Air Drying and Kiln Drying—How Long Do They Take?
The process of drying wood can take anywhere from a few weeks to a couple of years. Talk about a wide range of time!
It really depends on how you dry it, what kind of wood it is, and many other factors.
If you’re using a kiln, the process will go a lot faster, taking only three days to a couple of months. But air drying, with its many variables, could take more than a year.
And the wood may still not end up at the moisture content you need!
When you understand these variables, you can make an informed decision about drying your wood. We’ll answer the following questions:
- How long does wood take to air dry?
- How long does wood take to kiln dry?
- How does drying affect wood?
So, first, air drying.
How long does wood take to air dry?
Wood typically takes at least a year to air dry. Many wood professionals refer to the principle that wood requires one year per inch of thickness to dry.1
Air drying, as the name suggests, involves leaving the wood to dry outdoors in the open air, preferably with some kind of cover and stacked in a way that allows for optimal airflow.
Many factors affect the process, such as:
- The weather
- The type of wood
- The thickness of the wood
- Amount of airflow
- Desired moisture content
Here are some more details on each.
The outdoor relative humidity level and temperature can make a big difference in how quickly the wood dries.
In a humid environment, such as in the southern U.S., wood may take much longer to dry than in, say, Arizona, where there is much less humidity in the air.
The type (species) of wood
Different wood species have different densities that impact how quickly they release moisture. Hardwoods will generally dry more slowly than softwoods.
The thickness of the wood
This factor goes back to the rule of thumb mentioned earlier—one year per inch of thickness. That means that while a 1-inch thick slab of wood could take one year to dry, a 2-inch thick piece could take two years!
Amount of air circulation
The greater the airflow, the more quickly the wood will dry. That’s why air drying lumber works best when the wood is stacked up off the ground and spaced out; this allows air to flow underneath as well as around the boards. If you have the space to spread them out, even better!
Another way to speed up the process of air drying is to place the wood in a shed with good airflow and set up some fans to blow on it.2
Desired moisture content
The whole purpose of drying lumber is to get it to a certain moisture level.
For woodworkers, the moisture content (MC) matters because wood will shrink or expand to reach the moisture content of its environment (known as the equilibrium moisture content or EMC). Drying wood to that MC before working on the wood will keep the flooring, furniture, or cabinet from warping once it’s finished.
But air drying wood to the levels needed for an indoor environment (6–9% moisture content) can be very difficult or almost impossible if the outdoor air has a higher moisture content.
With wood used as firewood, this factor won’t matter as much. The Chimney Safety Institute of America states that firewood should have an MC between 15 and 25%, while the EPA says it’ll burn best under 20%. In this case, it may only take the wood 6 months to dry.
However, if you’re a professional woodworker or flooring installation specialist who’s working on projects for clients, kiln drying will be more feasible for you. Let’s talk about that next.
How long does wood take to kiln dry?
Kiln drying can take anywhere from about 2 to 8 weeks—or even less in some cases. The balance in kiln drying is finding the fastest drying speed possible without harming the wood.
Many woodworkers run sample pieces of wood through a kiln to experiment and determine how long a given batch of wood will take.
Heartwood Mills, a company that specializes in kiln drying, estimates their drying times anywhere from 4 days to 3 weeks.3
As with air drying, various factors come into play:
- The type of wood
- The thickness of the wood
- The beginning moisture content
- The type of kiln
- The temperature of the kiln
- The amount of air circulation
The type of wood
Similar to air drying, the wood species will affect the rate of drying. Hardwoods require slower drying times to prevent damage, such as warping or splitting.
The thickness of the wood
The thickness of the wood also still makes a difference. Thicker boards will take longer than thinner boards.
The type of kiln
There are four main types of wood kilns:
- Conventional kilns
- Dehumidification kilns
- Solar kilns
- Vacuum kilns
Learn more about how each one works by visiting our kiln drying page.
For now, suffice it to say that conventional kilns are usually faster than dehumidification or solar kilns because they run at higher temperatures. Vacuum kilns are also faster, but for a different reason—these kilns use lower air pressure, which allows the wood to release moisture more quickly at a lower temperature.
