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Tuesday, September 24, 2013

Wood Science 101(14) - Equilibrium Moisture Content

I'm retired (sort of) from the forest products industry and now have time for more serious activities such as archery hunting, fly fishing, grandchild watching and my furniture building hobby. I have some nice quarter sawn red oak that has air dried to 18% m.c. outdoors. I now have this surfaced stock in my shop in the house awaiting further drying before being used to make a bedroom set. I need help making a determination as to what I can expect this wood will equalize to given the 65 degree and 60% relative humidity of my house. The humidity will be closer to 30% during the winter.  
Can you direct me to an internet site where I can find this information? Any help will be greatly appreciated. 
Jack Buckler
PSU School of Forestry - 1969

Hi, Jack. 
Thanks for the interesting little problem in calculation. In effect, you are asking "What is the Equilibrium Moisture Content (EMC) of wood at 65F and 60% RH, versus 65F and 30%RH...correct? 
In the old days, we used to consult the EMC charts in the Wood Handbook or the Dry Kiln Operators Manual. Unfortunately, the Wood Handbook Figure 4-1 is a "contour" chart that you have to eyeball and guesstimate, while the DKOM Figure 1-7 assumes temperatures of 70F, 141F, and 212F. So we used to always just fudge our estimates from the nearest line that related to what we were trying to calculate. 
Nowadays, we have a better way to do it. I programmed the equations referenced in the 2010 Edition of The Wood Handbook, page 4-3, into an Excel spreadsheet, and after doing so plugged in your 65-60 and 65-30 conditions to find that in the first case, the wood would reach equilibrium at 11.1%, and in the second case, 6.2%. 
This is a nice illustration of to what degree wood shrinks and swells with the change of season. Hardwood is usually kiln-dried to about 6-7% MC for sale to furniture manufacturers. So your obvious solution in the absence of a dry kiln would be to store the wood in your home this winter until it gets down into this range, and then start your project. Make sure the wood is "stickered" (that is, stacked with open spaces between each board) so that each piece of wood gets fully and evenly equalized. How long it takes to get to these moisture contents varies depending on the type of wood...oak takes longer than most other species of wood. 
In case you're interested in playing around with the numbers, I'm attaching the spreadsheet I built for your analysis. You can plug in different temperatures (T) and relative humidities (h) and the resulting EMC will be displayed in the first column. I went ahead and plugged in a range of h's between .6 and .3 for your information. 
Good luck with the project. 
Equilibrium Moisture Content (EMC) is defined as the moisture content at which wood is neither gaining nor losing moisture, the ideal condition for the manufacture and use of wooden products. Many a wooden chair has been made from components machined prior to reaching EMC, and subsequently fallen to pieces due to the wood shrinking and pulling out of the joints. Wood's ability to pull apart in shrinking is absolutely amazing...I remember seeing a boat dock in Texas that had been built with deck screws (and improperly dried lumber) that had pulled itself apart and fallen in the lake in the middle of its first winter. The owner was not happy.

In case you're also interested in being able to calculate EMC for your wood project, I've uploaded the EMC spreadsheet I programmed for Jack to our Penn State WoodPro website. Just click here, then click on the link labeled "Equilibrium Moisture Content Calculator", and the Excel file will download to your computer after asking you where you would like to download the file to. I've added a second tab for you internationals who still insist on referring to temperatures by the Celsius temperature scale.

If you're interested in knowing what your wood will equalize to outdoors in your geographic region, this document has Tables 2 and 3 that list outdoors EMC in major cities in the United States (Table 2) and around the world (Table 3). Find the nearest city in the tables, and your outdoors EMC will be just about the same.

Wood working requires patience...make sure the wood is fully dry and equalized before you start your masterpiece.

Update: Bill Smith at SUNY-ESF just made me aware that Scott Leavengood at Oregon State has developed some neat tools for determining shrink and swell in wood that is species-specific. Click here for a great calculator that allows you to input a species code, the lumber orientation, and your initial and final moisture conditions, and you'll see how much your wood will shrink in both thickness and in width.

