Also known as seasoned timber, air dried timber is the traditional form of timber drying. The timber is stacked in an open warehouse or barn that is open to the environment but sheltered from rain. The stacks are arranged so air can easily flow around them which allows the moisture to evaporate. The duration of the drying period varies according to the thickness and species of the timber. However, the rule of thumb is that each inch of timber will take a year to dry. Therefore, air drying is a process that will take years rather than months or weeks.
The timber is classed as ‘dry’ when the moisture content of the timber is in balance with the moisture content of the surrounding area. In the UK, this is usually 20% – 30%.
Kiln dried timber is the more modern method of drying timber. The timber is stacked in a sealed warehouse (not open to the environment) and warm air is circulated in this sealed space. The moisture emitted from the timber is drawn away, further speeding up the drying process. Kiln drying shortens the drying process to one week per inch, and the final moisture content will only be 8% – 10%, much lower than air drying.
1, As the timber is already at Equilibrium Moisture Content (EMC) where the moisture content of the timber is balanced with the moisture content of the air. This means the air-dried timber is already suited to being installed in outdoor environments.
2, As the timber has been dried slowly and more ‘naturally’, air dried timber is more stable.
3, Again, air drying is a natural process and required no energy input. Therefore, it is more environmentally friendly than kiln drying.
1, The drying time is much longer which can impact timber availability, especially in unusual shapes and sizes.
2, Typically more expensive than kiln drying because the drying time is much longer.
1, The process is much more controllable than air drying. For example, some environments will require timber with a moisture content on 10%, but anything less than 18% is difficult to achieve through air drying. Theoretically, kiln drying could reduce the moisture content down to 1%.
2, Due to the reduced drying time, kiln dried timber is less expensive and more readily available.
1, Kiln drying is often more suited to indoor environments and less suited to outdoor environments. This is because most kiln dried timber has an 8% – 10% moisture content, which is equal to indoor environments but too dry for outdoor uses. Therefore, kiln dried timber will probably move and warp if placed outside more than air dried timber would.
2, Kiln dried timber is less environmentally friendly.
3, Sometimes the rapid moisture content changes during the kiln drying process can place undue stress on the substructure of the wood, which can cause weakness in the core of the timber.
Appearance: Over time, all timber weathers to a silvery grey colour. Air dried timber will tend to have this silvery grey colour because of the long duration of drying, whereas kiln dried timber will have the appearance of fresh sawn timber. If the silvery grey look is preferred, this may be an advantage of air dried. However, it is worth noting that kiln dried timber will eventually go silvery grey over time.
Stability: The moisture level of air-dried timber is balanced with outdoor natural humidity levels and will not be affected by fluctuations as much as kiln dried timber, which makes it a good choice for outdoor furniture and fittings.