits your needs.

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