What Makes Charred Wood Pest And Bug Resistant?

Charred Wood siding and fencing are uniquely durable among outdoor woods. While we have covered the fire-resistance of charred wood previously, an area we have not yet investigated is what makes charred wood pest and bug resistant.

The first thing we need to cover this that charred wood is not “bug proof.” It is however, bug resistant. Termites fed charred wood were shown to be less healthy than termites fed fresh pine shavings, although when given nothing else to eat, the termites still did eat the charred wood. The charring therefore, is a deterrent, but not a complete solution for termites and other wood pests.

There really isn’t one single solution for pests such as termites, other than just not using any wood in your construction. There are, however, many things you can do in the construction or remodeling of a building to help prevent pest damage, and using charred wood siding and fencing would be a part of that, but not the whole thing.

A thorough study of exactly why termites and other pests dislike charred wood has not been done, but we can speculate as to why the char layer is disliked by pests. Just like with fire resistance, the charring process drives off the lighter-weight and more volatile cellulosic compounds, leaving behind the blackened lignin which is harder and more stable. Like we explained in the fire resistance article, there are essentially two main components of wood – cellulosic compounds, and lignin. The cellulosic compounds in wood are more volatile, which means that they are less chemically stable, and therefore they are easier to break apart, releasing energy. The lignin is much more chemically stable, and therefore harder to digest.

The softer cellulosic components of wood do not necessarily contain more energy, but the energy is more easily accessible, so if the cellulosic compounds are largely vaporized and burned off during the charring process, then the wood is less nutritious to termites and other pests.

Additionally, while we will never know what tastes good to termites and other pests, the charcoal compounds left behind after the charring process are likely less tasty to pests. Numerous compounds formed during combustion are left in the charred layer, many of which are probably not particularly healthy to eat, even for termites.

Termite damaged cedar

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A Charred Wood Teahouse in Czechoslovakia

The Czech architecture firm A1Architects has really embraced charred wood siding, specifying it in many of their projects. They have done a beautiful job integrating and featuring this natural and striking material.

The siding they used has what we would call a “medium char” on it, with more of the browns and greys of the underlying wood shining through the char layer. This thinner char layer is less durable than a deep char, but provides its own unique and beautiful aesthetic. Furthermore, if weathering proves to be an issue with the medium char siding used in this project, it is always possible to add a clear-coat or stain to the siding once it has been installed.

Take a look at a beautiful garden teahouse they designed and built in Prague:

A garden teahouse with medium-char charred wood siding.

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What Is The History Of The ‘Shou Sugi Ban’ Charred Wood Treatment?

The term “Shou-Sugi-Ban” is Japanese (焼杉板) and literally translates to “burnt cedar board”. The term is commonly used to describe the centuries old Japanese technique of charring “Sugi” (cedar) planks used for residential siding, fencing, and decking projects.

Originally, Japanese carpenters looking for an artistic and unique finish that also improved durability used recovered driftwood from the coastlines of Japan. Because of the weathering process wood undergoes when it is subjected to the harsh environment of saltwater, surf, and sun, Japanese driftwood was prized for its unique appearance and durability in many different carpentry mediums.

Driftwood that had undergone the appropriate weathering process was in short supply while demand in Japan for such a product was high. So the Japanese turned to another weathering process to achieve the durability and aesthetic. Fire in this case provided the preservative, and the unique and artistic dimension Japanese homeowners and craftsman were looking for.

The practice of charring Sugi (Cryptomeria japonica) commonly referred to in the United States as Japanese Cedar has been commonplace in Japan since at least the 1700s, and likely earlier. In the last 50-100 years the practice has fallen out of favor in Japan due to the advent of modern plastic or cement based siding, decking, and fencing. Additionally, wood in Japan has been in short supply for quite a while, and most wood has to be imported, increasing its cost. These factors caused Shou Sugi Ban to become a “lost” technique.

In the early 2000’s, Shou Sugi Ban was “rediscovered,” first in Japan, but then it quickly gained the attention of architects and designers in Europe and North America, and started showing up in custom designed houses and buildings. In the last few years its use has really exploded, for all the same reasons that it was popular in Japan hundreds of years ago.

Charring cedar in the United States as a method of coloring, finishing and preserving siding and fencing is just catching on. Because Japanese Cedar is indigenous to Japan only, builders in North America have turned to American outdoor woods such as Western Red Cedar and Southern Cypress, and found that they work just as well, if not even better than the original Japanese Cedar.

We here at CharredWood.com use a sustainable and Northwest grown variety of cedar known as Western Red Cedar (Thuja plicata). It is harvested from working forests near our manufacturing facility, and is a fantastic high performing and beautiful outdoor wood even before we apply our charred finish to it.

Japanese Architect Terunobu Fujimori Creating Shou Sugi Ban the Traditional Way in JapanJapanese Architect Terunobu Fujimori Creating Shou Sugi Ban the Traditional Way in Japan
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How Does Charring Make Wood Fire Resistant?

To reduce woods natural affinity to catch fire, humans have been developing different techniques for improving its resistance for thousands of years.  From plant based waxes and coatings to petroleum based lacquers and paints, we have developed many different methods of keeping our homes safe from fire.

Interestingly, one of the oldest and most effective methods of protecting wood from fire is fire.  The process of applying heat and fire to the exterior of a wood product for a short period of time changes both the cellular structure and thermodynamic conductivity of the wood.

Chemically, you can think of wood as being made up of two components – cellulose and lignin. Cellulose is what the tree grows first; it is the compound that gives a young sapling its flexible strength, and the leaves and fresh shoots are primarily made up of cellulose. Lignin gets deposited into the tree tissue as it gets older, and the lignin is hard, tough and brittle. An old tree-trunk would have a much higher percentage of lignin than a fresh shoot at the tip of a branch.

When you burn wood, the softer, more reactive cellulose vaporizes and gets burned off, while the harder lignin takes a long time to burn. Furthermore, it takes a lot higher heat to get the lignin to ignite again. Then additionally the outer layer of charred wood acts as an insulator.

Therefore, when we char the surface of our cedar siding or fencing, what we are effectively doing is burning off the outermost cellulose and leaving only the blackened lignin behind. In order for the charred wood siding or fencing to re-ignite requires much higher temperatures and much longer in contact with a flame source.

The best way to think about this is if you were to light a campfire, and then when the logs are half-burned you put the fire out. If you come back later and try to re-light those logs again, it is actually much harder to get them to catch fire, because they don’t have any of the more easily ignited cellulose compounds left.

Burning House does not have fire resistant Charred Wood Siding

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