Photo by Jeremy Bitterman /JBSA. Architect: Mork-Ulnes Architects
Charred Japanese cedar wood known as yakisugi (or shou sugi ban outside of Japan) provides a variety of dramatic design and performance opportunities.
Wood is a desirable building product for many reasons including aesthetic appeal, sustainability, and being economical. While many different approaches have been used to improve its durability and longevity, particularly in exterior applications, the Japanese process of charring the surface of the wood has gained popularity in the United States in recent years. Referred to as yakisugi in Japan and often as “shou sugi ban” in the West, the material is being specified and used in residential and commercial designs due to its enhanced durability and the aesthetic appeal of the charred wood. This course provides an in-depth exploration of authentic yakisugi/ shou sugi ban and points out the differences with some similar treatment processes which can vary in their performance.
The content covers the history, science, and modern architectural applications of this sustainable material, emphasizing its environmental benefits and ability to protect wood from fire, moisture, and pests. By examining some example projects, readers can gain practical knowledge on incorporating charred wood into modern designs, balancing tradition with contemporary sustainability practices.
OVERVIEW OF YAKISUGI/ SHOU SUGI BAN
The term “shou sugi ban” is used in the Western world (North America, Europe, etc.) but is unknown in Japan. It is believed that the term was created by an American who misread the original Japanese characters related to a difference in the Chinese and Japanese languages. The original Japanese term is “yakisugi,” with “yaki” meaning burnt, charred, or heat treated, and “sugi” referring to Japanese cedar that is indigenous to the country. When speaking specifically of boards made from this charred or burnt species, the term “yakisugiita” is used. In order to be clear and accurate, in this course we use the original Japanese word of yakisugi to refer to the wood products being discussed, even though many may understand it to be what they have been colloquially calling shou sugi ban.
It is important to note at the outset that yakisugi is not a treatment process, rather it is a total product that yields long-term performance benefits and a range of aesthetics. It begins with selecting the proper species and grades of wood and cutting and milling it to the correct thickness. It is then subjected to burning or charring the wood in a controlled manner and allowing it to dry to the desired moisture content. It may also include some further finishing either at the time of manufacture or in the field. The combination of selecting the right wood and subjecting it to carefully controlled steps is what ensures a quality yakisugi product is developed suitable for use in buildings.
Historical Context
Yakisugi is believed to have originated in Japan several centuries ago, although the date of its first appearance has yet to be confirmed. Nonetheless, it has been successfully used to fire-harden homes constructed of wood in densely populated Japanese cities and urban areas that frequently encountered catastrophic structural fires. While the original users may not have fully understood the science, it is now known that during the heat treatment, hemicellulose, which is essentially a carbohydrate, is burned off. What’s left behind is the lignin, which is the structural component of wood. The end product is a hardened wood plank that is resistant to decay, insects, and weathering. Additionally, the soot layer increases the temperature threshold needed for combustion which dramatically reduces flame spread. Historically, yakisugi was made directly at the jobsite by the carpentry crew. However, the process has been refined after transitioning to manufacturing in lumber mills. This transition has led to more consistent increased longevity and dimensional stability for both interior and exterior use.
Photo by Ema Peter; courtesy of Nakamoto Forestry North America
Dark charred yakisugi is used on many residential and commercial buildings in North America.
In the United States, heat treating, or charring wood has been studied since at least 1927 as a method of wood protection. The charring process, for example, has been used commonly on the bottoms of wooden fenceposts to prolong their surface life. Interest in using yakisugi in the United States and Europe for other purposes has grown in the past two decades. For some architects, the appeal has been purely aesthetic as a way to add bold coloring to a facade. For others, the performance and longevity of the product caught their attention. For those interested in sustainability, the fact that it is made out of wood with a very favorable carbon reduction profile brought them to look closely at it. Others are attracted to the cultural and historical understanding including the dramatic transformation of the wood by fire.
Currently, yakisugi is used in the U.S. predominantly for building exteriors in residential and low-rise commercial projects. Most commonly, it is used as exterior cladding on vertical surfaces such as walls, fences, and similar applications. It has also been used for soffits and similar horizontal surfaces which may be less exposed to the elements. In building interiors, its use includes both vertical wall surfaces and horizontal ceiling surfaces. Note that the Japanese heat treatment process is never used for flooring or furniture. Japanese cedar is too soft for flooring, and yakisugi planks are too thin for most furniture applications. The burning of flooring, furniture, or counters has been done for artistic purposes, but it is not the Japanese yakisugi heat treatment.
