Wood and Moisture

INTRODUCTION
Wood is used in virtually all aspects of
construction, including, but not limited to
foundations, shoring, forms, siding, framing,
columns, beams, girders, joists, flooring,
trim, sills, plates, rafters, trusses,
blocking, and, nailers. A basic understanding
of wood and its characteristics will help
in its proper selection. As our forefathers
did, use care when selecting wood for roofing
applications such as rafters, trusses,
blocking, and, nailers.
Eighteenth and early nineteenth century
settlers and farmers had a keen understanding
of wood. They knew that from its
appearance to its performance, all wood
species are not the same. They knew that
one species was usually better suited for a
particular use than another. When selecting
wood, they considered the unique characteristics,
such as strength, hardness,
appearance, resilience, decay resistance,
and dimensional stability.
For instance, we have an early nineteenth
century rocking chair that is made
from four different woods. I’m confident that
the person who made that rocking chair
chose poplar for the seat to give softness;
hickory for the back to give spring; maple
for the arms to resist scratches; and chestnut
for the runners to resist floor dampness.
I remember a local ante-bellum mansion
that was built in 1827 and served its final
years as a mortuary. When the mortuary
closed, this perfectly sound mansion was
purchased by the city and demolished, and
the site was converted to a parking lot. As
the house was being demolished, I noticed
that, like our rocking chair, the mansion
was also constructed of different wood
species. For instance, the bottom sill and
plates were chestnut to resist moisture and
decay; roof trusses and exterior framing
were oak for strength; interior framing was
southern yellow pine heartwood; and the
floors were walnut and quarter-sawn oak
for dimensional stability. All trusses, sill
Figure 1: Bound Moisture Figure 2: Free Moisture
18 • I N T E R FA C E J A N U A RY 2005
plates, and corner posts were mortise-andtenon
with oak dowels. I requested a dowel
from one of the trusses for a souvenir.
However, because the oak trusses had, as
planned by the builder, shrunk tightly
around the dowel, it had to be pounded out
with a wooden mallet.
This article is intended to describe, in
general terms, the basic relationship
between wood and moisture. The subject is
divided into three discussion topics.
Detailed descriptions and the listings of
manufacturers, products, and trade names
are not within the scope of this article. The
topics are:
• Wood Structure: Basic structure
and makeup of wood.
• Reaction to Moisture: Wood’s reactions
to changes in moisture content.
• Protection from Moisture: Common
methods to protect wood.
WOOD STRUCTURE
When wood is in the form of a living
tree, wood and water are very compatible.
However, after the tree has been harvested,
moisture is one of wood’s worst enemies.
Wood is both hygroscopic and biodegradable.
Being hygroscopic – a natural property
of absorbing and releasing moisture –
wood can safely absorb large quantities of
water before reaching a moisture level that
may lead to decay. Also, since wood is
biodegradable, decay causes wood to
decompose readily under the proper conditions.
Wood is composed of long cells formed
by continuous fibers that have often been
described as resembling straws. When a
tree is living, these cells pull moisture and
nutrients up from the ground and transport
them throughout the tree. As shown in
Figure 1, moisture that is stored in the
fibers is called “bound” moisture. Figure 2
illustrates that, when the fibers become saturated,
the excess moisture is stored in the
cells and is called “free”
moisture. When the tree
is harvested, it immediately
begins to dry out,
as does a cut flower that
is not put into water.
The cells give up their
free moisture first.
However, this moisture
depletion has no effect
on shrinkage. When the
moisture in the cells has
completely evaporated,
the fibers contain all of
the residual moisture.
This is called the fiber
saturation point. Depending
on the wood
species, the fiber saturation
point occurs at
approximately 25 to 30
percent moisture content.
If the fibers lose
moisture and the moisture
content decreases
below the fiber saturation
point, the wood (log)
begins to shrink in
thickness but very little,
if any, in length. This is
a characteristic of all
wood species and
accounts for the lateral
dimensional changes of
the finished wood. When
the wood is cut to
dimension by the plain
sawn method, dimensional
changes will be in
width. As shown in
Figure 3, plain-sawn is
when the log is cut with
the annual growth rings
at an angle of 0 to 45 degrees. As shown in
Figure 4, when a log is quarter-sawn, the
log is first cut into quarters with the annual
growth rings at an angle of 0 to 45
degrees. Quarter-sawn wood is more expensive
and produces more waste, but it is
more dimensionally stable than plain-sawn
lumber. Because of its aesthetic affect on
the grain, quarter-sawn wood is commonly
used for furniture and
flooring.
