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.

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.