Wood Species and Treatment
The wood that is used for Japanese yakisugi is “Cryptomeria japonica,” which is commonly known as Japanese cedar or sugi cedar. These trees grow abundantly throughout Japan with planted forests in the southern part of Hokkaido and in Okinawa in the northern part of the main island. Sugi cedar has several characteristics, making it particularly suitable for this product while other species tend to fall short. In particular, sugi cedar is well-suited for the intense heat mineral-rich fiber. Species such as pines, larches, or firs have more pitch and less minerals, so they have a greater tendency to check when charred and are not as durable. Other species of cypress, cedars, or red- woods may have good durability but when subjected to the charring process, they do not retain the beneficial soot layer as long as the sugi cedar does.
Japanese or sugi cedar has a number of additional desirable characteristics, particularly for exterior siding products. It is straight-grained, fast-drying, flexible, tannin-rich, and strong. It features a thick, dense latewood growth ring, which burns to a more substantial, longer lasting soot layer. Compared to other species, its chemical properties respond well to fire, and it becomes incredibly dimensionally stable when milled, dried, burned, and quenched by traditional protocols.
It is important to be clear on the type of wood being used by a manufacturer of yakisugi since different manufacturers may use different types of wood that are less suitable than sugi cedar and can produce disappointing results when used in a project. It is also important to be aware that there are “yakisugi-inspired” products that try to be imitations but don’t have the same aesthetics or 100-plus years of demonstrated durability. A different process known as aburi is a surface burning treatment done using a torch to many species and used on furniture or beams for a darker appearance. Thermally modified wood is a separate low temperature process developed in Finland that penetrates more deeply into the wood and can be used on thick stock lumber for improved durability in exterior applications. Carbonization by the Chinese process is a high-temperature, hardening process used mainly for flooring. There is also chemically treated wood which relies on the chemicals used on various wood species to create its appearance which may be similar to yakisugi at first but will weather very differently over time.
Whenever assessing available heat-treated wood products, the specifics of wood species and the details of how it is manufactured are critical for understanding the true perfor- mance and design characteristics.
Photo by Jeremy Bitterman /JBSA. Architect: Mork-Ulnes Architects
Charred Japanese cedar wood known as yakisugi (or shou sugi ban outside of Japan) provides a variety of dramatic design and performance opportunities.
Wood is a desirable building product for many reasons including aesthetic appeal, sustainability, and being economical. While many different approaches have been used to improve its durability and longevity, particularly in exterior applications, the Japanese process of charring the surface of the wood has gained popularity in the United States in recent years. Referred to as yakisugi in Japan and often as “shou sugi ban” in the West, the material is being specified and used in residential and commercial designs due to its enhanced durability and the aesthetic appeal of the charred wood. This course provides an in-depth exploration of authentic yakisugi/ shou sugi ban and points out the differences with some similar treatment processes which can vary in their performance.
The content covers the history, science, and modern architectural applications of this sustainable material, emphasizing its environmental benefits and ability to protect wood from fire, moisture, and pests. By examining some example projects, readers can gain practical knowledge on incorporating charred wood into modern designs, balancing tradition with contemporary sustainability practices.
OVERVIEW OF YAKISUGI/ SHOU SUGI BAN
The term “shou sugi ban” is used in the Western world (North America, Europe, etc.) but is unknown in Japan. It is believed that the term was created by an American who misread the original Japanese characters related to a difference in the Chinese and Japanese languages. The original Japanese term is “yakisugi,” with “yaki” meaning burnt, charred, or heat treated, and “sugi” referring to Japanese cedar that is indigenous to the country. When speaking specifically of boards made from this charred or burnt species, the term “yakisugiita” is used. In order to be clear and accurate, in this course we use the original Japanese word of yakisugi to refer to the wood products being discussed, even though many may understand it to be what they have been colloquially calling shou sugi ban.
