Humidity, Moisture Content, and Lumber Storage

Lumber moisture content equilibrates to ambient relative humidity. Here are details. December 9, 2010

We dry our lumber to 6-8% and we are now storing it in a barn that can be cooled and or heated. I need to know what temperature we should keep the area in the summer and in the winter to keep the moisture the same so that it doesnít regain. We are located in Canada.

Forum Responses
(Sawing and Drying Forum)
From contributor T:
I store my kiln dried lumber in the upstairs of a barn. Store the lumber in a tight pile. As long as the roof is good on the building so no rain or snow gets on it you won't have a problem. My shed is not heated, and gets really hot on a hot day in the summer. I am in Canada as well.

From contributor Z:
Try to keep the barn with a relative humidity around 35%. That will keep your wood at 6% MC. If you wood is in a separate location from your work area it's probably less expensive to heat the lumber area even in the summer. Look at heating the area about 15-20 degrees F above the morning's low temperature.

From contributor F:
After the lumber is dry and deadpiled it should be fine unheated, as long as it stays dry. Most wholesalers that have millions of board feet in stock have at least part of the inventory in unheated buildings. You will need to get it acclimated to your shop if the lumber is real cold, especially if you are gluing or finishing.

From contributor S:
I've stored my own kiln dried lumber in an unheated but dry storage/sales area. A customer wanted his selections checked with a moisture meter and the surface showed about 10-12%, but when we crosscut a board the inside was 6-7%. Itís hot and dry here at the end of the summer, so lumber that comes out of the kiln won't pick up much moisture then. This time of year itís very damp with cool temperatures and plenty of rain, so I would expect the lumber to be picking up more moisture. So your answer depends on your particular climate and how long you lumber is stored in that environment. Itís my understanding that given enough time, K/D lumber could get to 12% even in the center of the boards.

From the original questioner:
Thanks a lot everyone. So this lumber I have is dead piled and going to be put into a storage building with a roof and walls. I was just wondering what temp and relative humidity this room needs to be in summer and winter to keep the lumber from gaining any moisture. If it doesnít matter since it is dead piled, then great, no need to heat or cool this room. This is also going to be holding about 50,000fbm.

From Carl Hagstrom, Systems Administrator at WOODWEB
This type of question is one of the most often asked at our Forums. The Moisture Content (MC) of your lumber is *directly* related to the humidity of the air where you store it, and will acclimate to room's humidity level. This is referred to as the Equilibrium Moisture Content (EMC). The chart below shows the relationship. You should search our Knowledge Base on "EMC", and you will find a chest-full of information on this. The Wood Doc may also chime in if we're lucky.

Also - the wood on the outside of the pile will be affected sooner than the wood within the pile. Gene has also commented many times that if you wrap your lumber tightly in plastic sheeting (non-permeable), there will be no exchange of moisture. Do a search, and read up - it's all explained in our Knowledge Base

Bottom line, though, is that lumber stored in an unheated building will eventually acclimate to a MC of around 10-11%, depending on what part of the country you're in.

19 to 25% RH = 5% EMC
25 to 32% RH = 6% EMC
32 to 39% RH = 7% EMC
39 to 46% RH = 8% EMC
46 to 52% RH = 9% EMC
57% RH = 10% EMC
65% RH = 12% EMC
74% RH = 14% EMC
80% RH = 16% EMC

Knowledge Base search results for EMC

From Gene Wengert, forum technical advisor:
Carl has indeed stated the facts correctly. Temperature is not a factor. It is only RH that matters. For most hardwood lumber, I like 32% RH storage (6.5% EMC). Tight piled bundles will not pick up moisture as quickly as a bundle on sticks, but the outside pieces of a tight piled bundle will quickly change MC if the RH is not at the correct level. After six months, the inside of a bundle will change MC if the RH is wrong. However, the core MC will not change MC very quickly at all, so checking the core MC will almost always give you the MC when the lumber left the kiln even if storage conditions are wrong. In a storage building, you can add heat to lower the RH and reduce heat to increase the RH.

A load wrapped tightly in plastic will not gain or lose any moisture because no moisture can get into or out of the package. Likewise, a shipping container that is fairly airtight will not allow any moisture into or out of the container. Note that even if the container is loaded on a high humidity day, the moisture in the air inside a container is so small that it will not affect the MC of the lumber.