Kiln Drying vs. Air Drying
Wood is a hygroscopic material, which means that it tends to absorb moisture from the air. This is due to the fact that wood has cells that contain both bound water and free water. Bound water is held in the cell wall while free water exists in the cell cavities. Free water comes in two forms: vapor and liquid. Even when a tree is cut down, the cells in the wood still retain moisture.
It's important to note that hardwood contains less moisture than softwood. This is because the cells in hardwood are more densely packed, which reduces the capacity for free water.
So how does wood gain or lose moisture after the tree is cut down? You can blame this phenomenon on something called relative humidity, which is a measure of how much moisture is in the air. As the relative humidity increases, the moisture content (MC) causes the wood to expand. On the other hand, when the relative humidity decreases, the moisture content also decreases and causes the wood to shrink.
Besides shrinking and expanding, moisture content in wood causes other problems when there are changes in humidity. Some of these problems include:
Warped and twisted lumber
Kicking while machining
Shifting or creaking in floorboards
Adhesive failures
Joints won’t fit properly
Discoloration
To avoid these moisture-related issues, sawyers should do whatever they can to help the wood reach its equilibrium moisture content (EMC). Sawyers can achieve this by either kiln drying or air drying the wood. But is one method more advantageous than the other? What are the pros and cons of kiln drying versus air drying? Let’s take a look at each method and find out!
Kiln Drying
A kiln is a type of oven that has a thermally insulated chamber. It produces temperatures to complete a variety of processes, which includes drying wood. For wood that is more than 50 percent MC, the temperature in the kiln needs to be between 100 to 130 degrees F. The temperature gets raised as the wood dries. In most cases, the kiln reaches a maximum temperature of 160 degrees F., although there are instances when the temperature gets raised to 200 degrees F.
One of the biggest challenges in kiln drying is the need to dry the wood as quickly as possible while avoiding severe drying conditions. Thankfully, modern kilns are equipped with computerized recorders and controls, which make it possible to set the most effective kiln schedule.
Pros of Kiln Drying
There are definitely many pros when it comes to kiln drying over air drying. Some of these pros include:
Takes much less time to kiln dry.
Requires less space.
Easier to control the moisture content.
Easier to reduce the number of defects in the wood.
Provides the ability to control the temperature, air flow, and relative humidity.
Higher temperatures reduce the amounts of fungi and mold in the wood.
Higher temperatures reduce the amounts of larvae and insects in the wood.
One more pro of kiln dried wood is that it can be finished in a shorter amount of time than wood that is air dried.
Cons of Kiln Drying
One of the major cons of kiln drying is the initial expense. The larger the kiln, the more board feet of lumber can be dried at once. Sawyers can expect to pay $4,000 for a brand-new entry-level kiln. Sawyers that want to produce up to 15,000 board feet of kiln dried wood can expect to pay around $38,000. Larger kilns cost up to $50,000. These prices do not cover the cost of installation. In some instances, the cost of installation is about as much as the kiln itself.
To avoid paying this kind of money, some sawyers make their own kiln. The biggest concern when doing so is knowing what size to build. For a single-stack kiln that is large enough to stack lumber four feet wide and 10 feet long, the size should be at least 8 feet x 12 feet. A two-stack kiln should be 12 feet x 12 feet and a three-stack kiln should be 12 feet x 16 feet.
While it might not be as expensive to make a kiln, it is be time-consuming. Some sawyers choose to either buy or build a solar kiln. This type of kiln is similar to a greenhouse and relies on solar power.
Here are a few more cons of kiln drying:
Requires a learning curve to kiln drying as there are various kiln schedules for different species of wood. In some cases, this learning curve is at least 6 months.
High temperatures can cause timber to lose up to 20 percent of its color.
Requires a source of energy, and a lot of it.
With all of pros of kiln drying, and the very few cons, it makes one wonder why sawyers would consider air drying. But for those who don’t have a kiln or have access to one, air drying is really the only other option to remove moisture. Plus, some sawyers simply prefer air drying over kiln drying.
Air Drying
This process for drying wood is exactly what it sounds like. When air drying, the wood is exposed to the natural elements until it reaches the optimum MC. While air drying sounds pretty basic, there is a process that goes into it. After the tree is cut down and the wood is attained, it should be cut and stacked as quickly as possible. This helps to prevent it from becoming damaged during air drying.
End grain coating as soon as possible is also important, which can be done with and end sealer like ANCHORSEAL. Since the ends dry the fastest, end grain coating will help prevent checking and splitting defects.
Here are some other things to remember when air drying wood:
The area on which the wood is stacked should be as level as possible.
Make sure water will be directed away from the stacked wood instead of it pooling underneath.
The foundation should be on cement or rock and preferably up to 12 inches off the ground.
Use place spacers, or stickers in between each layer of stacked wood.
The boards in a stack of wood should all be the same length and thickness.
If the wood is not stacked in an enclosed area, it’s important to cover the wood in order to protect it. Tarps, corrugated metal roofing, or even a sheet of plywood are all things that can be used to cover the stacked wood. Prevent warping and keep it flat by placing a weight on top of the stack. The recommended amount of weight is 50 pounds of weight per square foot. The MC of the stacked wood can be easily monitored with a moisture meter.
Pros of Air Drying
One of the biggest advantages of air drying is that there is not a huge expense associated with it as there is no need to fork out money for a kiln. In fact, sawyers don’t have to pay anything to air dry wood on their property!
Besides saving money, other pros of air drying include:
Doesn’t require an energy source.
As long as the stacked wood is covered, the color is not affected.
Less chance of cracking and warping.
Less of a learning curve involved.
These are definitely some great advantages when it comes to air drying wood. However, air drying also comes with its disadvantages as well.
Cons of Air Drying
Here are some of the major cons associated with air drying:
Takes much longer for the wood to dry – usually anywhere from 50 to 200 days.
No control over the weather and complete dependency upon the climate
Takes a lot of time, as well as physical labor to prepare and stack the wood.
Requires more space to air dry.
Lower temperatures will not kill insects or protect from fungi and mold.
Along with these cons, it’s important to note that air drying might not cause the wood to reach its EMC. For this reason, wood that is air-dried may not be suitable for indoor projects.
As you can see, there are definitely some amazing pros to both kiln drying and air drying. Each sawyer has to decide for themselves which method works best for them. Do you kiln dry or air dry? We would love to know! Be sure to like SawmillCentral on Facebook and post your answer in the comments below the article.