REACTION TO MOISTURE
After a tree has
been harvested, moisture
is wood’s worst
nightmare. Varying
moisture content leads to shrinking and
swelling of wood members, and high moisture
content can lead to decay. Moisture
content is a measure of how much water is
in a piece of wood relative to its dry weight.
Moisture content is expressed as a percentage
and is calculated by dividing the weight
of water in the wood by the weight of that
wood in a dried state.
The American Softwood Lumber
Standard defines “dry” as having a moisture
content of 19 percent or less, with an average
of 15 percent. Table 1 shows the relationship
between relative humidity (RH) and
optimum moisture content of wood (MC) for
several geographic locations in North
America.
Figure 3: Plain Sawn
Figure 4: Quarter Sawn
J A N U A RY 2005 I N T E R FA C E • 1 9
Since we are discussing the relationship
between wood and moisture, how do we
determine the moisture content? The first
and most accurate method is by using
tables and calculations that were prepared
by the United States Forest Service. The
second is by measuring moisture content
with a reliable moisture meter that either
has a wood scale or is specifically designed
for wood. Either kind of meter will measure
the actual moisture content of wood and
then display the measurement in percent
moisture. Meters are the simplest and most
common method of measuring moisture.
Measuring moisture and moisture meters
are topics for another article.
The following two important moisture
levels of wood should be kept in mind:
1. Nineteen Percent: Considered as
dry if wood has moisture content of
19 percent or less. Stamps denoting
the drying method and the moisture
content are mentioned later.
2. Twenty-eight Percent: This is the
average fiber saturation point for
wood where all the wood fibers are
fully saturated. At moisture contents
above the fiber saturation
point, water begins to fill the cells.
Decay generally begins if the moisture
content of the wood is above
fiber saturation for a prolonged period
of time. The fiber saturation point
is also the limit for wood swelling.
Wood shrinks or swells as its moisture
content moves above or below the fiber saturation
point. Shrinkage occurs when the
moisture content moves below the fiber saturation
point, and swelling occurs when the
moisture content moves above the fiber saturation
point. Wood used indoors will eventually
stabilize at 8-14% moisture content;
outdoors, wood stabilizes at 12-18 percent.
Another term not often seen or mentioned
is “equilibrium moisture content.” At
equilibrium, wood neither gains nor loses
water, because it has reached equilibrium
with the vapor pressure (produced by temperature
or humidity differentials) of the
surrounding atmosphere. A review of thermodynamics
shows that air (vapor pressure)
always moves from high pressure to
low pressure, high humidity to low humidity,
and high temperature to low temperature.
Changes in relative humidity and temperature
of surrounding air cause both seasonal
(long-term) and daily (short-term)
changes in the moisture content of wood.
However, long-term changes are gradual as
moisture slowly penetrates the wood, while
short-term fluctuations influence only the
wood surface. As a rule of thumb, a 50 percent
change in relative humidity produces a
10 percent change in EMC. Thus, a relative
humidity of 25 percent produces
an EMC of 5 percent and a relative
humidity of 75 percent produces
an EMC of 14 percent.
The following table shows
the relationship
between the EMC
of wood and relative
humidity at
various temperature
levels. This
relationship is
valid for both
interior and exterior
conditions
and can be used
to approximate
the moisture content
of wood.
From Table 2, wood exposed to ambient
conditions of 70 degrees F and 55 percent
relative humidity would have a moisture
content of 10.1 percent.
Standards prepared by The American
Lumber Standards Committee contain minimum
size requirements for green and dry
wood lumber. Lumber is distinguished from
timber in that timber is wood with finished
dimensions of 5 inches or greater.
Table 1: Lumber Use By Geographic Location
INTERIOR USE EXTERIOR USE
Optimum Indoor RH Optimum Moisture
Geographic Location Moisture Required to Content (MC)
Content (MC) Hold MC
Most of U.S., Ontario, and Quebec 5-10 % 25-55% 9-15%
Damp Southern U.S. Coastal Areas, 8-13% 43-70% 10-15%
Newfoundland, and Coastal
Canadian Provinces
Dry Southwestern US 4-9% 17-50% 7-12%
Alberta, Saskatchewan, and Manitoba 4-9% 17-50% 10-15%
Table from the USDA Forest Service, Agriculture Handbook No. 72
Temperature (degrees F.)