It is important to note at the outset that yakisugi is not a treatment process, rather it is a total product that yields long-term performance benefits and a range of aesthetics. It begins with selecting the proper species and grades of wood and cutting and milling it to the correct thickness. It is then subjected to burning or charring the wood in a controlled manner and allowing it to dry to the desired moisture content. It may also include some further finishing either at the time of manufacture or in the field. The combination of selecting the right wood and subjecting it to carefully controlled steps is what ensures a quality yakisugi product is developed suitable for use in buildings.
Historical Context
Yakisugi is believed to have originated in Japan several centuries ago, although the date of its first appearance has yet to be confirmed. Nonetheless, it has been successfully used to fire-harden homes constructed of wood in densely populated Japanese cities and urban areas that frequently encountered catastrophic structural fires. While the original users may not have fully understood the science, it is now known that during the heat treatment, hemicellulose, which is essentially a carbohydrate, is burned off. What’s left behind is the lignin, which is the structural component of wood. The end product is a hardened wood plank that is resistant to decay, insects, and weathering. Additionally, the soot layer increases the temperature threshold needed for combustion which dramatically reduces flame spread. Historically, yakisugi was made directly at the jobsite by the carpentry crew. However, the process has been refined after transitioning to manufacturing in lumber mills. This transition has led to more consistent increased longevity and dimensional stability for both interior and exterior use.
Photo by Ema Peter; courtesy of Nakamoto Forestry North America
Dark charred yakisugi is used on many residential and commercial buildings in North America.
In the United States, heat treating, or charring wood has been studied since at least 1927 as a method of wood protection. The charring process, for example, has been used commonly on the bottoms of wooden fenceposts to prolong their surface life. Interest in using yakisugi in the United States and Europe for other purposes has grown in the past two decades. For some architects, the appeal has been purely aesthetic as a way to add bold coloring to a facade. For others, the performance and longevity of the product caught their attention. For those interested in sustainability, the fact that it is made out of wood with a very favorable carbon reduction profile brought them to look closely at it. Others are attracted to the cultural and historical understanding including the dramatic transformation of the wood by fire.
Currently, yakisugi is used in the U.S. predominantly for building exteriors in residential and low-rise commercial projects. Most commonly, it is used as exterior cladding on vertical surfaces such as walls, fences, and similar applications. It has also been used for soffits and similar horizontal surfaces which may be less exposed to the elements. In building interiors, its use includes both vertical wall surfaces and horizontal ceiling surfaces. Note that the Japanese heat treatment process is never used for flooring or furniture. Japanese cedar is too soft for flooring, and yakisugi planks are too thin for most furniture applications. The burning of flooring, furniture, or counters has been done for artistic purposes, but it is not the Japanese yakisugi heat treatment.
Wood Species and Treatment
The wood that is used for Japanese yakisugi is “Cryptomeria japonica,” which is commonly known as Japanese cedar or sugi cedar. These trees grow abundantly throughout Japan with planted forests in the southern part of Hokkaido and in Okinawa in the northern part of the main island. Sugi cedar has several characteristics, making it particularly suitable for this product while other species tend to fall short. In particular, sugi cedar is well-suited for the intense heat mineral-rich fiber. Species such as pines, larches, or firs have more pitch and less minerals, so they have a greater tendency to check when charred and are not as durable. Other species of cypress, cedars, or red- woods may have good durability but when subjected to the charring process, they do not retain the beneficial soot layer as long as the sugi cedar does.
Japanese or sugi cedar has a number of additional desirable characteristics, particularly for exterior siding products. It is straight-grained, fast-drying, flexible, tannin-rich, and strong. It features a thick, dense latewood growth ring, which burns to a more substantial, longer lasting soot layer. Compared to other species, its chemical properties respond well to fire, and it becomes incredibly dimensionally stable when milled, dried, burned, and quenched by traditional protocols.
