Summer and Winter Air Drying of Lumber

How fast can lumber dry in winter, as compared to summer? January 28, 2014

From Carl Hagstrom, Systems Administrator at WOODWEB
Gene - over the years you've provided extensive information regarding air drying. There is one area I'm not entirely clear on, and I'm hoping you might be able to clear things up for me. You've mentioned that the rate of (air) drying depends, in part, on the temperature, with faster drying at higher ambient air temperatures, and slower drying at lower air temperatures. I'm trying to get a sense of a relative comparison between drying rates at, say, average temperatures in the teens (dead of winter) Vs. drying rates when average temps are in the 70-80's (peak summer). I can certainly understand that other factors are at play here - relative humidity and species, but if we considered all other factors to be the same, is it possible to quantify the comparative rate of drying in those two examples? In other words, would the drying rate be twice as fast in the summer then in the winter? Half again as fast?

On another somewhat related question, in a situation where lumber is sawn and will air dry for an extended period (six to ten months and no kiln drying), would one expect the drying loss to be less if the lumber were sawn in the fall and allowed to air dry at a slower rate through the winter, rather than sawn in late spring, and allowed to dry more quickly through the summer?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
The rate of drying does indeed relate to the quality of drying. So, hot summer conditions can result in more loss due to checking than in the fall, winter and spring. On the other hand, it is not unusual to find days with low Rhs and high wind in the spring, so there can be quality loss then too. In fact, I have seen beech lumber check in the winter. So, the answer to your second question is not a clear cut answer, but temperature is certainly a key consideration. Incidentally, rewetting partially dry lumber can make existing small checks worsen, so again, it is not 100% temperature related.

As a rough rule of thumb, for every 20 degrees F hotter, the wood will dry twice as fast. So, 50 degrees is 1/4 the rate of 90 degrees. Because the RH in the summer and the winter is the same (within a few points), the season RH change is not going to affect drying. However, in addition to the average RH, we also want to know the extremes. We often get some very dry days in the springtime, even though the average is the same as the summer and winter values. It is during the extremes that we can damage the lumber and then the average conditions the following weeks or months will not repair the damage and could even make it worse. Once checked, it is easy to make checks worse with "normal" conditions.

From Contributor S:
I'll add a note on "why". Water molecules are constantly in motion. They bounce off each other. The warmer they are, the greater the motion and bounce. They can bounce off hard enough to break the bonds that were holding them with the liquid water. They then evaporate.

From Contributor G:

When the temperature reaches freezing for those of us in the northern parts - due to the expansion of the water molecules, does it drive the moisture out of the wood in a measurable amount? Will the moisture be reduced further by successive thawing and freezing cycles, if enough heat is applied?

From Gene Wengert, forum technical advisor:
No, the expansion when freezing does not cause a significant loss of water. In fact, all of the free water in the living tree has a natural antifreeze that keeps liquid from freezing solid in the tree until well below zero F. The bound water never freezes, as bound water is not a liquid, vapor or solid, but is chemically held water in the wood structure as a single molecule or a small group of molecules.

From Contributor G:
In some research I found that hydroxyl may be the chemical you are referring to, which is a single Hydrogen and a single Oxygen bound in a covalent bond. Given water is H2O it seems to have derived itself from water. The research I have read suggests it is not stable. I have seen demonstrations of water freezing by the simple introduction of motion. In all other forms of drying how is this "chemical" removed from the wood? Or is it bound in the wood forever?

From Gene Wengert, forum technical advisor:
Actually, the water molecule is still H2O. It is held within the wall by hydrogen bonding. This bonding requires a little extra energy to break, so that is why bound water takes a little more energy for evaporation.

From contributor K:
I have read some old adventure stories from the far North, where they claimed on some really cold days certain trees would suddenly split open so quickly that they rang out like gun-shots. I have to admit I wasn't convinced, but tried to reason that maybe the air temp, along with the sun hitting a fully exposed trunk might heat the surface, along with the boundary level of air to cause enough evaporation over a period to cause that. Is there any evidence supporting that? Would it be like freeze-drying as used in food preservation? Could that be what it at play here?

From Gene Wengert, forum technical advisor:
As I mentioned the liquid water in the tree exists in two forms - bound water and free water. The free water is in the cell openings and not in the cell wall. This free water has natural antifreeze in it, but when the temperature gets way below zero, this free water can begin to freeze and turn into slush. If it gets cold enough that free water can become solid and as it expands and can crack the tree. Frost cracks are indeed well known. I am not certain about the role of sun heating, but it does make some sense. Note that commercial freeze drying of food is dome at cold temperatures and with a vacuum.

If all the free water is evaporated, but the bound water is present, the wood will be at 28% - 30% MC about. Some tropical wood will be as low as 22% MC. From a technical point, when a fiber has lost its free water but none of its bound water, it is said to be at the fiber saturation point. However, a fiber will seldom be at this exact point, as it will be losing both free and bound water, and not only free water as it dries. The same is true for a piece of lumber. Due to moisture gradients in the wood the outside fibers begin to lose moisture first, and they will eventually experience bound water loss even though the core has lots of free water. So a piece of lumber is never at the fiber saturation point. Individual cells may be at the fsp, but not a piece of lumber. So, anyone who talks about drying lumber reaching the fsp when the lumber is at 28% MC is incorrect. Incidentally, when bound water leaves a fiber, or cell, the cell wall shrinks.

From Contributor G:
This natural antifreeze exists while the tree is alive I assume? As once the tree is cut, the wood seems to freeze at 0 C or 32 F? I have heard many sawyers comment that sawing frozen wood is a dream, and splitting firewood when frozen is much easier. I am just wondering how quickly the antifreeze leaves the wood.

From contributor J:
"As a rough rule of thumb, for every 20 degrees F hotter, the wood will dry twice as fast. So, 50 degrees is 1/4 the rate of 90 degrees."

Does this pattern hold even at temps below freezing? Is the drying rate at 20F roughly half the rate at 40F?

From Gene Wengert, forum technical advisor:
I am not sure about below freezing, but I suspect that if there is ice and snow that must be evaporated, drying would be a bit slower