Acclimatisation of Hardwood

All 1600-plus wood species know in the world have their own shrinkage/swelling characteristics and this shrinkage/swelling is dictated by moisture content changes in the wood. In particular boards (sidings , flooring and decking) are susceptible to swelling and shrinking. We kiln dry (oven dry) all of our boards to 12% Moisture Content as to avoid any cracking, deforming or warping, yet it may be possible that boards may swell or shrink somewhat after kiln drying due to the wood's hygroscopic nature. Our company developed a system to cope with this natural phenomena.      

Wood shrinkage or swelling occurs when the wood’s moisture content (MC) levels oscillate due to changing ambient relative humidity (RH) and temperature.  Any combination of RH and temperature will yield what is called an equilibrium moisture content (EMC).  The hygroscopic nature of wood is the reason it can change moisture content due to changing EMC: or simply said; The open cell structure of wood means it contains inherent pathways for moisture to enter and leave the wood. thus swelling at a high humidity and shrinking at low humidity.
This implies that the wood’s MC will always try to acclimate  according to its surrounding environment (EMC) and this process can go on for years.

Shrinkage and swelling due to changing EMC is especially relevant to the process of installing wood sidings, floors and decks. When it is delivered, the wood parts are moved from one environment (our factory) to another (your building site). let alone that the wood may sit for weeks in a closed container without any ventilation. Once delivered, the wood's MC levels can ultimately (given exposure and time) equal the EMC of the building site. This type of change to the wood's environment can cause boards to shrink or swell, which can manifest as small gaps or cracks between the boards, even when properly kiln dried at the source.

Why? Because the ambient EMC changes will eventually also cause internal changes to the wood. Wood dimensions change due to the fluctuating MC of the wood boards. When ambient EMC rises, wood absorbs moisture from the air. Thus, wood dimensions expand. When EMC declines, wood dries out, and the wood will shrink. The more extreme the shift in the environmental (ambient) EMC, the more noticeable the change in the wood parts will be. Small changes will typically go unnoticed, but drastic changes may mean some moisture related problems in relation to the environment where the house is built. This fundamental knowledge is so vital that it urged us to develop a flexible board connection system with specially designed sills where equilibrium cracks cannot be seen.

Finally, our company will issue a kiln drying certificate, mentioning the period of kiln drying and the final achieved moisture content.

The moisture content of wood is tied directly to the relative humidity of the surrounding air. The higher the relative humidity, the higher the MC of the wood. Period. If you’re installing wood that’s recently been transported, or installed on a job, it might take a little while for the material to reach its equilibrium moisture content (EMC) with the air—in other words, for the wood to accommodate to the humidity level for the climate around the wood: the wood may take on more moisture or it may dry out. For example, if wood at 10% MC is exposed to 25% RH, the wood will dry to 5% MC (and shrink as it dries).

The EMC helps us understand the response wood will have to relative humidity, whether it will shrink or expand. For wooden house prefab manucfactureres like us, the EMC is more helpful than RH. The simplified chart shown below shows the EMC values of wood when stored at the humidity and temperatures indicated.