Air drying or kiln drying—which one?

Perhaps you have some beautiful walnut wood you’d like to have dried for your next woodworking project. But the question is, How should you dry it?

Kiln drying involves placing wood in a special type of chamber—called a kiln—to dry. The chamber is a temperature- and humidity-controlled environment where warm air circulates around the wood and brings its moisture content down.

This process is different from air drying, which involves leaving wood outdoors to dry.

On this page, we’ll cover various aspects of kiln drying so you can be confident in deciding how to dry your wood. We’ll explore:

• How kiln drying works
• An overview of the process
• How much it costs
• How long it takes
• When to use kiln-dried wood
• Why choose kiln drying

Let’s get into it!

How kiln drying works

Kiln drying involves placing wood in the main chamber of a kiln and using a heat source and fans to remove moisture from it. As the moisture content goes down, the operator can raise the temperature by increments.

But increasing the temperature requires caution because doing it too quickly can damage the wood, resulting in checks or splitting.

The process of drying wood in a kiln requires some effort and education to do it properly, but here we’ll cover the basics.

According to Hank Stelzer, a forestry state specialist at the University of Missouri, wood needs three factors to dry¹:

• Dry air
• Heat
• Wind (air circulation)

Kiln drying combines all three of these factors using the following elements:

• Main chamber
• Heat source
• Humidifier
• Fans

In a kiln, temperature and humidity are typically controlled based on the dry-bulb temperature (regular air temperature) and wet-bulb temperature (temperature that considers both heat and humidity).

The process depends on the kiln used. We’ll look at four main ones that are outlined by Salim Hiziroglu, a wood products specialist at Oklahoma State University:

• Conventional
• Dehumidification
• Vacuum
• Solar

1. Conventional

A conventional kiln uses steam coming through pipes to heat the chamber. The heat causes the water to evaporate from the wood.

Typically, conventional steam kilns are gas-operated and can have temperatures from 120° F to 190° F.² They are popular in commercial operations.

2. Dehumidification

Dehumidification kilns are one of the most common types. Rather than causing the water in the wood to evaporate, they use a heat pump to condense the moisture and remove it.

They run on electricity but are a little more efficient than conventional kilns because they recycle heat.

They usually have temperatures around 95° F to 100° F, though some can become nearly as hot as conventional ones.

3. Vacuum

Vacuum kilns dry wood within a compartment that has all air pressure removed.

How does it work?

It’s similar to being at high elevations where air pressure is lower. Water boils at lower temperatures in those locations. Likewise, the lower pressure in the vacuum causes moisture to leave the wood more easily at lower temperatures.

The result is a much quicker drying process.

But the downside to vacuum kilns is that they tend to be more expensive because they run on electricity.

4. Solar

Solar kilns use sunlight to heat the drying chamber and fans to circulate the air. Because temperatures are warmer in the daytime and cooler at night, they can take much longer to dry wood. However, they’re also less expensive and more environmentally friendly to run than the other options.

These wood kilns run at lower temperatures, though some can reach up to 130° F.

Some woodworkers even build their own solar kilns. The Virginia Cooperative Extension provides a helpful guide for building and operating a solar kiln.

Now that we know the four different types of kilns, let’s get a rundown of the drying process.

The kiln-drying process

Here’s a look into kiln drying to help you decide whether it’s the best option for your wood.

We cover the overall process mentioned in the Dry Kiln Operator’s Manual published by the USDA Forest Service. But take a look at the detailed instructions in the manual if you plan to operate a conventional one yourself.

Prepare the wood.

Typically, the lumber should at least be rough cut before drying. Right after, seal or coat the ends to prevent moisture from escaping more quickly through the ends and causing the ends to split.³

Commercial products are available for sealing the ends of lumber (such as AnchorSeal), although wood glue or old latex paint can do the job, too.

Some woodworkers recommend allowing green wood to air dry for a while to get it to release some initial moisture. This can shorten the amount of time it needs to spend in the kiln, depending on how much the moisture content has come down already.⁴

Decide on a drying schedule.

The Dry Kiln Operator’s Manual provides numerous kiln schedules based on wood species, thickness, starting moisture content, the part of the tree the wood is from, and the way the wood has been cut.

The schedules can follow a timetable or moisture content measurements. Though they are designed for conventional kilns, the manual includes instructions on modifying them for a dehumidification kiln, too.

If the wood is green, the drying schedule will specify a lower temperature or dehumidifier setting to prevent damage in the beginning. As the wood dries, that temperature can be raised.

Load the wood into the kiln.

Stack the pieces of wood with stickers (spacers) in between. These spacers are typically narrow pieces of wood that are about an inch thick.

The stickers should be placed near enough to each other so that the wood can’t bow or warp. One woodworker, who operates his own dehumidification kiln, recommends placing stickers at least every 24 inches.

Then, place a weight on top or strap the wood down.

Dry the wood according to the schedule.

The temperature and humidity levels of conventional and dehumidification kilns are controlled by dry- and wet-bulb temperatures. Following the drying schedule, the operator will raise these temperatures as the wood dries.

