You may recall from previous posts that I decided to use fallen oak trees for the wood in the townhome that I’m renovating. Most of these trees would have been ground to pulp if I had not used them. This is the first time that I had used actual trees as a building material, and so I thought I would share some the details
I made 9 trips to haul fallen trees to a sawmill in Michigan. The mill turned my tree trunks into boards, but that was only the start of the process. Believe it or not, trees consist mostly of water. In fact, the oak trees that I processed were almost 70% water. If you used these boards immediately they would shrink and distort in place, resulting in a construction disaster.
There are a number of ways to dry lumber. The simplest way is to air dry. In this process, green boards are stacked outdoors. Small pieces of wood called sticks are used to create spaces between each layer of wood. The goal is to reduce the moisture content from well over 50% (depending on the species) to a final moisture content below 10%. This process can take many months to several years depending on the thickness of the wood; how much moisture you can naturally remove is dependent on the humidity of the environment.
Another method is to combine a period of air drying with kiln drying. In this scenario, wood is air-dried for a number of months. It is then moved into a hot air kiln for a month or more. This process speeds up the curing process to 6 to 9 months depending on the thickness of the wood.
The fastest method to dry green lumber is to use a vacuum kiln. To understand this process you need to understand just a little bit of physics.
Water exists in three forms, solid (ice), liquid (water), and gas (steam, fog, or mist). If you leave a saucer of water out it will eventually evaporate. In this process, the energy in the environment slowly energizes the liquid water and converts some of it into a mist. Eventually, all of the water will be evaporated. If you add more energy to the water it will evaporate faster. This happens when you boil a pot of water on the stove.
At sea level water boils at 212 F, but at higher elevations, it boils at lower temperatures. This is because there is less atmospheric pressure at higher elevations (the air is “thinner”). Less atmospheric pressure makes it easier for liquid water to turn into a gas. It takes less energy (in the form of heat) because it is easier for the water molecules to escape the liquid water, as the atmosphere isn’t pushing them down as much. OK, you can now put away your calculators.
The fastest way to turn green wood into useable lumber is to use a vacuum kiln. This is a sealed box that pumps out some of the air creating a partial vacuum. This vacuum means that the air pressure in the box is lower than the regular atmosphere. It takes much less heat (energy) for the liquid water in the wood to vaporize and be removed. The vacuum kiln has hollow metal plates that circulate hot water like an old fashioned radiator, heating the apparatus. These are placed between each layer of wood. The combination of mild heat and vacuum allows the green wood to be dried in 7-10 days instead of a year or more for natural drying. When done properly this gentle process causes less cupping (distortion) and splitting of the wood.
See the photos below.