Will Stacked Lumber Gain or Lose Moisture?

Lumber in storage may gain or lose moisture, but very slowly. Wrapping the pile in plastic will slow or stop the moisture change. March 29, 2006

Do I have to be concerned with lumber deadstacked in an unheated garage absorbing moisture from the air? I don't know if this is an issue in the winter in central Wisconsin. The lumber is soft maple, and at 6% now.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
I would think the lumber would lose moisture throughout the winter. The air is much drier in the wintertime here in the South. We have a humidification system in our factory. You wouldn't know it in the summertime, but come winter, it looks like a rainforest.

From Professor Gene Wengert, forum technical advisor:
The outside air in almost the entire USA and Canada averages about 65% RH, which is 12% EMC. It is not true that the outside unheated air is drier in the wintertime. However, when this moist air is heated, then the RH is lowered, which accounts for the low RH and EMC in the wintertime. As your storage is unheated, the wood will try to achieve about 12% EMC. Because of the temperature, any changes will be very slow. You probably will see no change at all in any pieces except at the ends and the top and bottom layers for 3 months. Is it too late to wrap the entire pile with plastic, including the bottom? If the wood is enclosed, then no moisture can enter or leave, and therefore the MC will remain constant.

From contributor C:
Over the years we have successfully used 6 mil polyethylene film as a vapor retarder for this purpose. As Gene says, "including the bottom." Many people do not think of the bottom.

From contributor R:
Doc, is there an easy way to wrap an entire unit of lumber in plastic so that no air enters or escapes? Will this procedure work in the Arizona desert to help keep wood from over-drying?

From Professor Gene Wengert:
I do not know what you mean. Put down a piece of plastic, put the lumber on top of it. Put another piece on top and then hopefully the excess on the sides and edges will allow you to completely cover the edges and tape the top and bottom pieces together to make it fairly airtight. It takes 4 gallons of water per 1000 bf to change the moisture by 1% MC, so the plastic does not have to be perfectly sealed. This idea is temporary storage.

From contributor C:
For whole units, we precut the poly, roll the edge on the side to be forklifted from like a blueprint; the other side can just lay flat. Once we set the lumber on the poly with a forklift, we unroll one side, lap over with the other side, then do the ends like wrapping a present. The problem then is if you have to move the unit with a forklift, you can tear the poly easily, so we sometimes lay the poly on a pallet the size of the unit for easy moving.

From contributor R:
Thanks for the tip. Makes sense. Doc, my concern is that in Arizona, the RH can be as low as 10%. I've gotten MC's on 6/4 cherry as low as 4%. I was asking if wrapping the unit in plastic when the wood is at 6-8 % would keep the wood at that constant level.

From Professor Gene Wengert, forum technical advisor:
It takes 4 gallons per 1000 BF to change the moisture by 1% MC and this is the same whether you are gaining or losing. So, yes, it will work if the environment is too wet or too dry.