30 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3
40 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3
50 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3
60 1.3 2.5 3.6 4.6 5.4 6.2 7.0 7.8 8.6 9.4 10.2 11.1 12.1 13.3 14.6 16.2 18.2 20.7 24.1
70 1.3 2.5 3.5 4.5 5.4 6.2 6.9 7.7 8.5 9.2 10.1 11.0 12.0 13.1 14.4 16.0 17.9 20.5 23.9
80 1.3 2.4 3.5 4.4 5.3 6.1 6.8 7.6 8.3 9.1 9.9 10.8 11.7 12.9 14.2 15.7 17.7 20.2 23.6
90 1.2 2.3 3.4 4.3 5.1 5.9 6.7 7.4 8.1 8.9 9.7 10.5 11.5 12.6 13.9 15.4 17.3 19.8 23.3
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Relative Humidity (Percent)
Table 2: Moisture Content at Various Temperatures and Relative Humidity Levels
From Agriculture Handbook, No. 72 by the Forest Products Laboratory of the U.S. Dept. of Agriculture
20 • I N T E R FA C E J A N U A RY 2005
Because green lumber (lumber with a
moisture content higher than 19 percent)
has not reached its EMC, it is produced in
dimensions that are slightly larger than dry
lumber (lumber with a moisture content 19
percent or lower) to account for the green
wood’s eventual shrinkage as it reaches the
EMC point. This will allow the green wood
and dry wood to be approximately the same
dimension at the same EMC. The same
design values can be used for both green
lumber and dry lumber; however, the two
should not be mixed within the same construction.
Actual moisture content is
stamped on the lumber.
The following are examples of some of
the stamps that denote moisture content
that may be found on structural lumber:
• S-GRN: Moisture content greater
than 19 percent.
• S-DRY: Kiln-dried or air-dried to 19
percent maximum.
• KD: Kiln-dried to 19 percent maximum.
• MC15 or KD15: Kiln-dried to 15
percent maximum.
• KD•HT: Kiln-dried to 19 percent
maximum and heat-treated to 133
degrees F for 30 minutes.
Effects of Moisture
The Forest Products Laboratory of the
U.S. Department of Agriculture has developed
dimensional change coefficient values
for various wood species with EMC ranging
from 6 to 14 percent. These values are constants
that are unique for that particular
wood species and can be used to calculate
dimensional changes (shrinkage or swelling)
of wood. Table 3 shows the dimensional
change coefficient values for some common
wood species.
The formula to calculate the dimensional
change is: DC = (MC1-MC2) x CE x W
Where:
DC = Change in Width of the Wood
MC1 = Beginning Moisture Content
MC2 = New Moisture Content
CE = Change Coefficient from Table 3
W = Actual Width of the Board
Example: The moisture content of a 2 x
6 (actual dimensions are 1-1/2 inches x 5-
1/2 inches) southern yellow pine board
increases from 9 to 12 percent.
9-12 = 3 x 0.00265 x 5.5 = 0.0437 inch.
Since the moisture content increased,
the board width would increase by 0.0437
inch (approximately 3/64 inch). Conversely,
if the moisture content had decreased from
12 to 9 percent, then the board width would
shrink by 0.0437 inch. In a floor application,
it is obvious that the dimensional
change would be significant over a 16-foot
wide room (16 ft x 12 inches/5.5 inches =
34.9 boards x 0.0477 inch/board = 1.665
inches) (approximately 1-11/16 inches)
movement.
If the EMC is known, then ideally, wood
should be dried to a moisture content within
2 percent of the equilibrium moisture
content of the location where it is to be
used. However, this may not be practical for
applications where the EMC is extremely
low, and provisions
must be
made in the
field assembly.
Table 4
shows moisture
content of
wood according
to its use.
Mold and Moisture
Moisture often suggests the “M”
word – Mold. Mold and moisture
(M&M) are usually mentioned in the
same breath, and moisture has virtually
become synonymous with mold.
M&M: moisture begets mold. If mold spores
are present in the atmosphere, there are
four basic requirements for the spores to
grow: (1) food, (2) moisture, (3) oxygen, and
(4) suitable temperature. Eliminate any of
the four requirements, and mold growth is
discouraged. Wet wood provides two of the
four requirements – food in the form of cellulose
and moisture. Dry wood provides
food. Consequently, controlling moisture
offers the best opportunity to prevent mold
growth on wood products.
