When lumber is first cut from a log, it is
said to be "dead green". For the hardwoods, moisture content ranges
from about 50% to well over 100% depending on species and the presence of
heartwood and sapwood. Moisture content is calculated by the following
formula:
Moisture Content = Original Weight - Oven Dry Weight X 100
Oven-Dry Weight
Original weight is the weight of the wood at
the time of the test. Oven dry weight is the weight after the wood
has been placed in an oven at 212 degrees Fahrenheit until it achieves a
constant weight.
After sawing, most hardwood lumber is put up on sticks to air dry.
Sticks are strips of wood placed between each coarse of boards, thus allowing
air to circulate between boards. Air drying is done outside with a cover
over the pile or under an open shed. The lumber will eventually dry
to about 15% moisture content.
If the lumber is to be used inside where heating occurs, it will be kiln
dried to about 6-8% moisture content. If lumber is not properly kiln
dried and then made into furniture for our homes, it will dry when in use.
As it dries, shrinkage occurs and it is usually accompanied by warping.
When inside surfaces are exposed and critical joints exist, an unacceptable
product will likely develop.
Wood moisture meters are available to evaluate
the moisture content of lumber. Check in any wood trade magazine or
with your wood supplier for sources. Inexpensive thermometers and
relative humidity sensors are available in some building
supply stores, usually in the heating section. These sensors can be
used to estimate the equilibrium moisture content (EMC) of wood. The
EMC is the moisture content wood will eventually come
to at a particular temperature and relative humidity. It may take
several weeks for this to happen.
Lumber kiln dried to 6-8% moisture content will regain moisture to the air-dried
level if stored in an unheated facility. A storage facility must be
15 to 20 degrees warmer than the outside temperature in order for the lumber
to stay at 7-8% moisture content. Many hardwood lumber users and retail
outlets are not aware of this potential problem. The old wives' tale
that the lumber must be dry because it has been in the barn for the last
twenty years is incorrect. This lumber is no drier than recently air
dried lumber, unless the barn is heated. Proper storage of kiln-dried
lumber is very important.
