Q&A / 

Kiln Lumber Drying

DEAR TIM: You mentioned moisture content in new construction can continue to dry and cause cracks in walls. Can you elaborate on moisture content? If kiln dried, is moisture reduced to 10%, etc? How long for moisture to get to 0%? Dave G, Red Bluff, CA

DEAR DAVE: Lumber is a hygroscopic material. This means it can absorb moisture in both liquid and gas form through its very open cellular structure. Think of a tree when it's standing up as a massive bundle of very tiny straws packed next to one another like the straws jammed into a canister at an old-time soda shop. Cut off the end of a piece of lumber exposing a fresh edge and then paint the end with some water. You'll see the water disappear into the wood as if there was a vacuum attached to the other end of the piece of lumber.

Kiln-dried lumber has the moisture baked out of it. It's not much different than the oven in your home. I can't tell you how long it would take in a kiln to get the moisture to zero percent. It doesn't matter because once you remove it from the kiln, the moisture content of the lumber will start to rise as it absorbs water from the air. It will stabilize to the humidity where the lumber is.

It's impossible to air dry lumber and get it to zero percent moisture content. There are very few, if any, places on Earth where the relative humidity is always zero. You may get it close if you set a piece outside to dry in the Atacama Desert.


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