One of the key things everyone involved with woodworking should understand is how humidity(moisture in the air) affects your woodworking project. Wood absorbs or loses moisture depending on the surrounding humidity which causes the wood to expand or shrink. If you don’t plan for this the results could ruin your project.
The ideal moisture content of wood for woodworking is between 5% and 9% depending on the typical relative humidity of the location where the piece will be used. Wood moisture content during construction should be within 2% of the EMC(equilibrium moisture content) for that final location. This chart gives an example of which percentage to use.
Humidity of the final location of use
Within 2% of EMC
How to dry wood for woodworking
Over half of a green freshly milled tree’s weight is water(actually up to 200% for some species) and as it dries over time the wood will shrink by about 10% in diameter. If you make anything out of green wood than when the wood naturally dries any joints or other surfaces will warp, crack and otherwise fall apart.
This video by fine woodworking gives one of the best explanations of how to dry wood. The two methods are either air drying or kiln drying the wood.
Air drying wood is usually a good option if you have a big tree that falls down and you want to use the wood for your own projects. Its cheaper than buying wood at a lumber yard but you’ll need to have patience because it’ll take on average about a year per inch thickness of the wood to dry.
Typically a woodworker will have a couple year supply of wood in the process of drying due to the cost savings of drying it yourself. It just takes time which is where a kiln comes into play.
Kiln drying wood takes a couple of weeks or less depending on the kiln and wood thickness. Most wood available at lumber yards is kiln dried. The process involves putting the wood in a strictly controlled environment with forced air where the humidity and temperature is changed based on a schedule for the specific type and thickness of wood.
Most lumber yards will kiln dry your wood for a fee or you can buy a smaller kiln if you want to get into that aspect of woodworking. The USDA forest service forest products laboratory has a lot of good information on their website (USDA Forest Service link)
How to measure moisture level in wood
Measuring moisture levels in wood can be performed using either a moisture meter or by oven dry testing. Each of the choices has its owns pros and cons.
Electric Moisture Meter
There are two types. moisture meters
- A pin type with electrodes that poke 2 electrodes into the wood and determine moisture content by using the electrical resistance between the two pins.
- The other type is a pin less one which uses an electromagnetic sensor to scan the wood to determine moisture content.
The Electromagnet meter does not damage the wood in any way which is good for more expensive species of wood or smaller pieces.
How to use a moisture meter:
- Hold the measuring plate against the wood surface and initiate a scan per the meter instructions. You might want to scan the wood in several places to get an average of the content.
Pin type moisture meter:
- Put the pins in the wood surface per the meter instructions(should specify best location and how far apart)
- Typically you want to measure with the grain and not across the grain for the best measurement.
- Take a measurement per the meter instructions.
I bought a Dr. Meter moisture meter from amazon(Link Here), and have not been impressed with its performance. The Mini-Ligno, which I’ve used in the past, is a much better meter because of its higher accuracy.(Link to Mini-Ligno on Amazon here)
Oven Dry Testing
A slice of wood is dried in a small kiln designed for dry testing. The piece of wood is stuck in the kiln and dried until the weight stabilizes. The initial and final weight are compared to determine the initial moisture content.
The disadvantages of oven dry testing:
- Costs associated with the kiln.
- The wood sample is destroyed increasing costs.
- The process takes a couple hours or more.
Should you use a moisture meter or oven dry to test moisture levels
A moisture meter will cost $30 for a basic low quality meter to a couple hundred or more for a high quality meter. This type works well for any hobby woodworking.
A drying oven doesn’t make sense unless you are processing larger volumes of wood. Of course if I’m wrong let me know!
How accurate are wood moisture meters?
A high quality moisture meter has an accuracy of better than 0.1%. This level of accuracy is necessary when you are making something that requires a correct moisture content on the wood.
Low price moisture meters, which you can purchase for around $40 at your local hardware store, are not that accurate and can cause you to make errors with your woodworking due to thinking the moisture content is different than it actually is. The standard accuracy for meters in this price range is 0.5% to 5%.
A higher quality moisture meter should have an accuracy of 0.1% in the critical range of less than 10%. These will typically cost a bit more. I have a review of the different moisture meters at this link(Best wood moisture meters for the buck).
If you intend to make projects that last forever checking the moisture level of your wood is a good idea in certain locations. If you let your wood sit in a temperature and humidity controlled environment prior to working than its not as important.
What moisture level is allowable for woodworking?
This depends on the local climate and what the final product is going to be used for. Wood moisture content is known as MC. The MC along with EMC(equilibrium moisture content) are the main parameters used when talking about wood moisture.
The normal acceptable wood moisture levels are about 7% for interior items and about 10% to 13% for exterior end use items.
A rapid change in humidity and temperature is the most dangerous condition change for your project. Wood is highly hygroscopic so easily loses or gains water causing swelling and shrinking. If the humidity changes quickly joints and other joined pieces can warp and get damaged.
If your garage or shop where you do your woodworking is at a different humidity than the final end use location you need to take precautions to ensure your project isn’t ruined.
The best way to set up your workspace is to control the humidity with air conditioning and, if the humidity is high, a dehumidifier that vents outside. Try to maintain a constant environment at all times if weather conditions where you live are not favorable.
Try to buy the wood for your project a month prior to use and store it in your controlled environment to lower the moisture content.
How do you reduce moisture in wood?
If your like me and don’t have a lot of time for preparing wood for woodworking one of the questions you have is what is the fastest way to dry wood for wood working.
The fastest way to reduce moisture in wood is to kiln dry the lumber. This process takes a week or two. Air drying wood takes a year per inch of thickness.
Some lumber yards will kiln dry wood for you for a fee. You’ll have to call to see what policies they have for the process.
The typical relative humidity levels in homes in the US is about 35% except for the desert areas which is lower and the more humid coastal areas. 35% Humidity puts your EMC(equilibrium moisture content) at about 7% plus or minus 2%. Exterior goods are usually about 5% higher than than interior items.
The best way to ensure that the wood you are using for your woodworking project is at a good moisture level is to ensure your work location is set up correctly. This includes ensuring your workshop has a controlled environment without large daily swings in humidity.
Woodworkers in the more humid states(southeast, coastal) swear by running a dehumidifier 24/7 during the summer months. If you have problems controlling humidity levels consider changing the types of projects you work on based on the time of year.
Table tops and intricate tightly fitted joints can cause the most problems when the humidity levels aren’t correct when you make a project.
Kiln dried lumber moisture content is typically about 10%. This is the typical value you want for furniture making projects.
Types of Moisture damage:
- Uneven surfaces can be caused when gluing pieces side by side with different moisture content. when they both reach equilibrium they will warp.
- Joints might have a sunken look due to the wood absorbing moisture from the glue.
- End splits due to wood shrinking if the moisture content was too high when you started making something
- Curved parts due to the one side of wood drying before the backside dries which can sometimes be caused due to different finishes on each side.
- Lumber that is too dry will occasionally split or chip when working on it with tools.
- too much moisture will occasionally cause a fuzzy grain that is due to wet wood fibers bending instead of getting cut.
- Glue or laminating failure due to too much moisture in the wood preventing the glue from working correctly.
- Bad finish which could be due to wood expansion or contraction cracking the finish.