It is important to be clear on the type of wood being used by a manufacturer of yakisugi since different manufacturers may use different types of wood that are less suitable than sugi cedar and can produce disappointing results when used in a project. It is also important to be aware that there are “yakisugi-inspired” products that try to be imitations but don’t have the same aesthetics or 100-plus years of demonstrated durability. A different process known as aburi is a surface burning treatment done using a torch to many species and used on furniture or beams for a darker appearance. Thermally modified wood is a separate low temperature process developed in Finland that penetrates more deeply into the wood and can be used on thick stock lumber for improved durability in exterior applications. Carbonization by the Chinese process is a high-temperature, hardening process used mainly for flooring. There is also chemically treated wood which relies on the chemicals used on various wood species to create its appearance which may be similar to yakisugi at first but will weather very differently over time.
Whenever assessing available heat-treated wood products, the specifics of wood species and the details of how it is manufactured are critical for understanding the true perfor- mance and design characteristics.
Manufacturing Process
The means to produce yakisugi involves several steps outline as follows:
- Log Selection: Harvested trees are reviewed and graded for quality, straightness, and size (diameter). Grading quality The wood that is used for Japanese yakisugi is “Cryptomeria japonica,” which is commonly known as Japanese cedar or sugi cedar. These trees grow abundantly throughout Japan with planted forests in the southern part of Hokkaido and in Okinawa in the northern part of the main island. Sugi cedar has several characteristics, making it particularly suitable for this product while other species tend to fall short. In particular, sugi cedar is well-suited for the intense heat mineral-rich fiber. Species such as pines, larches, or firs have more pitch and less minerals, so they have a greater tendency to check when charred and are not as durable. Other species of cypress, cedars, or red- woods may have good durability but when subjected to the charring process, they do not retain the beneficial soot layer as long as the sugi cedar does. Japanese or sugi cedar has a number of additional desirable characteristics, ultimately leads to boards that include standard select (some knots) or premium clear (no knots). Straightness of the logs helps with the internal integrity of the wood and the final grain of the boards. Size determines the yield of the log into different sizes of boards.
- Rough Milling: The graded logs are fed into milling and cutting machines to create rough moulder blanks. Their size will vary depending on the need, but they are the rough boards that will eventually become the finished planks.
- Drying: Once the rough blanks are produced, they are then stacked and separated (e.g. “stickered”) so they can be air-dried. Some wood siding manufacturers use kiln drying over several days at low heat. However, that is more energy intensive and may or may not produce the best results compared to air drying which can slowly and completely dry the wood.
- Finish Milling: Once dried, the rough boards are ready for their final profile milling. Widths can vary by standard lumber sizes (i.e., nominal 4”, 6”, 8”, etc.). In Japan, the standard thickness is 3/8” (10mm), but 9/16” (15mm) is also available in the U.S. since that is as thick as can be heat treated with good yield. Heat treating a plank thickness over 5/8″ (e.g., 3/4″) by the traditional Japanese charring process doesn’t work well since the interior moisture content will stay around 12% while the surface moisture content goes down to 0%. This results in a higher incidence of crooking, warping, twisting, or checking. Milled profiles typically include shiplap or square-edged shapes. Tongue and groove profiles are not produced since they are not recommended for good weathering of the wood.
- Heat Treatment: Once fully dried, the boards are ready for heat treatment. This step is done in a kiln, preferably engineered to use an afterburner so natural gas consumption is minimized and carbon emissions are reduced. Enough contained heat is applied to the boards so that the surface hemicellulose wood fibers are burned off. The result is an inky black, thermally modified surface. Note that all yakisugi is burned deeply to heat treat the wood. Cosmetic “shallow” or “light” burning is similar to aburi, a different traditional Japanese wood darkening technique done with a torch on furniture or interior millwork. This carbonizes the less dense earlywood growth rings only, not the dense latewood.
- Surface Treatment: After burning, different surface treatments can be used to achieve varied appearances. There are three common choices. First, the sooty, textured surface can be left alone (i.e., unbrushed) for protection and character. Unbrushed yakisugi generally takes 40-50 or more years for the black color to weather in a gradual erosion process into looking like brushed yakisugi. It will start black then gradually turn various colors over this period and may or may not stay black. Second, the boards can be wire- brushed lightly for a smooth and more refined appearance exposing the grain while keeping a moderately dark color. Third, it can be further wire-brushed to show more of the grain texture and color producing a distinctive, topographic texture that is lighter in color than the other two options.
