It’s all about energy

Let's start with a sealed chamber containing only a beaker of water. Some of the water will evaporate to water vapour. To keep things simple we will assume that there is no temperature change during this thought-experiment. Water will continue to evaporate until the volume of the chamber is saturated, ie 100% relative humidity. This phenomenon is due to the second law of thermodynamics, which says that the entropy of a closed system increases to a maximum at a given set of conditions. Entropy is a measure of randomness.

Water vapour is more 'random' than liquid water because the water molecules in the vapour state are more mobile. As a consequence, the water vapour also has more energy than the liquid water and it is this energy level which is important with regard to wood.

Now let's introduce a dry piece of wood into the chamber, next to the beaker of water. We all know that the moisture content of the wood will increase; eventually, it will obtain an equilibrium moisture content of around 32%. In other words a new stasis will develop in the chamber; the air will still have 100% humidity, the wood a moisture content of 32% and the beaker will have slightly less liquid water in it than before. Since the wood is not in contact with the water in the beaker, there must be some reason why the water vapour in the air is attracted to the wood.

The reason is energy level. All things, especially students(!), try to attain a low energy state. The water vapour molecules have high energy relative to the water in the beaker, so if they attach themselves to the hydroxyl (-OH) groups in wood, through hydrogen bonding, then they will have a lower energy state (see above). As they do so, a small amount of heat is released. In fact, the 'heat of wetting' can be observed with a very accurate bomb calorimeter. Therefore, the water vapour molecules will tend to saturate the hydroxyls in the wood, of which there are a limited number, and so the maximum moisture content of wood is linked to the number of hydroxyl groups present.

Therefore, the moisture content of wood has nothing to do with the fact that it is a porous material, or natural. Rather, it is a question of energy levels. If you really understand the interactions between water and wood, then you can call yourself a wood scientist