We recently visited a company which is involved in the drying of wood, and learned a bit about wood drying. This company had bought a temperature and humidity logger for monitoring their drying environment.

Timber Drying in General

Wood is probably one of the oldest building materials on the planet. But before wood can be used as a construction material, whether it for structural support in a building or to manufacture furniture, it has to undergo treatment to gain the required properties defined by the application in which the wood is used. The first and most important treatment is the drying process.

A timber frame for a barn.

The fastest and most effective way to drying timber is in a Kiln. Kiln drying is done in a closed chamber in which air temperature, relative humidity and airflow can be controlled to dry timber to a specified moisture content. The temperature for the drying is usually between 40-90°C depending on type, size and the intended use of the timber. There are many different types of kilns such as vacuum systems, traditional heat and vent type kilns and radio frequency dryers. The cost of installing and maintaining a kiln may often be prohibitive unless a large amount of timber can be processed. However, if the value of specific species is high enough, it becomes more feasible to kiln dry green timber.

Wood in a drying kiln.

Some other drying options timber include: Solar drying where the green timber gets put into a glass house. This option is more often used for drying small amounts of timber. For bigger amounts the Air drying option tends to be used more often. Both drying options are only controllable to a very limited extend since they strongly depend on weather conditions.

Facts & figures:

One cubic metre of freshly felled oak contains approximately 540 litres of water.

Examples for air drying times:

Softwoods: 25mm thick Scots pine that is stacked in April can reach 20 % moisture content by July to August if the summer months are warm and dry.

Hardwoods: 25mm thick English oak if piled in early autumn can reach 20 % moisture content in about 10 months.

A 75mm thick log of wood can even take 3 years to reach equilibrium moisture content.

Why the need to measure humidity?

Controlling humidity during the timber drying process is essential for many factors. An incorrect level of % Equilibrium Relative humidity (ERH) in wood can have the following effects on product and process:

When damp, wood is easily damaged.

Dimensional changes

A controlled drying process prevents the timber from unacceptable shrinkage after the installation. But since wood is a natural hygroscopic product it will always change its size to a minor extend.

Strength

Drying the timber below a water contents of 25 % to 30 % will maximise the mechanical strength. dry wood is nearly twice as strong and twice as stiff as green wood.

As moisture content of wood decreases, the strength increases.

Decay

After drying, timber maintaining less than 20 % moisture content is unlikely to be attacked by wood decaying fungus.

Preservation

To increase the effectiveness of preservative treatments. Many preservatives should only be applied when the humidity of the timber has been reduced.

Corrosion

Drying timber prevents the corrosion of metal fixings such as  nails and screws.

When wood is wet, it may corrode metal fittings.

Weight

Dry wood is much lighter in weight than wet wood. For many species, dry wood is nearly half the weight of wet wood.

Philip Robinson                                                                                                           Rotronic Uk