How Fast Does Kiln-Dried Oak Regain Moisture?

      Advice on how to measure the increase in moisture content of wood stored in a covered shed after kiln-drying. June 13, 2014

How many days will it take for 4/4 red oak on sticks dried to 7% MC to reach an EMC of 12% in a shed with open ends and half sides?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
Do you mean how long for the ends of the lumber, or for the average of the entire piece to reach 12% MC? What is the outside condition (average RH)? In most of the US it is about 65% RH average, summer and winter, so that is 12% EMC, so KD lumber will never reach 12% MC but will come closer to 11% MC. The ends might reach 11% MC in a few weeks (give or take) of warm weather (at 65% average outside RH); the core might take a year. (Note: The lumber will reach 11% MC at an EMC of 12%. That is, the EMC refers to air and not to lumber.)

From the original questioner:
It's about 65 to70% RH. The reason I ask is the past two charges of red oak were dried to 7% MC, put in the shed for about a week and then graded. The lumber was put back in a closed shed for a day or two. When it was tallied to be shipped the MC was running from 8 to 12%. The lumber was cooled before checking MC and the same moisture meter was used both times.

From the original questioner:
If you use a pin-type meter and use insulated needles then you can measure the core MC by driving the pins halfway into the lumber (thickness-wise). The core MC does not change for many months, so it will be the MC when the lumber left the kiln. If the test is done at room temperature, then the meter will be accurate to within 1/2% MC. From your description, I doubt that the lumber regained that much MC in storage. Rather, it looks like your sampling did not get these wetter pieces of wood and so the lumber was pulled too soon.

From contributor P:
I am currently in the process of testing for moisture regain. We have taken four samples of different species of lumber; 5/4 red oak, 4/4 South American mahogany, 8/4 poplar and 8/4 Spanish cedar (cedro). We took freshly dried kiln samples 48" long and placed them in our storage facility (closed sheds with no climate control. The red oak regained 1.3% moisture in a matter of 30 days.

From Gene Wengert, forum technical advisor:
A similar study was done for the Midwest Wood Seasoning Asociation ten years or more ago. I do recall that after a month the average gain in red oak 4/4 was fairly small, as you have noted. Are you checking shell and core as well as the average weight gain? Are your samples end coated? If so, and if it is still possible, you might use a 24" long piece without end coating. This will show the effect of exposed end grain. Often the lumber will have a higher MC only at the end of the piece, while the average MC is still close to the original value, and this high MC end will go into the plant and create problems with a small number of pieces. In fact, on a tight piled pack, we found almost all the weight gain was due to gain in the ends.

From contributor P:
To answer your question we did do shell and core and normal wafers to determine our MC. After 90 days we plan to dissect the board every inch and do a wafer than shell and core. We also took another sample of material that had a predetermined time in storage and also dissected that at intervals and found most of the regain happened at the edges of the board. On a side note, the 8/4 poplar has regained the fastest and the Spanish cedar the slowest. We determined the MC and weigh our samples periodically. We also note temperature, RH and EMC daily at the location of test samples. As far as increases I have noticed a decrease in red oak as well when we had some drier days.

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