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.
