What are the advantages of air dried vs. kiln dried oak and how do you choose between them? The simple answer is, it depends on your project. However, there are a few factors that should be taken into account when deciding on which one.
Oak timber used for construction comes in two forms – green or dried. Green oak is sometimes misleadingly referred to as ‘wet’ oak. Oak naturally has a low moisture content compared with other timber. In this case ‘wet’ is used to indicate the low moisture content of dried, or seasoned oak. Green oak will naturally dry over time, shrinking together to form a tight, stable structure. However, sometimes a project makes this undesirable and the initial construction must remain unchanged over time.
Oak is naturally insect resistant, but some species such as the Ambrosia Beetle can still be a problem. Once the moisture level drops below 30%, they will die and reinfestation will not occur. Dried oak will also not react with metal components abutting it due to the low moisture levels in the timber. This could otherwise cause corrosion and staining.
Before considering the strengths of the two types, it is worth looking at the process of seasoning oak.
A traditional form of oak drying used for centuries. In air drying the timber is stacked, covered, in an area exposed to the environment such as a barn or open warehouse. The stacks are arranged so air can flow easily around them, allowing the moisture to naturally evaporate. As the seasons pass, the timber expands and contracts with the temperature and moisture variations in the environment. The drying takes one year per 25mm of thickness and should take a minimum of three years. Once finished, the timber produced will have a moisture content of 20% – 30%.
In kiln drying, the timber is stacked in a sealed unit and warm air circulated mechanically around it. The moisture emitted is drawn away and vented, speeding up the process significantly. This shortens the drying process to one week per 25mm and results in a moisture content of only 8% – 10%. Kiln drying should be controlled carefully to avoid placing undue stress on the substructure of the wood as it dries quickly. This can cause weakness in the core of the timber that will not be externally visible.
Strengths and Weaknesses
1. Cost and availability
Both air dried and kiln dried oak can be more expensive than green oak due to the processes involved in producing them. Due to the time invested in air drying, the timber will not be available for purchase for quite some time. This can lead to limited availability as once stocks are depleted they can only be replaced when the next batch is ready. While Kiln dried timber will be available in a shorter space of time, the industrial processes involved will naturally lead to a higher price. It is also worth bearing in mind that dried oak is pre-cut before drying, so may not be available in larger lengths or thicknesses.
Green oak will naturally age over time, changing its appearance as it silvers and develops natural splits and cracks. Dried oak will remain virtually unchanged over time. For restoration projects or where a more aged, weathered look is preferred, Air dried oak is perfect. As it has naturally dried, it is effectively ‘pre–aged’ and will complement the existing woodwork perfectly. For a more modern, clean look, kiln dried oak is preferable. it will change little over time as the low moisture content and hardening of the wood prevent environmental factors affecting it. This will preserve the clean, straight lines and ensures the look remains unchanged over time.
3. Ease of Use
Dried oak will always be harder to work with due to its increased hardness. There are a couple of factors to consider for kiln dried vs. air dried lumber for furniture and similar projects. Kiln dried is virtually guaranteed not to warp or twist after the piece is finished, especially for interior fittings. However, it is hard to turn the wood on lathes as the blades will struggle and quickly become blunt. Air dried oak is far easier to turn and causes less wear and tear on tools and is always better for exterior furniture as the elements will not affect it as much. Due to its extreme dryness, kiln dried timber can react to water based glues used in furniture construction. This can cause swelling and damage joints and must be taken into consideration or other methods such as pegging used.
4. Interior and Exterior Use
This is the main factor to consider when assessing the suitability of air dried or kiln dried oak for the job in hand. The type of timber used must have a similar moisture level to the environment it is used in, otherwise it will absorb water, leading to swelling, or dry out further causing shrinkage. This can seriously affect the structural integrity and stability of structures. Joints can expand and split or shrink and become loose with possibly dangerous consequences. At best, this can lead to an ugly, unsightly end product and a waste of effort and valuable materials. Air dried oak has already been exposed to the elements for an extended period of time. Its internal moisture level is balanced with the natural humidity levels outdoors and will not be affected by their fluctuations. This makes it the ideal choice for external features such as oak gate posts, gazebos, or external cladding with oak boards.
For internal uses, kiln dried oak is usually the best choice as its low moisture content suits the lower humidity of the environment. Green oak is usually used for oak trusses or structural oak beams due the larger sizes of timber available and the lower cost. However, for elements such as floor joists and oak flooring, kiln dried oak is preferable. There is little or no chance of shrinkage and the increased strength and load bearing capacity mean they should never need replacing. It is also perfect for features such as decorative oak beams as it will not shrink or split, damaging their appearance. It is also possible to combine both materials in the same structure. For example, an inner wall of oak posts and cladding can be combined with external cladding of air dried oak. This allows a completely oak built building to withstand the elements while preserving a beautiful, hardwearing interior.
Hardwoods Group are established kiln dried and air dried oak suppliers with a wide range in stock. For advice on which of these superior materials is most suited for your current project or for any trade enquiries, contact us today.
Properties of Oak Wood
What is Green Oak