The timber of living trees and freshly felled logs contains a large amount of water. When wood is used as a construction material it will absorb or release moisture until it is in equilibrium with its surroundings. The equilibration process (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if the process occurs too rapidly. The process of equilibration needs to be controlled in order to prevent damage to the wood. There are two methods of controlling this process: air drying and kiln drying.
Air drying is the drying of timber by exposing it to the air. Timber is stacked with the boards separated by sticks on raised foundations in a cool, dry place. The rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged.
The main disadvantage of air drying is that all pieces of a given stack of timber will not necessarily dry to the same moisture content. Boards on the inside of the stack may have completely different moisture content to those on the outside. In other words, when air drying timber you don't have the same control that kiln drying offers. Another disadvantage of air dried wood is that is usually needs acclimatising when delivered to site to prevent further shrinkage once installed. Furthermore, air dried wood may not be sufficiently dry for a coating to be applied whereas kiln dried wood can be coated with a wood finish (paint or stain) immediately. There are dangers in air-drying timber as well. If allowed to dry out too quickly, case-hardening or honeycombing will ruin the structure of the wood.
Kiln drying can be thought of as controlled air drying. Modern dry kilns allow tight control of air flow, temperature and humidity so that all the timber dries uniformly and to the precise moisture content required. Kiln drying often means a more uniform product and the problems often associated with poorly air-dried timber are avoided. Kiln-dried timber can be machined to a superior finish than air-dried timber and therefore problems such as raised grain are virtually eliminated. Kiln dried timber can be installed with minimal acclimatisation on site and is also suitable for finishing immediately with a paint or stain which means that the wood can be protected without delay.
Clearly there are significant advantages gained by using kiln dried cladding and trim rather than air dried. It should be mentioned that there are still a number of traditional timber suppliers that successfully air dry certain species such as oak and elm, a process that can take many years to do properly. However in our experience kiln drying is a superior process, especially when dealing with premium cladding products such as Western Red Cedar and Siberian Larch. Only by drying these species in state of the art kilns prior to machining, can a superior quality finish and consistency be achieved. For this reason all of Silva Timber’s cladding and trim products are kiln dried at source by experts in the drying of each particular species.