Why Does Wood Hold Moisture?

7/1/2021 (Permalink)

A fundamental fact is that wood is hygroscopic. This means that wood, almost like a sponge, will gain or lose moisture from the air based upon the conditions of the surrounding environment.

But not only does wood gain or lose moisture, but it will also expand or contract according to the magnitude of such changes; and it is this swelling and shrinking in finished wood products—often referred to as the wood’s movement in service—that is responsible for so much mischief and so many malfunctions in woodworking.

When a tree is first felled, it is considered to be in the green state, and contains a very large amount of moisture. This moisture exists in two different forms: as free water that is contained as liquid in the pores or vessels of the wood itself, and as bound water that is trapped within the cell walls.

Once a fresh log or piece of lumber is cut and exposed to the air, it will immediately begin losing free water. At this point, the wood does not contract or otherwise change in dimension since the fibers are still completely saturated with bound water. It is only once all the free water has been lost that the wood will reach what is called the fiber saturation point, or simply FSP.

Below the FSP, the wood will then begin to lose moisture in the form of bound water, and an accompanying reduction in the wood’s volume will occur. At this point, the wood is no longer considered to be in the green state, but is now in a state of drying.

Just how much bound moisture is lost during the drying phase will ultimately depend upon the temperature and relative humidity (RH) of the surrounding air. At 100%?rh, no bound water will be lost. At 0% RH, all the bound water in the wood will be lost, a condition known as ovendry—so-called because a kiln or oven is typically required to completely drive out all moisture.

The amount of water in a given piece of wood is expressed as a percentage of the weight of the water as compared to its ovendry weight. Some species of trees, when they are initially felled, may contain more water by weight than actual wood fiber, resulting in a moisture content (MC) over 100%.