Monitor moisture content with a wood moisture meter.

A wood moisture meter will indicate how the wood is drying and whether it has reached the desired moisture level.

Without one, it’s impossible to know when the wood has dried enough.

If the kiln operator is following a moisture content–based schedule, the moisture meter will help keep the process on track, indicating when the temperature should increase.

Allow the lumber to cool.

When the wood has reached the desired moisture content, the operator will turn the kiln off and let the wood cool there. Or, if the production schedule is tight, the wood will be moved to a controlled environment where it can cool.

After the wood has dried, be careful of leaving it outdoors or in an area with high humidity. The wood can still absorb and release moisture as it did before, and you may risk it changing in moisture content again.

How much does kiln drying cost?

If you’re taking your wood to a kiln dryer, it can cost anywhere from $0.25 to $3.00 per board foot of wood. The higher prices typically apply to vacuum drying.

The price depends on many factors, including:

• The initial moisture content of the wood
• The type of kiln being used
• The thickness of the wood

Reach out to your local sawmill or drying operation for prices specific to your wood.

How long does kiln drying take?

When drying wood that is green (above 60%) to a moisture content suitable for indoor woodworking (about 6–8%), kiln drying can take from a couple of weeks to a couple of months.

This is a pretty significant range because many factors impact the drying process. The exact number will depend on:

• The species of wood (hardwoods dry slower than softwoods)
• The thickness of the wood
• The initial moisture content
• The type of kiln
• The temperature used
• The amount of airflow between the stacks of wood

For example, Cory, who does custom drying at Hill Farms, explains that some types of wood, like pine and poplar, may only take a week to dry. Oak, on the other hand, can take anywhere from four to eight weeks.

Time-based drying schedules can help you to estimate how long your wood will take to dry.

When should you use kiln-dried lumber?

Almost all the time!

Kiln-dried lumber is best for nearly every project because it has been dried in a controlled environment. You can be sure it’s at the moisture content you need (typically, the equilibrium moisture content, or EMC) to prevent warping later on.

In fact, it may not even be possible to get air-dried wood to an MC suitable for the indoors because of higher relative humidity levels outdoors.

Some outdoor projects are an exception, though.

For example, wood for decks, house framing, or fence posts won’t need to be at an EMC suitable for the indoors. And building contractors actually prefer working with wetter wood when framing.

Firewood can also be air-dried, although kiln-dried firewood is becoming more popular because of how well it burns.

But if you’re planning to use your wood for any of the following projects, opt for kiln drying:

• Cabinet making
• Furniture building
• Flooring installation
• Lutherie

Here’s why.

Why choose kiln drying

Kiln drying wood has many benefits for your woodworking project:

It saves time.

Because your wood is drying at a controlled temperature and humidity, you won’t have to deal with seasonal fluctuations that would affect the air-drying process. The raised temperature and forced air circulation help move the process along much more quickly.

You’ll save the many months—or even a year—that it would take to air dry the wood!

It saves space.

Having to store wood for months on end while it’s air drying can be a hassle. With kiln drying, you won’t have to do that for a long period of time. Once the wood comes from the kiln, it’s ready to be worked on!

It increases the wood’s durability.

Wood that has been kiln-dried can handle moisture better. According to woodworker Shannon Rogers, wood becomes more stable once it has been dried to 8% moisture content.

Furthermore, when wood is drying outdoors, it could become damaged because it’s harder to control the wood’s environment.⁵

It kills pests.

The high temperatures can help ensure that any pests—whether bugs, algae, fungi, or mold—are destroyed. As a result, the wood won’t require other chemical treatments, as it would if it were air-dried.

It can help prevent damage to your wood project.

Wood adapts to the moisture content of its environment, so the goal is to get the wood to the moisture content of its final location so that it’s less likely to shrink or expand.

Kiln drying helps your wood reach the moisture content needed to prevent it from warping later on. Less worry for you as a woodworker!

Begin your project knowing that your kiln-dried wood is ready

Kiln drying will be your best bet for getting wood that’s at the right moisture content for your project—wood that can ensure a high-quality project.

If you’re the DIY type, you might already be figuring out how to build your own kiln. If not, you still have the information you need to work with a kiln operator and get your wood properly dried.

But don’t assume your wood is ready for your project just because it’s been kiln-dried.

It’s up to you to know the desired moisture content (equilibrium moisture content) for the final location of your project. And it’s up to you to check the wood and make sure it matches that number.

Then, you can start your project with confidence.

A wood moisture meter will be your best friend in this process.

1. Stelzer, Hank, “Considerations in Drying Hardwood Lumber,” University of Missouri, Extension. (↑)
2. Ibid. (↑)
3. Introduction to Kiln Drying, Wood-Mizer Kiln Drying Systems, p. 5. (↑)
4. “Drying Wood with Dehumidifier Wood Kiln – How to Dry Wood,” WoodWorkWeb.com. (↑)
5. Introduction to Kiln Drying, Nyle Systems. (↑)