The temperature of the kiln
Typically, the higher the temperature of the kiln, the faster the wood will dry. But that doesn’t mean you can just crank up the heat to the highest level.
It’s a balancing act.
Drying the wood too quickly can cause damage, so kiln-drying professionals try to find the highest possible temperature that’ll also preserve the wood.
The USDA’s Dry Kiln Operator’s Manual contains numerous sample kiln schedules for different types of wood. They can help you estimate drying times.
The beginning moisture content
Kiln drying green wood (above 60 or 70% MC) will take longer than kiln drying wood that has already dried some. Allowing your wood to air dry for a while can speed up the kiln drying process and save some money. But again, it’ll depend on how much time you have.
The amount of air circulation
Air circulation matters for kiln-dried lumber, too. Stacking the wood with spacers (known as stickers) can ensure that the greatest amount of air moves between the wood.
Now that we know a little more about wood drying times and the factors that affect them, let’s look at how drying affects the wood and ways we can make the best use of drying time.
How does drying affect wood?
In short, drying wood causes shrinkage as the moisture leaves the piece of wood.
But this shrinkage doesn’t occur right away because there are two types of water in the wood:
- Free water
- Bound water
Free water is in the cavities of the cells of the wood. Bound water, on the other hand, is actually within the walls of the cells.
Thus, the free water typically dries first. Once it’s all or mostly gone, the bound water remains—this is a point we call the fiber saturation point (FSP). It’s usually around 27 to 30% moisture content.4
Then, as the bound water dries, water begins coming out of the cell walls, causing the cells’ “microfibrils [to] move closer together.”5
That’s when shrinkage occurs.
Shrinkage in wood
Wood shrinks and changes as the bound water dries up. But this process doesn’t happen evenly throughout the wood. Instead, it experiences different amounts of shrinkage in three different dimensions:
- Radial shrinkage: This is shrinkage that occurs on the radial plane of the wood—the plane you would see if you were to cut across a log and look at the wood rings radiating outward. The wood shrinks about 3 to 5% in this dimension.
- Tangential shrinkage: Most of the wood’s shrinkage takes place on the tangential plane of the wood. It can be anywhere from 3 to 12%.
- Longitudinal shrinkage: Very little shrinkage occurs lengthwise in wood. It’s usually around 0.1 to 0.2%.
Because most shrinkage occurs on the tangential plane, flatsawn wood has the potential to warp much more than quartersawn wood.
Possible drying defects include:
Thankfully, proper drying techniques can go a long way to prevent these issues.
Protect your wood from damage while drying
As a woodworking professional, you no doubt want to be efficient with your time, which means drying the wood as quickly as possible while avoiding damage. Here are some practices that can help you keep your wood in the best possible condition throughout the drying process:
- Seal the ends of your wood as soon as possible after sawing and planing to protect them from cracking. You can buy end-grain sealers for wood on the market, though many wood professionals use glue, latex paint, paraffin wax, or polyurethane.
- Use wood cut farther from the center of a log (the pith). The pith is most prone to shrinkage.6
- Avoid wood with knots because they can cause warping.7
- Place stickers no more than 24″ apart. Having the stickers (spacers) close to each other will keep boards from bending or bowing in the areas between.
- Put weight on the wood after you’ve stacked it for drying. This weight can help prevent warping.
- Don’t dry the wood too quickly.
And that last tip may be the most important.
You can avoid drying your wood too quickly by following a kiln drying schedule and checking the moisture content of the wood. The moisture readings will inform you of how the wood is drying and when you have reached the EMC.
The best way to get those readings?
- Meier, Eric, “Drying Wood at Home,” The Wood Database. (↑)
- “Considerations in Drying Hardwood Lumber,” University of Missouri, Extension. (↑)
- “Kiln Drying Wood: Is It Worth It?” Heartwood Mills. (↑)
- Schmidt, Udo, “Predicting Wood Movement,” Woodcraft Magazine. (↑)
- Ibid. (↑)
- Meier, Eric, “Drying Wood at Home.” (↑)
- Ibid. (↑)