About Moisture and
Wood
The durability of wood is often a function of water, but that doesn't
mean wood can never get wet. Quite the contrary, wood and water usually
live happily together. Wood is a hygroscopic material, which means it naturally
takes on and give off water to balance out with its surrounding environment. Wood can
safely absorb large quantities of water before reaching moisture content levels that will
be inviting for decay fungi.
Moisture content (MC) is a measure of how much water is in a piece of wood relative
to the wood itself. MC is expressed as a percentage and is calculated by dividing the
weight of the water in the wood by the weight of that wood if it were oven dry. For
example, 200% MC means a piece of wood has twice as much of its weight due to water than
to wood. Two important MC numbers to remember are 19% and 28%. We tend to
call a piece of wood dry if it is at 19% or less moisture content. Fiber saturation
averages around 28%.
Fiber saturation is an important benchmark for both shrinkage and for decay. The
fibers of wood (the cells that run the length of the tree) are shaped like tapered
drinking straws. When fibers absorb water, it first is held in the cell walls themselves.
When the cell walls are full, any additional water absorbed by the wood will now go to
fill up the cavities of these tubular cells. Fiber saturation is the level of moisture
content where the cell walls are holding as much water as they can. Water held in the cell
walls is called bound water, while water in the cell cavities is called free
water. As the name implies, the free water is relatively accessible, and an accessible
source of water is one necessity for decay fungi to start growing. Therefore, decay can
generally only get started if the moisture content of the wood is above fiber saturation.
The fiber saturation point is also the limit for wood shrinkage. Wood shrinks or swells as
its moisture content changes, but only when water is taken up or given off from the cell
walls. Any change in water content in the cell cavity will have no effect on the dimension
of the wood. Therefore, wood only shrinks and swells when it changes moisture content
below the point of fiber saturation.
Like other hygroscopic materials, wood placed in an environment with
stable temperature and relative humidity will eventually reach a moisture content that
yields no vapor pressure difference between the wood and the surrounding air. In other
words, its moisture content will stabilize at a point called the equilibrium moisture
content (EMC). Wood used indoors will eventually stabilize at 8-14% moisture content;
outdoors at 12-18%. Hygroscopicity isn't necessarily a bad thing
- this allows wood to function as a natural humidity controller in our homes. When the
indoor air is very dry, wood will release moisture. When the indoor air is too humid, wood
will absorb moisture.
Wood shrinks/swells when it loses/gains moisture below its fiber saturation point. This
natural behaviour of wood is responsible for some of the problems sometimes encountered
when wood dries. For example, special cracks called checks can result from stresses
induced in a piece of wood that is drying. As the piece dries, it develops a moisture
gradient across its section (dry on the outside, wet on the inside). The dry outer shell
wants to shrink as it dries below fiber saturation, however, the wetter core constrains
the shell. This can cause checks to form on the surface. The shell is now set in its
dimension, although the core is still drying and will in turn want to shrink. But the
fixed shell constrains the core and checks can thus form in the core. Another problem
associated with drying is warp. A piece of wood can deviate from its expected shape
as it dries due to the fact that wood shrinks different amounts in different directions.
It shrinks the most in the direction tangential to the rings, about half as much in the
direction perpendicular to the rings, and hardly at all along the length of the tree.
Where in the log a piece was cut will be a factor in how it changes shape as it shrinks.
One advantage of using dry lumber is that most of the shrinkage has been achieved
prior to purchase. Dry lumber is lumber with a moisture content no greater than 19%; wood
does most of its shrinking as it drops from 28-19%. Dry lumber will have already shown its
drying defects, if any. It will also lead to less surprises in a finished building, as the
product will stay more or less at the dimension it was upon installation. Dry lumber will
be stamped with the letters S-DRY (for surfaced dry) or KD (for kiln dry).
Another way to avoid shrinkage and warp is to use composite wood products, also
called engineered wood products. These are the products that are assembled from
smaller pieces of wood glued together - for example, plywood, OSB, finger-jointed studs
and I-joists. Composite products have a mix of log orientations within a single piece, so
one part constrains the movement of another. For example, plywood achieves this
crossbanding form of self-constraint. In other products, movements are limited to very
small areas and tend to average out in the whole piece, as with finger-jointed studs. |
