Fresh cut logs right out of the forest are full of
water. Water content can be as much as 200% of the weight
of the wood substance, or as low as 30%, depending on
the type of wood. As wood dries after being cut, it
experiences changes in shape and size.
For wood used in construction, dimensional changes
have to be compensated for, or minimized by using some
method of accelerated drying, usually either natural
air (stack) drying or oven (kiln) drying. The purpose
of drying is to stabilize further changes by reducing
moisture content to approximately the same conditions
that the wood will experience in it's final application.
Some log companies state that their logs are dried
to a particular moisture content, such as 19% or 15%,
and explain why it is the ideal value.
In fact, things are not quite quite as simple as that
when it comes to log moisture percentages.
Let's look at some of the facts:
- The ideal moisture content of a log is its "equilibrium"
content which is based on the temperature and humidity
of the environment in which it is placed in service.
For most areas of the U.S., the average value (for
exterior wood) is about 12% according to the U.S.
Department of Agriculture.
- Few log home companies achieve this level of moisture
content. It's too expensive and time consuming —
and not necessary. Shipping levels of 20% or less
are close enough to allow logs to seek their own equilibrium
level and not have to change so dramatically as to
cause significant dimensional changes.
- Moisture content of logs vary from inside to outside,
heartwood to sapwood. The thicker the log, the more
difficult it is to achieve a consistent moisture level
throughout. Even after drying, content can vary from
log to log. Consistency between batches of logs is
practically impossible to achieve.
- After being placed in service, logs will attempt
to adjust to changes in their environment, meaning
that their moisture content is never constant and
will change a few percent from season to season. Moisture
content will be different in logs used in dry climates
than in humid climates.
- Log surfaces on the outside of a wall will experience
different environmental conditions than those on inside
walls, meaning that moisture content can vary on different
sides.
- Logs dried at the manufacturing plant will experience
some changes in moisture content while being shipped
and while stacked awaiting use.
- The effects of environmental changes on moisture
content can be minimized by sealants and preservatives
applied to logs.
About Drying Methods
Two methods are used for drying logs for log homes:
air drying and kiln drying. Log home companies generally
one or the other of these methods, although some companies
use a combination of both.
Air Drying
Air drying involves stacking logs on exposed racks or
pallets in a covered shelter such that air can easily
flow around each log. Sometimes fans are used to accelerate
the drying process.
Air drying can require longer periods of time than
kiln drying to achieve a relatively low moisture content,
and is somewhat more difficult to control due to the
variable environmental conditions in which the logs
are aired. Weather, season, temperature, humidity, and
other factors can affect air drying in unpredictable
ways.
Air drying proponents claim that the slower, more natural
method reduces the possibility of stresses and defects
caused by rapid kiln drying. Many log home companies
use air drying not so much because they necessarily
believe it is a better process, but because they feel
that it is a perfectly acceptable alternative to the
higher cost of kiln drying. Properly done, air drying
can be about as effective as kiln drying.
Kiln Drying
Drying kilns ("ovens") are essentially closed
buildings in which batches or cants of logs are placed
to be dried using heat sources and fans. Temperature
and humidity are managed according to the type of wood,
moisture content, drying rate, and thickness.
Kiln drying requires specialized equipment and fuel,
and is more expensive than air drying. It must be carefully
monitored and controlled to prevent overdrying, induced
defects, and burning or discoloration of logs.
Proponents of kiln drying readily point out its many
advantages. It is faster, can produce lower moisture
content levels, produces more consistent results, and
kills mold, fungi, and insect larvae in the wood. It
also reduces or eliminates natural staining. Oven temperatures
of 130 - 180 degrees, will crystallize and harden resins
in softwoods such as pine. They also point out that
all commercial construction lumber is kiln dried and
that it's even more important for log construction.
Summary
It is clear that low moisture content is important
in milled log home construction (not as important in
handcrafted construction) and that some drying method
be used to achieve that objective. Most log home companies
agree. What they may not agree upon is which drying
method is best. You must make that decision for yourself,
hopefully based in part on the information we've provided
here.
