Log Drying Methods Explained

Understanding Moisture Content

Fresh cut logs right out of the forest are full of water. Water content can be as much as 200% of the weight of the wood substance, or as low as 30%, depending on the type of wood. As wood dries after being cut, it experiences changes in shape and size.

For wood used in construction, dimensional changes have to be compensated for, or minimized by using some method of accelerated drying, usually either natural air (stack) drying or oven (kiln) drying. The purpose of drying is to stabilize further changes by reducing moisture content to approximately the same conditions that the wood will experience in it's final application.

Some log companies state that their logs are dried to a particular moisture content, such as 19% or 15%, and explain why it is the ideal value.

In fact, things are not quite quite as simple as that when it comes to log moisture percentages.

Let's look at some of the facts:

- The ideal moisture content of a log is its "equilibrium" content which is based on the temperature and humidity of the environment in which it is placed in service. For most areas of the U.S., the average value (for exterior wood) is about 12% according to the U.S. Department of Agriculture.

- Few log home companies achieve this level of moisture content. It's too expensive and time consuming — and not necessary. Shipping levels of 20% or less are close enough to allow logs to seek their own equilibrium level and not have to change so dramatically as to cause significant dimensional changes.

- Moisture content of logs vary from inside to outside, heartwood to sapwood. The thicker the log, the more difficult it is to achieve a consistent moisture level throughout. Even after drying, content can vary from log to log. Consistency between batches of logs is practically impossible to achieve.

- After being placed in service, logs will attempt to adjust to changes in their environment, meaning that their moisture content is never constant and will change a few percent from season to season. Moisture content will be different in logs used in dry climates than in humid climates.

- Log surfaces on the outside of a wall will experience different environmental conditions than those on inside walls, meaning that moisture content can vary on different sides.

- Logs dried at the manufacturing plant will experience some changes in moisture content while being shipped and while stacked awaiting use.

- The effects of environmental changes on moisture content can be minimized by sealants and preservatives applied to logs.

About Drying Methods

Two methods are used for drying logs for log homes: air drying and kiln drying. Log home companies generally one or the other of these methods, although some companies use a combination of both.

Air Drying
Air drying involves stacking logs on exposed racks or pallets in a covered shelter such that air can easily flow around each log. Sometimes fans are used to accelerate the drying process.

Air drying can require longer periods of time than kiln drying to achieve a relatively low moisture content, and is somewhat more difficult to control due to the variable environmental conditions in which the logs are aired. Weather, season, temperature, humidity, and other factors can affect air drying in unpredictable ways.

Air drying proponents claim that the slower, more natural method reduces the possibility of stresses and defects caused by rapid kiln drying. Many log home companies use air drying not so much because they necessarily believe it is a better process, but because they feel that it is a perfectly acceptable alternative to the higher cost of kiln drying. Properly done, air drying can be about as effective as kiln drying.

Kiln Drying
Drying kilns ("ovens") are essentially closed buildings in which batches or cants of logs are placed to be dried using heat sources and fans. Temperature and humidity are managed according to the type of wood, moisture content, drying rate, and thickness.

Kiln drying requires specialized equipment and fuel, and is more expensive than air drying. It must be carefully monitored and controlled to prevent overdrying, induced defects, and burning or discoloration of logs.

Proponents of kiln drying readily point out its many advantages. It is faster, can produce lower moisture content levels, produces more consistent results, and kills mold, fungi, and insect larvae in the wood. It also reduces or eliminates natural staining. Oven temperatures of 130 - 180 degrees, will crystallize and harden resins in softwoods such as pine. They also point out that all commercial construction lumber is kiln dried and that it's even more important for log construction.

Summary

It is clear that low moisture content is important in milled log home construction (not as important in handcrafted construction) and that some drying method be used to achieve that objective. Most log home companies agree. What they may not agree upon is which drying method is best. You must make that decision for yourself, hopefully based in part on the information we've provided here.

+++