Hardwood lumber is often called either “Air Dried,” “Kiln Dried,” or “Green”: What’s the difference?
First let’s start with how we get from tree to board. First, trees are harvested. A harvested tree is called a log. A log is taken to a sawmill, and sawn into boards. A freshly sawn board is called a “green” board, as it still has the moisture from the “water” in the tree. Typically these boards will look and feel wet. Once the lumber is sorted for length and grade, it is stacked with spacers between each layer of lumber to start the drying process. The first part of drying is typically done outside.
Once material is outside drying on these spacers, it is referred to as “Air Dried.” The process of Air drying can take anywhere from 2 weeks to many months depending on lumber thickness and species. Why so long? The cost of kiln drying is very expensive, so lumber manufacturers use the natural sunlight and wind to start the drying process. It’s very similar to hanging your laundry on the laundry line (free) vs. drying in a clothes dryer (cost for electric or gas to run dryer).
After the lumber is around 30% moisture content (30% of weight is “water”, 70% is the wood), manufacturers will load their kilns to start the kiln drying process. This process can take anywhere from 1 to 8 weeks depending on species and the thickness of the lumber species. A kiln is essentially like an oven. Large fans within the kiln move air that is heated through the kiln. A kiln operator also can add or remove moisture from the kiln to insure that each piece of lumber is drying at a similar rate. At the end of this process, lumber will be between 6-10% moisture content, and referred to as kiln dried.
Most companies that make furniture, cabinets, flooring, and many other products utilize only kiln dried lumber. Kiln dried lumber is less likely to cup, warp, twist, and break, which enables manufacturers to make a product that can be enjoyed for years to come!
- Kiln dried lumber is often used for furniture, cabinets, and flooring.
- Air dried lumber is often used in patio furniture, fencing, and decking.
- Green lumber is mostly utilized in the pallet industry.
Hopefully, that can help you to understand those different terms. Please let us know if you have any questions below in the comments section!