They also have a neat map of the United States that shows the EMC in each state by month. Click here to use the map. The map indicates what the outdoor EMC of wood under conditions that prevail in each state during each month are. This is useful if you happen to be storing wood outside for a future can figure out how much the EMC will change when you move it indoors for working and use, or you can figure out how much the EMC will vary in outdoor wood applications in your area over the course of a year.

One last point...I've had inquiries about EMC relative to different species. It is true that EMC will vary slightly not only by species but by different specimens within the same species. This is due to the variability of wood...different pieces have different cellular arrangement, different levels of extractives, tyloses, etc. So the EMC results you obtain from these tools should be treated with recognition that ultimate EMC will vary somewhat from the theoretical numbers. The best way to know when your wood is equalized is to simply monitor the moisture content on a regular basis and note the rate of change. When the rate of change flattens out, then your wood has reached EMC for those conditions. And remember, if the temperature and relative humidity conditions keep changing, such as when the wood is outdoors or in a building where fluctuations are not tightly controlled, then the EMC of the wood will change right along with them.


Jennifer Darling Durant said...

There are plenty of people who know how to work with wood while it is still green as well. Certainly some projects require it to be dried properly, but sometimes green is best. Just ask the Artisans at Plimoth Plantation, or a local boat builder.

Anonymous said...

The DKOM shows different EMC's for those setpoints than your equation. Table 1-6 shows 65-60 with about 11% EMC and 65-30 with around 6.2 EMC. Wood Handbook also shows similar numbers in Table 3-4.

Chuck Ray said...

Thanks for pointing this out, Anon. Samuel Glass at the USFS Forest Products Lab in Madison found the error in the spreadsheet, and it is now corrected and reflects the numbers you state. I've added a permalink to the corrected spreadsheet in the right-hand column of the blog.

Thanks for catching it!

Ron said...

I really enjoy reading your posts. As far as EMC calculations go there is a calculator at the
is web sit that I have been using for years.
Why recreate something that already exists.

Gene Odato said...

During the years that I was in the log home manufacturing business we dried our logs to 12% EMC. south central PA. the log walls were very stable as a result.

When I use air dried lumber for furniture or cabinets I put the boards in the attic with a fan on them and let them sit until they stop losing weight. That is about as dry as I can get them. otherwise the boards will shrink after creating your furniture.

Mandt Lofthaug said...

Perhaps my thinking is off center, but according to the Figure 3-2 in The Wood Handbook the equilibrium moisture content varies significantly depending on whether the wood in desorbing or adsorbing.
It makes a big difference when wood is milled with a variance of 6% to 12% and an allowance of 5% out-of-range material, and then equilabrates at 8%. Is it any wonder that one finds floor planks of significantly difference widths within the same lot.
I've never come across a discussion on this.
Mandt Lofthaug, Inspector
Maple Ridge

Chuck Ray said...

Not off center at all, Mandt. Your point is well taken. From the WH...

"The rate of adsorption EMC to desorption EMC varies with species, RH, and temperature, with a mean value of about 0.8 near room temperature (Stamm 1964, Skaar 1988)."

On the graph you refer to, we see that the adsorption EMC at 50% RH is around 8%, while the desorption EMC at 50% is around 11%, which is close to the 0.8 ratio quoted above.

That means that wood settling at EMC from lumber that has been first dried to 6% (and is therefore in adsorption, will achieve a lower EMC than a piece that is approaching EMC from a piece milled at say, 12%. The resulting difference in EMC will cause a differential in shrink/swell (as you can test on the Oregon State website), and ultimately, result in pieces of slightly different dimensions even though they were milled at precisely the same dimension. Not a critical difference in most applications, but as you point out, can be very critical in flooring planks and other such strip applications.

Chuck Ray said...

Thanks for the link, Ron. Had not seen it before, and that at least partially answers your question...the more times something is posted on the internet, the easier it is to find.

Another advantage of downloading the spreadsheet from Go Wood (Penn State WoodPro) over all the online calculators, is that the user has the freedom to create his or her own gradient range data (as I've done from 65/60 to 65/30 in the spreadsheet template) and the resulting tables and graphs right in the spreadsheet, and then export those data/graphs to another package (such as PowerPoint) or print them out. A little more flexibility in case one is studying a specific problem.