As previously mentioned, when wood
dries, it gives off moisture. Some of this
moisture may become trapped and condense
on the surface of the wood. While the
moisture is on the surface, food and oxygen
are also present. If the temperature is suitable
(above 50 degrees F.) and if there are
spores present, mold may grow. However,
once the moisture content of the wood
decreases to less than 20 percent (the range
to which lumber is typically dried), mold
growth is not likely.
As previously discussed, green lumber
has a moisture content greater than 19 percent.
Green lumber will eventually dry naturally
to match ambient conditions.
Frequently, this drying occurs after the
Table 3: Dimensional Change Coefficient
WOOD SPECIES DIMENSIONAL CHANGE COEFFICIENT
Southern Yellow Pine 0.00265
Douglas Fir 0.00267
Maple 0.00353
White Oak 0.00365
Red Oak 0.00369
From Agriculture Handbook 72 by the Forest Products Laboratory of the U.S. Department of Agriculture
Table 4: Wood Use According to Moisture Content
MOISTURE CONTENT USES
16% Outdoor furniture
12-14% Occasionally heated rooms
11-13% Normally heated rooms
9-11% Continually centrally heated rooms and general level
for southern and coastal regions of USA
8% Most northern and central regions of USA
6-8% Radiator shelves and arid southwest USA
Table from American Woodworking Institute “Quality Standards”
22 • I N T E R FA C E J A N U A RY 2005
wood has been installed in the structure.
Again, residual moisture can move to the
surface. If there are no provisions, such as
airflow, to carry the moisture away quickly,
mold may form. In warm climates with good
air movement, wood framing can dry to
below 20 percent moisture content in several
days. However, under the right conditions,
mold can begin to grow in 24 to 48
hours.
Just because wood has been properly
dried, there is no assurance that mold will
not grow. Since wood is hygroscopic, mold
will grow under the right conditions. Dry
lumber may become wet from precipitation
(rain, snow, sleet), condensation,
improper storage (placing
lumber bundles near puddles or
other water sources). Since stacked
lumber will dry from the outside inward,
it is also possible for moisture
to be produced
by the
green or wet
lumber that
is stacked
inside a
bundle that
has dry lumber
on the
outside.
Wood scheduled to be pressure treated
should be dried to a moisture content of 19
percent before treatment. Promptly after
treatment, the wood should be dried again
to the required moisture content, either 19
percent or 15 percent. This drying should
be done prior to shipment. Treated wood
that has been dried after treatment is usually
identified by a tag that is attached to
each piece of lumber. The tag should show
the AWPA standard, the amount of retention,
and final moisture content. If “DRY” is
printed on the tag in lieu of a percentage,
the wood has been dried to 19 percent. Not
all lumber suppliers that provide treated
lumber can provide it properly dried. When
I have confronted contractors about treated
wood without the proper tag, a frequent
reply is “It’s not available locally,” or “I can’t
find it.” Insist on the label!
PROTECTION FROM MOISTURE
How do we protect wood from its worst
enemy? Some wood species are more resistant
to decay than others, so they do not
need as much protection. These decayresistant
woods include cedar, cypress,
locust, mahogany, redwood, black cherry,
black walnut, and teak. However, virtually
all wood should be protected from moisture
and organisms by treating, painting, or
using it in a protected area. This will ensure
the longevity of the wood when it is exposed
to conditions that promote moisture or to
destructive organisms and insects.
Treating Wood
In many ways, wood’s relationship to
decay is analogous to metal’s relationship to
corrosion. Some metals have a natural
resistance to corrosion. These metals,
which include aluminum, chromium, zinc,
tin, and lead, require little or no protection.
Other metals, such as iron, readily corrode
(rust) and must be protected. In a similar
fashion, there are many wood species that
do not offer natural resistance to decay,
such as pine, poplar, and fir. These can be
treated with chemical preservatives.
Preservatives can be applied by pressure
treatment or by topical treatment. Pressure
treatment is a factory process that impregnates
the wood fibers with a chemical
preservative. Preservatives that are applied
by brush, roller, or spray to a local area
such as end cuts and holes are called topical
preservatives. They are field-applied,
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J A N U A RY 2005 I N T E R FA C E • 2 3
have little or no penetration,
and are usually
short term, especially
when exposed to
weather. Since wood is
very susceptible to
moisture through the
ends (remember the
straw-like construction of wood cells), all
end cuts should be treated with a topical
preservative recommended by the wood
treater.
