Kiln drying

WOOD WARPING NIGHTMARES.........AND WHY THE WOOD FOR OUR PRE-FABRICATED WOODEN HOUSES IS THEREFORE KILN DRIED (OVEN DRIED).

Improperly dried or stored wood may suffer from cracks and deformations due to shrinkage, while it may twist, bow, crook or warp caused by a combination of shrinkage and swelling. This is caused by the moisture content (MC) of the wood. The higher this content the greater the chance that one or more of these negative effects will occur. In addition wood taken from the outer rings of the log (called sapwood) is more sensitive to distortion than wood taken from the core of the log (heart wood). A moisture content can be higher than 100% !!  See note at the end of this page. 

ALERT
Most countries require that at least the boards shall be kiln dried to a moisture percentage of maximum 15%. in such case our company needs to issue a certificate proving that kiln drying has been applied indeed. Without this certificate you may be unable to import the wood. We strongly advise you to contact the local authorities to verify this requirement (normally the ministry of Forestry or Agriculture). Nonetheless our company will automatically issue a kiln drying certificate, whether or not required by the authorities.   

Related article: <acclimatisation of hardwood>

The moisture content is measured as the ratio of the weight of the free water in a given piece of wood to the weight of the wood when it is completely dry (oven dried, or kiln dried) and is usually expressed as a percentage. The "green" wood of a freshly felled tree may have an MC anywhere in the range 30% to over 200%, depending on the species (hardwood (1) is far less receptive for a high MC than soft wood). Click on the picture for an enlargement

Cracks and warping will occur in the saturation zone (between 30% and 100%, shown in red in the graph above), NOT in the bound water zone (yellow), provided the wood is properly dried in an autoclave (kiln dried), a process that may take 4 to 7 weeks, depending on the type of wood. Air drying, however, is largely preferred over kiln drying. The set back is that such will take significantly longer, a process which we often use during the progress of prefabrication when a until takes months to built.    
Furthermore, wood is a cellulose material which behaves somewhat like a sponge, so that even wood which has been kiln dried down to say 7% may in fact later reabsorb water from the atmosphere so as to reach an equilibrium state. Actually all wood is constantly gaining or losing water to or from the environment, in other words, the moisture content of wood changes as the relative humidity changes. Coats of varnish or paint can slow the process but cannot stop it.
As the moisture content of improperly dried wood (or green wood) changes, so does the wood expand or contract, potentially producing all manner of disastrous defects (wood warping and wood cracking nightmares).
Most of the wood for our bungalows and cottages is kiln dried to an MC of some 12%. When arriving at destination this MC may have increased to a maximum of 20% depending on the humidity of the environment. This MC percentage of 20% is still 10% lower than saturation point, thus in the bound water (technically dry) zone and in conclusion safe against cracking and warping.

NOTE: When building in an seasonal environment interspersed with low humidity and low temperatures in winter and higher humidity and high temperatures in summer, all boards must have an MC of 10%, whereas beams and columns shall be dried up to 20 % to 30%. When this is not achieved boards will crack and warp, while beams and columns will crack and/or deform. However, the set back is that a number of hardwoods like Bangkirai and Merbau cannot be kiln dried to a lower percentage than 12%. Lower percentages wil cause natural cracking. The remedy is air drying, but this will take much longer, up to 4 months < read more >  

NOTE: The degree of wetness in wood is called moisture content, and it's expressed as a percentage. But that percentage often can exceed 100 because it represents the ratio of the weight of the water in a piece of wood to the weight of the same wood when it is completely dry. For example, a piece of green wood weighs 22 kg.; dry, it weighs 9 lbs. That means that the green wood contained 13 kg. of moisture, or 13/9 th. of its dry weight was water. As a percentage (13 divided by 9 equals 1.44), that's a moisture content of 144 percent!