Moisture Regain in Dried Lumber
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Should lumber be end-painted after it's been dried? May 11, 2005
Question
Is there much of a difference between hardwood species in how quickly lumber will pick up moisture if exposed to high humidity? For example, will a red oak board and a maple board, at 7%, pick up moisture at the same rate if exposed to an 11% EMC?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Moisture regain is related to how easily vapor will move in the wood. This is related mostly to density. Hence, oak and maple would pick up moisture at about the same rate. The lower density woods might pick up a little faster. Actually, I did my PhD thesis about this subject, so if you want to get technical...
From the original questioner:
I asked this question because recently I have been trying to re-evaluate why we do some of the things we do at our lumberyard. Currently, for example, we end paint red oak after drying to prevent moisture regain, but we don't end paint maple or poplar. Should we be end painting more species or do we need to end paint at all?
From Gene Wengert, forum technical advisor:
As I understand it, you are end painting (Gem Paint?) dried lumber. If so, then this is only for beauty and not for moisture benefits. The end paint for moisture would likely cause finishing problems later. But the pickup of moisture by the ends of lumber is not as much of an issue as the gain through the faces. It would probably be cheaper to wrap the pile in plastic than to paint.
From the original questioner:
Yes, we are using Gem Paint, and I understand that this has been done somewhat for cosmetics but mainly to prevent moisture regain. Some customers want us to end paint and others do not want us to end paint our red oak.
Some of our maple regained 3% moisture after kiln drying. The maple sat over the summer in an enclosed storage shed, near the entrance, for approximately 6 months. Should we possibly end coat maple as well?
From Gene Wengert, forum technical advisor:
End coating will not stop moisture regain. The only thing that will stop regain is storing the wood in a dryer condition (lower RH).