Some chemical preservatives have been
considered harmful and have been prohibited
by the Environmental Protection Agency
(EPA). Chromated Copper Arsenate (CCA) is
one of the most common chemical preservatives
and has been prohibited by the EPA
for residential use as of January 1, 2004.
Continued availability of CCA for commercial
and industrial uses is questionable.
Recent studies indicate that replacement
preservatives for CCA may be more corrosive.
Two of the more common replacement
preservatives are CA (Copper Azole) and
ACQ (Ammonical Copper Quat). Whether
one is more corrosive than the other is
questionable. However, because of moisture
and chemical retention and the fact that
treated wood is normally used in a moist
environment, all fasteners and metal in
contact with treated wood should be stainless
steel or hot-dipped galvanized steel per
ASTM A153 and G185. Stainless steel is
preferable.
When selecting treated wood, refer to
the tag that is attached to each piece. Each
tag will state intended use “Above Ground”
for nailers, blocking, decking, etc., or
“Ground Contact” for posts, sills, etc. The
tag will also indicate moisture content. A
more detailed discussion of wood treatment
methods is the subject for a future article.
Painting Treated Wood
Can treated wood be painted? Treated
wood can be painted, provided the paint
manufacturer’s instructions and precautions
are followed closely. Usually, paint
manufacturers recommend a good, highquality,
oil-based product, such as transparent
or semi-transparent products. Highsolids
products will lie on the wood instead
of penetrating the fibers and will not move
as the wood moves. This results in peeling.
Consumer Reports published findings on
painting treated wood in the June 1999
issue.
Pressure treatment with water-borne
chemical preservatives such as the prohibited
CCA and its replacement ACQ leaves
moisture in the wood. This moisture can
affect the penetration and drying of stains
and paint. For optimum performance of a
paint or stain, ensure the treated wood is
dry and clean prior to painting or staining.
As already discussed, drying time depends
on the time of the year, and on ambient conditions.
The paint manufacturer’s recommendations
depend on dryness. If possible,
test for wood dryness with a moisture meter
or apply the paint or stain on a scrap piece
of the wood to see if the finish spreads,
cures, and adheres properly after a full
cure.
CONCLUSION
Prior to being harvested, wood, in the
form of living trees, relies on moisture for
sustenance. After harvesting, when trees
are converted to lumber, moisture becomes
its worst enemy. Moisture can reduce the
strength and dimensional stability of wood
and will promote mold growth and decay.
While some species have a natural resistance
to decay, most wood must be protected
from moisture by treating with preservatives,
painting, or using in a protected location.
REFERENCES
Agriculture Handbook 72, U.S.
Department of Agriculture, Forest
Products Laboratory.
Grading Standards, American Lumber
Standards Committee.
Quality Standards, 7th Edition,
Architectural Woodwork Institute,
1997.
Water and Wood, Technical Publication
P200, National Wood Floor Association.
Wood Basics, Western Wood Products
Association.
Wood Preserving Standards, American
Wood Preservers Association.
www.treatedwood.com, Chemical Specialties,
Incorporated.
ACKNOWLEDGEMENTS
I extend my sincere gratitude to Charlie
Martin, AIA, of McMillan Smith and
Partners, Architects for creating the graphics
that show the details of wood; Angela
Napolitano of McMillan Smith and Partners,
Architects for adding computer graphics;
Dick Canon of Canon Consulting Engineers
for his technical review; and to my wife,
Linda, for editorial review. Their time and
contributions to this article are truly appreciated.
24 • I N T E R FA C E J A N U A RY 2005
Joseph L. (“Cris”) Crissinger, CSI, CCS, CCCA, is a construction
materials specifier with 22 years of experience. As a
partner with McMillan Smith and Partners, Architects, in
Spartanburg, Greenville, and Charleston, SC, he evaluates
new products and develops all construction specifications for
the firm. His responsibilities also include facility assessment,
field investigations, and the coordination of internal training
programs. Crissinger is a Certified Construction Specifier, a
Certified Construction Contracts Administrator, and a member
of the Construction Specifications Institute, the Building Performance Committee of
ASTM, the Design and Construction Division of the American Society for Quality, and
serves in his community on the Board of Directors for the Spartanburg Boys’ Home
and the Camp Croft Restoration Advisory Board. McMillan Smith and Partners specializes
in the design of education, office, sports, healthcare, and church facilities and
provides full construction contract administration services.
Joseph L. (“Cris”) Crissinger, CSI, CCS, CCCA