Air Dried or Kiln Dried Oak?

First of all, let’s take a look at why the drying process is so important and what it involves…

■ The Drying Process

Seasoning or drying of timber refers to the controlled process of reducing the moisture content to the point that is stable and suited for its intended use and final environment.

Oak is a timber that dries slowly and is prone to developing splits and checks (very slight splits) during the drying process, as well as shrinking (as most timbers do) as it dries out. Shrinkage is usually allowed for in the initial sawing process by cutting it oversize, and splitting is minimised by ensuring the environment remains correct throughout the process so that the moisture content does not reduce too quickly. The stacking and storage of the timber during the process is also critical to the final shape and condition of the timber.

The duration of the drying period and the final moisture content achieved, varies according to the thickness of the timber (see table below). For natural seasoning, the rule of thumb is that oak needs to dry ‘in stick’ for one year per 25mm of thickness. Today, this is backed up by regular testing of the moisture content throughout the process.

■ Air Dried (AD) | Evara

Air dried (Evara) oak is seasoned naturally in a well-ventilated outdoor environment, typically under cover such as an open-sided barn. Sticks are placed between the boards to allow air to circulate. Because natural seasoning offers less control than kilning, it takes longer and the boards are slightly more prone to surface checks.

At one year per 25mm of thickness, this process brings the moisture content down to 16–23%, depending on the thickness of the board. The result is a well-balanced, durable material that is particularly suited to external applications such as gates, doors, cladding, balustrades and decking.

■ Kiln Dried (KD) | Aura

Kiln dried (Aura) oak undergoes a controlled drying process in a closed chamber, where heat, humidity and airflow are carefully regulated to reduce the timber’s moisture content.

Before kilning, all oak must first undergo a period of natural seasoning to slowly reduce its moisture content to 20–30%, depending on the thickness of the boards. Kilning must then be carried out carefully, because drying oak too quickly can cause internal stresses and splitting.

After kilning, the oak will be at 10–15% moisture content. This makes it suitable for dry and heated internal environments, including staircases, furniture and fitted internal joinery.

■ What happens when Air Dried is used inside or Kiln Dried outside?

Where the timber is not at a suitable moisture content for the environment it’s placed in, it will often react: either shrinking (drying out further), expanding (taking in moisture) or just slightly changing shape in situ.

Depending on the extent, movement can have major consequences to the point that seals breakdown or joints pull or are pushed apart (blow), causing joinery to completely fail. In most cases it is only slight but can still make a finished piece of joinery very unsightly with open/uneven joints.

So it’s worth getting it right!

■ An internal and external face?

For completely internal or external joinery it is an obvious decision: for external joinery such as gates, you should use air dried; for internal joinery such as a staircase you must use kiln dried. But where the joinery has an internal and external face it gets harder to define. It’s a judgement you have to make. Ask yourself questions about the property:

How dry is the interior likely to be? How constantly/well heated is it?

For example, a well-insulated, efficiently heated house will likely have a low humidity level and the timber must be well suited to this (KD), whereas for an old church or outbuilding that is well ventilated, the humidity level is likely to be closer to an outdoor environment, so AD will suit better.

You must equally think about the outside face:

How exposed are the outside faces to the elements? Is the property particularly subject to heavy or changing weather? If so, air dried is likely to react less than Kiln Dried, being at a natural moisture content to the outdoors.

Some situations allow you to take advantage of both - for instance a south facing door straight into a heated interior - it may be best to make the frame from kiln dried and the boards from air dried.

Please note - this article focuses on joinery. For beams and structural frames, fresh sawn oak (Verde oak) is commonly used. It shrinks as it seasons in situ, by an average of 6% in width and thickness, and this shrinkage is used to tighten the joints of an oak frame. Where greater dimensional stability or a naturally aged look is wanted, seasoned beams (Vieux oak) that have been air dried over one to three years are an alternative.

We hear about many varied situations and sometimes there is not an obvious answer, but being aware of the risk and if you consider the factors above, you are very likely to come to the right conclusion.

If you'd like to discuss a project or situation then we'll be glad to talk it through with you. We're here to support!

(Sources: Geoffs Woodwork, TRADA)