Updated: Jul 17
By Erich Ebert, Urban Wood Lab Store Supervisor
At the Urban Wood Lab we get questions all the time about using wood that’s been air dried. Usually the slab or lumber has been in someone’s barn for years. If wood isn’t properly dried, you run the risk of it shrinking or warping, and your project won’t be a reflection of the hard work you’ve put in. Here’s what you need to know about proper air drying, kiln drying, and moisture content etiquette. Download in printable format.
What is moisture content?
Moisture content is a measure of how much water is in the wood. Living trees can contain more than 66% water by mass, and some species have much higher water content. View average moisture content by species.
When trees are felled the moisture content starts to decrease, and that can cause movement in the wood such as cracking, warping and shrinking, especially in large or thick slabs. Movement in wood that has not been properly dried can reduce the quality of your woodworking project. It’s important to know that for projects that will be used outdoors such as patio furniture, wood that has been properly air dried will work fine. For projects that will be used indoors, be sure to use wood that has been kiln dried.
What is air drying?
With air drying, milled slabs and lumber are placed in the open outdoors to dry. Because wood is a material that reaches moisture content equilibrium with the surrounding environment, it is critical to air dry where there is a good exchange of air. The closed environment in barns and garages makes them less than ideal for air drying wood. Air drying wood indoors is also not ideal because the wood can dry too fast, resulting in movement.
How do I air dry wood properly?
Sticker stacking with compression is a best practice for air drying wood. This involves stacking the lumber or slabs out in the open on a level surface, leaving space side to side between the pieces for air flow. Stickers – usually 1” x 2”s – are placed on top of each layer to create space for air flow between the layers. Then, the entire stack is compressed with straps or weights to prevent the wood from twisting as it air dries. If straps are used, they’ll need to be tightened periodically because the wood will shrink as it dries. Air drying takes a minimum of three to six months, and some species taken even more time. For instance, oak requires six to twelve months to air dry.
How is moisture content checked?
Digital moisture detectors have a probe or small pins that are inserted into the wood to check moisture content. Users select the species of wood being checked on the tool. At the Urban Wood Lab we use both types of detectors. (We also use a temperature and humidity gauge inside the kilns.) You know your wood is air dried properly when it reaches a moisture content around 19%.
Why is wood kiln dried?
Wood for projects that will be used indoors should be dried to a moisture content between 5% and 7% and to reach that moisture content level, a kiln is used.
Here is the kiln drying process we use at the Urban Wood Lab:
First, lumber and slabs are air dried using the sticker stacking method described above.
Next, the stacks are placed in the kiln. The kiln is a large, insulated space that can be heated and includes fans to circulate the air. The stacks are dried at about 110°F for four to six weeks, until the moisture content is near 7%. The length of time depends on the species of wood being dried.
Then we turn the heat up to about 140°F for a few days to finish the process and kill any mold or bacteria left in the wood. The kiln space is monitored with a temperature and humidity gauge, and a digital moisture detector is used to check the wood.
From there, the kiln dried wood is delivered to the Urban Wood Lab Store and Milwaukee-area ReStores for purchase by you.
The Urban Wood Lab carries kiln dried wood in a wide selection of species and sizes in slabs and dimensional lumber. We also carry some air dried wood for use in outdoor projects. If you’d like to learn more about the ins and outs of wood drying, stop in and talk with us soon.