THE KILN DRIED DIFFERENCE
(by Joseph C. Folker In recent years the log home industry has seen a steady increase in the number of log home manufacturers offering it's clients kiln-dried logs as an option. Some companies have even made the commitment to offer only kiln-dried materials. This movement was initially challenged by skeptics who claimed that moisture could be removed from the external sapwood, but not deep inside the heart of the timber. Thanks to a few kiln-drying pioneers in the log home industry, these initial assumptions are now changing.
The process of kiln-drying lumber is the science of carefully controlling the extraction of moisture from wood at a rate which will not cause damage to the wood. When drying large dimensional lumber, a stack of cants is placed in a sealed building with the equipment necessary to control the conditions needed to season the timbers. These conditions are the same for air-drying lumber, namely; air movement, temperature, and humidity. Once the material is in the kiln, the temperature is slowly raised to an eventual 170 degrees Fahrenheit. Heated air is used primarily to speed up the evaporation of the moisture in the wood. Warmer air will absorb a greater amount of moisture as it flows over and around the green lumber.
Large reversible fans circulate the heated air and change the direction of the air-flow at regular intervals to help maintain a consistent drying rate throughout the kiln. The moisture laden air is removed from the kiln by way of an automatic ventilation system and large dehumidifiers. The drying rate of the timbers is carefully monitored because the outside perimeters of the cants naturally tend to dry faster than the centers. Uncorrected, this imbalance would cause severe checking (cracking), so the final stage of kiln-drying large timbers requires a steaming process. This adds moisture to the outside portions equalizing the moisture throughout the wood.
As mentioned earlier, kiln-drying lumber is a science. It is not a random act of loading material in a kiln, flicking a switch, and coming back at some later time to retrieve it when a timer rings. It is a carefully monitored procedure. Before the lumber is placed in the kiln, a kiln schedule must be prepared. This schedule will outline the different sets of conditions to be carefully followed in order to achieve the desired moisture content, without damaging the wood. The kiln operator will modify the conditions in stages based on the moisture content of the lumber or by time-intervals. Sample logs in the kiln are connected to sensitive measuring devices outside the kiln chamber, and monitored for temperature and moisture content. Once it has been determined that the desired moisture content (15%) has been achieved, cross-section samples are taken from the center of randomly selected cants in the kiln, and tested.
Moisture content of the timbers can be measured in a variety of ways, however, the most accurate method is by use of the "oven-dry-ratio" analysis. This is done by first weighing the kiln-dried sample on a set of sensitive gram scales, then drying them down completely in a small oven. The ratio between the original kiln-dried weight and the final oven-dried weight represents the moisture content of the timber, inside and out. Another device used to measure moisture is the moisture probe, which measures only the outer fractional portion of the timber. It can not penetrate the wood deep enough to give an accurate reading of the moisture content throughout the log, but only the outer portion, which will tend to be the driest portion of the timber.
After testing to verify the desired moisture content (average 15%), the cants are removed from the kiln, ready for milling to the finished log profile. This process of drying a green 6"x 8" timber generally requires a 4 to 6 week period (on average) in the kiln, to achieve the 15% moisture content.
Kiln-drying provides several very important benefits; primarily, it produces logs that are uniform and stabilized to match the actual home environment. Wood must be conditioned to the moisture content it will assume during it's service life. Otherwise, it will have a tendency to shrink and twist to some degree. Since the exterior wall normally will range 10%-30% moisture content throughout their yearly cycles, kiln-dried logs, at 15% moisture content are as dimensionally stable as possible for most areas. Companies that kiln-dry their material down to an average 15% have the advantage of removing logs that exhibit severe stress changes such as splitting and warping, prior to the final milling and grading processes. The customer can see the final beauty and integrity of each log before it is placed in the wall, and not have to worry about what the home is going to look like after the logs have finally dried-out.
Most manufacturers provide only air-dried logs. These logs are seasoned by simply storing them for an extended period of time. The important point here is to make sure the customer asks the manufacturer of the air-dried materials to guarantee the moisture content levels. It's technically possible to air-dry logs to 15% moisture content. From a practical standpoint, however, it is not usually done. Under normal conditions, logs would need years of air-drying to match kiln-dried standards, however, still lacking the additional kiln-dried benefit of sanitized logs.
Briefly, some of the other benefits associated with kiln-drying are:
a) Kiln-dried logs are preshrunk before milling to a final profile. Uniform and stable - no severe shrinking or warping. Tight joints.
b) Careful drying in a controlled environment minimized checking (cracking), which occurs in the kilns, before milling and final construction.
c) Warping and twisting occur during kiln-drying, before construction. Defective logs can be graded out by trained inspectors. Logs built into the home should then be stamped with the NAHB certification for Stress Grades and Moisture Content.
d) Logs are sanitized when heated to 170 degrees Fahrenheit in the kiln. Such heat kills mold, fungi which causes wood decay, as well as any insects, their eggs or larvae.
e) Pitch in the wood is crystallized during kiln-drying, preventing the sticky substance from seeping to the surface.
f) Interior and exterior finishes can be applied immediately following construction for more convenient and immediate protection. Applications absorb deeper and last longer.
g) More than 10,000 pounds of water are removed from the typical home, significantly reducing the weight of the logs. Even the longest logs are easily handled by two people.
In recent years more prospective log home buyers are asking questions, considering, and choosing kiln-dried logs when building their home. With this apparent, many log home manufacturers are now offering kiln-drying as an option. It is very important for those considering kiln-dried logs to understand how the term can often be misinterpreted.
Kiln-drying is not achieved by running cants through a pre-dryer, which are used to reduce the surface moisture of wood with large fans. This process is intended as a superficial treatment only as warm air is circulated by fans. The temperature in pre-dryers does not approach the heat generated by a dry-kiln, and the humidity is controlled only to the extent of venting some of the moisture-laden air. Pre-dryers DO NOT sanitize the logs.
If a log home buyer is shopping for a kiln-dried log home, they should ask the manufacturer the following questions:
1. Who, where, and how are the logs dried?
2. What temperature levels are achieved in the kiln?
3. What moisture content are the logs dried, and is the moisture content guaranteed in writing?
4. What process is used in determining the moisture content (moisture probe which measures the level on the exterior portion of the log only, or the "oven-dry-ratio" method)?
The benefits of kiln-drying is not limited to the United States. According to the International Trade Council (ITC) of the American Forest Products Association (AFPA), effective June 1, 1993, European Community regulations require that all coniferous lumber be heat-treated (santitized) and be accompanied by a phytosanitary certificate. Shipments not meeting these requirements may be rejected.
The regulations go a step further, forbidding the import of green lumber, even if accompanied by a phytosanitary certificate because European Community regulations require heat-treatment of all coniferous lumber. Imports of large-dimension lumber that has not been heat treated are also forbidden.
Given the information available, it is the opinion of many, that kiln-drying materials is essential to building a quality log home. Regardless of the method used to dry the logs, the important factor is to make sure the logs going into a home are at a low, stable moisture level. A comprehensive kiln-drying program will accomplish this, and provide additional benefits not shared with other conventional drying programs.
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