The term ‘kiln-dried’ means that the moisture of the wood has been removed or evaporated in a chamber where the circulation of air, the level of humidity, and the temperature were properly controlled. The term kiln-dried also implies to moisture content (MC) lower than that in softwood construction lumber.
What is the kiln-dried wood moisture content?
The Kiln-dried moisture content (MC) is the amount of moisture in the wood after the drying process. Kiln-dried wood normally has a six percent to eight percent moisture content in it. A higher moisture content than 15% has a negative impact on the corrosion of metal fasteners and even higher moisture can reduce the strength.
The moisture content of wood
In the words of Dr. Eugene Wengert, a professor and an extension specialist in wood processing at the University of Wisconsin-Madison’s Department of Forestry,
“Freshly forest-cut “green” wood may have a moisture content (MC) of thirty percent to more than two hundred percent, depending on the species of the tree that you have cut. Before using any wood for the construction of buildings or furniture, it needs to be kiln-dried to reduce its moisture content (MC). The “ideal” moisture content (MC) depends upon the use of the wood and the annual average relative humidity (RH) at the place where the kiln-dried wood is to be used. The wood you work with must be dried down to a moisture content MC within two percentage points of the equilibrium moisture content (EMC) of the in-use location. The equilibrium moisture content EMC of air is numerically equal to the moisture content that will eventually be attained by any piece of wood when stored indefinitely at a particular humidity. The temperature has no direct effect on moisture content or equilibrium moisture content (EMC).
The relative humidity (RH) in most homes and offices in the U.S. (except in some coastal areas and exceptionally dry areas like the desert Southwest) averages thirty percent to forty percent. This is six percent to seven percent equilibrium moisture content (EMC), which means that wood in interior locations will average six percent to seven percent moisture content (MC). Therefore, lumber intended for interior use should be dried to six percent to seven percent moisture content and should be kept at this moisture content before and during manufacturing.
Note 1: Softwood
Softwoods machine better at a little higher moisture content. They shrink and swell less than hardwoods when the moisture content changes. Therefore, often the target moisture content for softwoods is eight percent
Note 2: Furniture
The term kiln-dried has no special indication of moisture content for furniture, cabinet, or millwork manufacturers; don’t specify kiln-dried lumber without also adding the actual moisture content that you want.
Why do we kiln-dry wood?
There are 8 main reasons for kiln-drying wood.
1. Dry faster:
To make the wood dry much faster compared to other methods of drying wood.
2. Make wood lighter:
to lighten the wood for shipping to other places for the construction of furniture, etc, Dried wood is much lighter in weight than a fresh one, and its transportation, loading, and handling costs are reduced to a great extent.
3. Make it stronger:
Dried wood is much stronger than green, fresh wood in most strength properties.
4. Dry it correctly:
Wood for impregnation with preservatives has to be properly dried, particularly in the case of oil-type preservatives.
5. Dry to certain moisture level:
In the field of chemical modification of wood and wood products, the material should be dried to certain moisture content (MC) for the appropriate reactions to take place.
6. Easier to use:
Dry wood generally works, machines, finishes and glues better than green timber (although there are exceptions; for example, green, fresh wood is often easier to turn than dry wood). Paints and finishes last longer on kiln-dried wood.
7. Improved properties:
The electrical and thermal insulation properties of wood are improved by kiln-drying.
8. Remove organisms:
To kill wood-borne organisms.
Kiln-dried softwood moisture content
For softwood lumber of one-inch width or thickness the goal is to have the moisture content (MC) under twelve percent (12%). Softwoods are used for construction in higher relative humidity conditions than hardwoods, suppliers of wood don’t need to get their moisture content lower than twelve percent (12%), and this level can be easily achieved after air-drying the wood. Softwoods used in interior locations are dried to about eight percent moisture content.
Wood with a higher moisture content than 12% to 15% can cause the wood to become brittle and face many other problems during the machining process. According to Dr. Steven Doggett, the founder of Built Environments, wood moisture content as high as fifteen percent can cause corrosion of metal fasteners and at sixteen percent it may lead to fungal growth. When it comes to the moisture content of plywood or dimensional lumber, a moisture content (MC) of seventeen percent to nineteen percent reduces the overall strength of plywood and moisture content of twenty percent or more reduces the strength of dimensional lumber (lumber cut to certain predefined sizes, such as two x four).
A study conducted by two experts namely, Imamura and Kiguchi showed that wood moisture content above twenty percent can cause a five percent loss of nail shank diameter in 4 years and a projected twenty-five percent loss in thirty years. The same study showed a forty percent loss in joint strength and concluded that a twenty percent moisture content may significantly compromise shear resistance of exterior walls.
How to kiln dry at your house
After chopping down a tree into bits, you can bring the wood indoors to finish the process of drying. To build the kiln indoors yourself, you must lay poly (clear plastic roll) on the ground inside your house and then build a frame with 2×4 studs on top of it for the wood to rest on. You must leave enough space to be able to place a standard household dehumidifier at one end of the pile of wood, and a small fan at the other end of the pile of wood.
The fan circulates the air to even out the drying process of the wood. You can also design it to pull air from below, then blow the air down a plastic tube to the other end of the pile of wood. This way you will be sure that there is no stagnant air or dampness trapped inside the kiln. The dehumidifier must also be trapped inside the kiln and must be set to maximum.
Some models have a hose that runs out of the kiln and fills a bucket. The kiln is built around the stacked and bound wood that is placed over a lightweight wooden frame which sits on a plastic roll. All seams must be properly sealed with vapor barrier tape to hold the moisture inside.
You must cut a few small access holes to poke your fingers in to be able to control your dehumidifier and to test the woods moisture content in different places. You must tape up these holes after you have done using them. The wood will need to stay in the kiln for at least four months to reach an acceptable moisture content level of eight percent.
How to measure moisture content (MC) level in kiln-dried wood?
Resistance and Dielectric meters are used to measure the wood moisture content with the same accuracy over the same moisture range. Let me now compare some of the advantages and disadvantages of these 2 types of meters.
With pins that can penetrate the wood, resistance meters will leave unwanted and ugly holes in the wood. Dielectric meters use surface contact electrodes that leave no marks on the wood. The use of insulated pins with resistance meters allows the measurement of wood moisture gradients which is useful to kiln users.
Dielectric meters give the average moisture content of a larger area. They are more reliable on wood that has a fairly uniform moisture content than on wood with substantial moisture gradients.
Dielectric meters can read lower moisture content than resistance meters. Resistance meters are not sensitive to differences in wood density. Although dielectric meters provide specie corrections that are based on average species density, the moisture content readings are sensitive to within-species variation in density.
Resistance meters are used with a separate hammer/electrode probe, making them bulkier to carry around on the factory floor. These electrodes need frequent care due to broken or loose pins or broken cables and can be sensitive to static electricity in cold and dry weather. The moisture sensing element is built into the case of the dielectric meter, it is more compact and easier to carry around the trees.
To sum up this post, the moisture content of kiln-dried wood is six percent to eight percent.