Why is properly dried lumber so important? In a word, it’s because of movement. As a hygroscopic material, the fibers of wood run along a board, absorbing and shedding moisture as needed. This kind of capillary action causes swelling as atmospheric humidity increases and causes shrinking as the weather becomes drier. While this characteristic of wood is part of what makes it so resilient and allows it to last throughout the seasons and the years, it can also make working with wood a little tricky. Proper spacing and expert installation are imperative when you’re working with such dynamic materials. While some applications require tighter clearance than others, and there’s a difference in what you need for exterior versus interior projects, the fact remains that lumber drying is important.
Significance of Proper Drying
Not only does properly dried lumber move less significantly, but it also offers greater stability and predictability. In fact, we believe that when you consider lumber quality, you’re not only referring to the initial characteristics of lumber but also to the manner in which it’s been dried. A lumber supplier that’s careful about details will ensure that lumber has been dried carefully. Lumber drying can’t be rushed; it requires a multi-step process in a controlled environment. But it’s truly worth the wait!
Ideal Moisture Levels
The ideal moisture content of your lumber depends largely on your climate. The North American standard for kiln-dried lumber intended for interior applications is between 6% and 8%. By contrast, Europe’s wetter climate means that the European standard is between 12% and 15%. But the numbers aren’t the only aspect of drying that’s important; method matters, too.
Pros and Cons of Kiln Drying
Kiln drying, when done properly, can create greater stability, which is a very good thing. However, it’s it done incorrectly, kiln drying can actually lead to less stability. Let us explain. Kiln drying isn’t just faster than air drying; it also serves to harden the lignin, the parts of wood between the cell walls. When lignin is set in place, it can still absorb and expel moisture, but the cell walls become less flexible, displaying a reduced ability to move. The result is greater stability — as long as kiln drying is done properly.
If kiln drying is done too quickly, though, case hardening can result. Case hardening occurs when the inner layers aren’t allowed to dry as quickly as the outer layers; the problem is that the moisture becomes trapped inside, with outer layers unable to move. As you might already guess, this leads to highly unstable wood that can move dramatically when the wood is cut. Discoloration and cracking are telltale signs of case hardening.