The Science Behind Flame Retardancy Of Shou Sugi Ban/Yakisugi

Over the past 10 years yakisugi (or shou sugi ban) has been rapidly gaining popularity in the West, not only for its beauty and longevity in exterior applications, but also for its reputation of being resistant against fire.  But is increased fire resistance really true?

The Japanese have maintained for hundreds of years that shou sugi ban is fire-resistant, claiming that houses clad with it do not catch fire as readily as non-heat-treated wood cladding. Our recent lab tests of correctly made shou sugi ban corroborate these findings for the first time in the Western hemisphere.  How exactly does shou sugi ban effectively fight fire with fire? What is the science behind cypress testing to a Flame Spread of ASTM/UL Class C, but our heat-treated cypress testing to Class A? We did some research to find the answers.

Typical flame spread laboratory test result of Nakamoto Forestry Shou Sugi Ban Suyaki

What Causes Wood To Combust?

The two main components in softwood lumber are carbohydrates, like cellulose and hemicellulose, which make up 65-90% of the non-water mass, and lignin, which makes up the other 10-35%. Cellulose is made of a linked chain of glucose molecules, or sugar. Hemicellulose is also made of sugar, but in addition to glucose, it’s made with a range of different polysaccharides. Lignin doesn’t contain any sugars and is the main rigid, structural component of wood. The basic premise of heat-treated wood longevity is that the carbohydrate portion is burned off, leaving the structural lignin, and therefore depriving fungi and wood-eating insects of what they metabolize to survive.

In addition to being insect and microbe food (as well as hygroscopic), those carbohydrates are also fuel for fire to consume. In order for wood to ignite, its temperature must be high enough that pyrolysis takes place and the chemical reactions of combustion start. Ignitability of wood is dependent on the thermal properties of the species, moisture content and dimension of the piece, and the way heat is applied to the material.

The factors affecting the ignition of wood are generally well known: wet wood is more difficult to ignite than dry, thin kindling ignites more easily than thick logs, and softwood species ignite at a lower temperature than hardwood. Less common knowledge is that wood burns in stages. Cellulose is the first component of wood to ignite and it burns away quickly, leaving mostly lignin and other sugars behind, which become charcoal. Charcoal is the last component of wood to burn, as it requires higher temperatures than cellulose to ignite. This is the key to why shou sugi ban is naturally flame resistant: the cellulose has already been burned away, leaving a surface that requires much more extreme heat than non-heat-treated cypress to ignite.

When wood burns, flames spread across the surface, and if the heat released is high enough, it ignites more material. Therefore, flame spread is governed by the same factors as ignition. The heat released by a burning area has an effect on flame spread rate, directly raising the temperature of the surrounding material. Thus, the factors governing the rate of heat release are essential for the flame spread.

Fire retardants used on wood generally have a borate base, and are applied to the wood in solution with a water carrier for penetration.  Borate embedded in the wood swells dramatically from heat during a fire, expanding and constricting the flow of oxygen into the wood.  This prevents flame spread and is the mechanism behind most lumber fire retardants.  One theory is that case hardening from the Japanese heat treatment also constricts the flow of oxygen into the wood during combustion, slowing down flame spread.  While we are not sure of this it might also be a factor.

Testing the flammability of our shou sugi ban charred wood before sending off to the lab

Trial By Fire

Last summer we experienced the flame retardancy of our product in the real world when the Tubbs Fire burned around a house with our siding on it at a vineyard just outside of Calistoga, California (View Project). The fire came right up the side of the house and the homeowner is convinced that the heat-treated wood prevented their house from igniting and burning to the ground. The only noticeable damage to the structure from the extreme heat of the wildfire was that the resin we use on the backsides of the planks to hold and fill loose knots melted and bled down the face of the exposed wall (Cypress characteristically has some loose knots as a species as opposed to cedar which has tight knots.  This is due to how slowly or quickly the lower branches die and rot as the trees mature).

Unfortunately a lot of recent demand for our siding comes from projects within the Wildlife Urban Interface zones in California and other western states due to concern about increased wildfire exposure. It has been interesting to get laboratory proof of the fire resistance shou sugi ban has and to find that it’s vernacular reputation is accurate.

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