It’s a very common question: why does the moisture in my logs matter? It’s a good one, since the amount of moisture in your logs will have a big effect on how your house performs with energy efficiency, tightness against weather, maintenance, and comfort.
First a little bit about how water is stored in wood fibers. Water in wood can be divided into two types: free and bound water. Free water is the moisture held in cells and is most easily given up in the drying process. Bound water is held in the wood cell walls and is slower to disperse. As the bound water disperses, shrinkage will occur until the log reaches an equilibrium with the humidity levels in the air around it. At that point, the log will become stabilized and the shrinkage will generally have little further effect. In most areas of the United States, excepting the very dry areas of the Southwest, logs will reach an equilibrium at about 12% moisture content. Once that equilibrium is reached the effects of shrinkage are not significant.
One way to think about wood and its reaction to moisture is to compare it to a common sponge. Wood acts very much like a sponge, absorbing water and releasing it as it dries. If you squeeze a sponge you can release quite a bit of water. The sponge still retains its shape even though much of the excess water has been squeezed out. But if you allow the sponge to continue to dry, it begins to shrink and distort in shape, often at different rates in different directions. When the sponge dries enough to reach equilibrium with the humidity of the surrounding air, it may look and feel quite different than it looked when wet.
To extend the comparison of the effect of moisture to wood, green pine (like the logs at left) starts out with a moisture content of about 50%. A course of drying in a kiln can take the moisture content to about 30%, but the log has still not given up all of its free or bound water, and has not yet begun the shrinking process that brings the log to its final equilibrium of approximately 12% moisture content. Even though a pine log has been treated through the kiln process, in many cases the log has not begun to shrink.
A dry log is a good log
So how can this affect your log home? First, the moisture content when the log is green is an important factor, as this is its naturally occurring state. When a tree is first cut, the amount of moisture in a green log can vary by species. As noted, pine starts at about 50% moisture when green. Northern White Cedar is one of the driest used in log homes, with a naturally-occurring water content in green wood of about 24%, based on Katahdin’s in-house testing. So it is apparent that the processes that we need to reduce the moisture levels to bring a cedar log to equilibrium is much more easily attained when the initial moisture level in green cedar is nearly half as much as pine.
Kiln drying is common, and depending on the season and the ambient humidity, Katahdin sends logs into the kiln for finishing, to reach the optimal average of 12% moisture level. This kilning happens only after the logs have been air dried, which allows the shrinkage to occur at a natural rate and provides less stress on the cellular structure of the log. This extended air drying allows the cells to gradually release the free and bound water, allowing for natural shrinkage. Because the temperature and humidity allows the cedar log to reach equilibrium more slowly, the abrupt shrinkage on a cellular level from high-heat kilning is less likely to appear.
The result? A log home constructed on low-moisture Northern White Cedar will be subject to considerably less shrinkage, settling and cracking than the much wetter pine log as it is already close to equilibrium with the surrounding air.
Another important benefit is that Katahdin is able to ship these low-moisture logs ready for the application of the sheet insulation included in our Energy Envelope System®. That stable cedar log platform keeps the insulation in place and enables us to offer a highly energy efficient log home. If the same insulation is applied to a wetter log, the subsequent settling and shrinkage would cause the insulation to buckle and pop off the walls.
That’s why it’s the log of choice at Katahdin Cedar Log Homes.