Air Seasoning of Timber Compared to Kiln Drying

Air seasoning and kiln drying are the two main methods used to dry timber. Air seasoning is inexpensive and requires little equipment, while kiln drying is much faster and can produce lower moisture contents. This page compares the advantages, disadvantages and typical drying times of both methods to help you decide which is most suitable.

The table below summarises the main differences between air seasoning and kiln drying.

Comparing Air Seasoning and Kiln Drying

Air seasoning and kiln drying each have advantages depending on the required moisture content, drying time and production costs.

Typical Drying Times

It is often the case that timber is first air seasoned (down to the minimum that can be obtained in this way) and then, if required, kiln dried (the following relates to drying 1 inch (2.54 cm) stock):-

Species	Time (days)
Oak	240 – 360
Beech	150 – 200
Spruces	45 – 90

... and then Kiln Dried (20 - 6%):-

Species	Time (days)
Oak	6 - 16
Beech	5 - 8
Spruces	3 - 5

This can be compared to Kiln Drying only (green to 6%):-

Species	Time (days)
Oak	20-30
Beech	12-15
Spruces	4-7

Table comparing the time taken (in days) to dry 1 inch (2.54 cm) stock. Note, it is usual to air season to 20% and then kiln dry down to 6%.

Advantages of Kiln Drying Compared with Air Seasoning

• A major benefit of kiln dried timber production is that kilns can dry to lower moisture contents than can be obtained by air seasoning – in fact we must use kilns in countries having cool, moist weather conditions (such as in the U.K.) for most indoor (heated) applications. In the UK the lowest moisture content achievable by air seasoning alone is about 18%.
• Kiln drying is much faster than air seasoning, so “Rush” orders can be met, and regular supplies can be ensured regardless of the season of the year.
• Air seasoning is at the mercy of the elements. Correctly controlled, the drying conditions produced and maintained in a kiln reduces losses of timber due to degradation (i.e. drying “degrade” (defects) is reduced).
• Another important advantage of kiln drying is that kilns can sterilise insect or fungus infected timber.The more rapid drying also reduces the propensity to bio-degrade.
• The temperature used in kilns sets or hardens gums and resins which might otherwise cause processing or finishing problems.
• Desirable, attractive colour changes may occur using kilns: for example it has been claimed steaming in kilns improves the colour of beech.
• Kiln dried timber can be easier to machine and provide better stability of products to moisture variations

Advantages of Air Seasoning

• Undesirable, unattractive colour changes may occur when using kilns.
• There is no chance that timber will suffer the expensive damage that can occur when kiln drying.

Disadvantages and Costs

• To realise the full benefits of kiln drying timber it is vital that the kilns are carefully controlled otherwise expensive financial losses can be incurred and the timber might be rendered valueless.
• Mistakes made when kiln drying timber can very rapidly have very expensive consequences: the stock can become damaged and command low prices.
• Kilns are very expensive (high initial investment). They also have high energy costs. Consequently they must be fully utilized – running 24 hours a day, 7 days a week.

However air seasoning also has its costs:-

• Capital is tied up in the necessary large stocks of slowly drying timber.
• The large stocks held must be insured. Fires can cause expensive loss of stock (N.B. use fire-breaks in drying yards to limit losses).
• The large yard will incur a purchase or rental cost.
• A lot of expensive stickers are required to stack large volumes of timber when air seasoning.

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