In this article we compare common timbers used, to determine which may be considered the best timber for external doors and windows.
The timbers we will be comparing will be Western Red Cedar, New Guinea Rosewood and Accoya. We will be comparing them based on multiple factors such as stability, longevity and coatability.
The first character we will be comparing is hardness. This is an indicator of the surface durability of a timber and is measured using the Jenka system.
When comparing the three timbers we are investigating, we find that Western Red Cedar Typically has a low Janka hardness of 2kN, New Guinea Rosewood has a moderate Janka hardness of 5kN and Accoya has a moderate Janka hardness of 4.1 kN. This makes Rosewood the hardness option with Accoya close behind, whilst WR Cedar is quite soft. Due to it’s softness, Cedar may mark quite easily whilst Accoya and Rosewood will not mark as easily.
Estimated Service Life
The estimated service life of a timber is the timespan it is expected to perform it’s purpose to a satisfactory standard. Timbers are classed by their estimated Service Life with Class 1 being timbers with a service life of 40+ years, Class 2 being timbers with a service life of 15-40 years, Class 3 being timbers with a service life of 7-15 years and Class 4 being timbers with a service life of 0-7 years. These periods being based on outdoor use of the timber, above ground.
Both Western Red Cedar are Class 2 Timbers with an estimated service life of 15 – 40 years. Accoya on the other hand is a Class 1 timber with an estimated service life of 40+ years. Thus, out of the timbers investigated, Accoya has the longest Estimated Service Life.
When considering how suitable a timber is for painting, we need to look at multiple factors including how stable the timber is (Will it move and split the paint) and how likely it is to bleed tannins which may discolour the paint.
One way we can determine how well a timber will support a painted coat is by looking at the radial shrinkage of the timber. This is the amount the timber may move along the growth rings (Shrink and expand) which can result in the painted surface cracking. Rosewood has a radial shrinkage of 1%, Western Red Cedar has a radial shrinkage of 1.5% and Accoya has a radial shrinkage of 0.4%. This makes Accoya the most stable when it comes to shrinkage.
When we look at tannin and leaching, both Western Red Cedar and New Guinea Rosewood may stain the paint causing discolouration. For this reason they are best sealed and painted with an oil based paint. Accoya on the other hand, does not bleed or leach and so it can also be painted with water based paints.
With these factors considered, Accoya is the most suitable timber for painting.
Considering the above factors, we find that Western Red Cedar is the least hard, durable and suitable for painting of the three timbers investigated. Rosewood is the hardest, but is not the most suitable for painting and is only a Class 2 timber. Accoya proved to be the best for painting and is a class 1 timber which also boasts a hardness not far behind Rosewood. For these reasons, we conclude that Accoya is the best Timber for external doors and windows.
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