4. For each change in EMC, the
wood tries to shrink or swell by 1/4
percent, although when going from
drying to an increase in MC, there
is an almost 1 percent EMC “dead
band.” This means that wood that is
fairly dry can easily tolerate a 1 or 2
percent increase in MC ( 12 percent
RH increase), while any more than a
1 percent drop in MC ( 6 percent RH
drop) creates shrinkage that can easily
result in cracking. Further, cupping
is the result of drying during or after
5. Wood will naturally cup toward
the bark side of the lumber. Sometimes
we hear that alternating the grain of
the individual staves up and down will
lessen the cup, which is partly true, but
this approach fails to address the main
issue. The main issue is that cupping
means that the wood has dried fur-ther…that is, the MC was wrong. (See
more below.) Having said this, another
cause of cupping is that the face side is
drier than the back side, or vice versa.
An excellent finish, face and bottom,
will prevent this.
Conclusions and what to do
So, the issues you mention are the
result of having the wood dry further
Step 1. Require your supplier to
provide wood that is no higher than 7.0
percent MC. Likewise, wood no drier
than 5.0 percent MC. Use a moisture
meter (costing more than $250 and
made in the U.S.) to double-check the
Step 2. You can have the correct MC
in lumber coming into your facility, but
the MC can change if you do not store
the lumber at humidities in the low 30
percent RH range for lumber that you
will be using in the wintertime. If you
cannot store lumber in a dry humidity-controlled environment, wrap incoming
lumber in plastic when you get it. The
plastic assures that there can be no loss
or gain of moisture during any reasonable storage conditions.
Step 3. Check the moisture again
when you begin to use it.
Step 4. Check the RH in your plant.
Small battery power hygrometers are
available that will do the job, but their
life can be shortened by dust in the air.
Step 5. Wrap the finished product
in plastic so that its moisture cannot
change at all before it is installed.
Step 6. Include a note (large print on
red paper) that states that your products
are designed to be exposed to and used
in 30 percent RH to 50 percent RH conditions. If the homeowner has drier or
wetter conditions, there is no warranty.
Further, you can make products for use
outside these humidity levels if so indicated when the order is placed with you.
Step 7. To make higher MC products, make them in the summertime
and then store them, wrapped in plastic for shipment at any time of the year
to humid locations. Similarly, for very
dry locations, even in the summertime,
make dry wintertime products and
then wrap them and store them until
“Wrap the finished product in plastic so that its
moisture cannot change at all before it is installed.”