Relative Humidity in the Wood Storage Area

Advice on what humidity level to maintain where rough lumber is stored, and how to accomplish that. September 23, 2008

Im setting up lumber racks in a new location and I am concerned about the humidity level in the room. In my regular lumber rack location the humidity stays pretty consistent at 50%. In the new location it has been between 55% and 60%. Does anyone have any thoughts on the moisture content effect of the 10%+ increase in humidity?

In the main lumber racks poplar, etc. typically measure at around 9% to 11% moisture content after weeks or months of storage. In time I'll find out what the moisture content turns out to be in the new rack location, but it would be nice to hear what others have to say about this. I have a dehumidifier I can run in new rack space if needed.

Forum Responses
(Architectural Woodworking Forum)
From contributor C:
I've included a link below that will display the search page in our Knowledge Base. If you search for "MC Relative Humidity" you'll find lots of good reading on this topic (as you'll see). I quickly scanned a couple articles, and found what is likely the salient info you're looking for (shown below). Hope this helps.

58-64 RH -11%
52-58 RH -10%MC
46-52 RH - 9% MC
39 -46 RH - 8% MC
32 -39 RH - 7% MC
25 -32 RH - 6% MC

19-25 RH - 5% MC

Relative Humidity

From contributor B:
We ran an air conditioner controlled by a Honeywell Humidity Control Unit. We set it at 50 percent since 35 or 40 would be too hard to do with the door opening for deliveries. The unit hung from the ceiling (22 high) and distributed the air through a 3 diameter sock about 60 long. Since the wood was stored in the same room as the moulder, we enjoyed the temperature benefit was well. On a 95 degree day the shop might get down to 80 but with 50% humidity it felt great. We put two big pleated air filters with pre filters on the a/c unit and let the fan run all day to act as an air cleaner. We did not notice much of a difference in the electric bill (about 800 a month) from summer to winter.

From Gene Wengert, forum technical advisor:
The previous responses are perfect. Let me add that if the lumber is kiln-dried to a different MC than the EMC of the room, you will effectively be drying or adding moisture back to the lumber in storage. This will set up gradients and could cause some warping later on.

As most homes and offices run about 38% RH (30% RH wintertime; 50% RH summertime), many people would target that same level for lumber storage so that the MC does not change after manufacturing.