- Finishing: Yakisugi does not require any additional finishing or oiling and historically oiling has not generally been done. However, most yakisugi in modern construction has an oil finish applied during manufacturing that bonds the soot onto the wood surface so that it does not come off when touched. Additionally, the boards will change color over time as the charred surface naturally erodes and creates a patina on the wood. Some designers or building owners prefer to have a particular color secured and choose to have an oil or stain added to the surface. This can be done by a manufacturer as the boards are produced, in the field during construction, or several years later by the building owner when the desired patina color is achieved.
Photo by xoMe Photography
The use of yakisugi wood in buildings can be creative, dramatic, and sustainable.
Photo courtesy of Nakamoto Forestry North America
Properly manufactured yakisugi produces very straight, durable, and hardened wood planks that can be used for exterior and interior applications.
Understanding the manufacturing process above, there are different ways to tell heat-treated yakisugi vs “faux” finishes that are lightly burned or embossed. One is the striped grain patterns that are negative images of each other on the surface of the wood. Another is how surface checking from full burning has a different texture than lightly burned, although a solid coating can conceal that. Such faux finishes can be appropriate for interior uses, but on exteriors, they may weather quickly or have unknown aesthetics over time.
DESIGN CONSIDERATIONS
With an understanding of the yakisugi products, we can turn our attention to the different ways they can be used in a design, whether for exterior or interior applications on residential or low-rise commercial buildings. A fundamental design choice will be the orientation of boards related to their use. Yakisugi planks can be installed as exterior siding in a horizontal, vertical, or diagonal pattern whether across an entire facade or broken up into different sections with different patterns as appropriate to the building design. The same is true for interior paneling and horizontal surfaces; the pattern can be at the discretion of the designer.
The next fundamental design decision is the finished look of the boards. That starts with the three different surface treatments. Is the dark look of the full charred wood desired or one of the wire-brushed options? They each have their own aesthetic appeal and can be used singularly or in combination. In Japan, the dark, charred siding is often used in conjunction with white stucco on different portions of a building facade. Other designs may be more suitable for the less dark or even the lighter appearance with bold wood grains.
Finishing Choices
The choice of using an oil pre-finish or adding oil later on comes down to the personal preference of either the building owner or the designer. Historically 80% or more yakisugi was unoiled/ untreated while in modern construction about 80% is oiled. If left untreated, weathering from the elements leaves a patina, or what the Japanese call a “wabi-sabi” aesthetic. While this concept can be different than Western views of aesthetics, it is born of a cultural sensitivity that values things as they age. The “wabi” refers to a beauty from variations or imperfections in the world that offer their own contributions just as they are. The “sabi” refers to the passage of time and change. Wabi-sabi is described as finding the beauty behind something, its atmospheric feeling, not just its external appearance. There is an element of loss, of transience, but also abundance and transformation. This traditional cultural view leads to an aesthetic for the wood, which is ever-changing, accepted, and embraced.
Photo courtesy of Nakamoto Forestry North America
Traditional Japanese use of yakisugi often uses unfinished charred wood which erodes and patinas over time and is embraced as a “wabi sabi” aesthetic. It is often used in conjunction with other materials such as white stucco.
Photo courtesy of Nakamoto Forestry North America
Traditional Japanese use of yakisugi often uses unfinished charred wood which erodes and patinas over time and is embraced as a “wabi sabi” aesthetic. It is often used in conjunction with other materials such as white stucco.
If a different appearance is sought, there are also several dozen other different aesthetic options (as many as 32 from at least one manufacturer). Any of the surface treatments (charred, lightly brushed, or double brushed) can be left unfinished and allowed to patina to a color or appearance that is desirable or finished with a coating. Color change can also be slowed down by applying oil finishes at the outset. All products can be re-oiled at any time in the future to freshen up the color and sheen as desired. There are no fast rules for how wood weathers in color since every site has different conditions. However, in general, over time, siding tends to mellow reddish brown on south-facing elevations and bleach to a gray tone on north-facing elevations.
What are the reasons to consider oiling the wood? First, to achieve the particular color the designer or owner wants with a pigment. Secondly, preserving the look longer than untreated wood. Third is to glue in the residual soot as part of the finished surface.
Note that unoiled wood will have better fire resistance since there will be no combustible oils present. Eliminating the oiling also saves money offering better overall cost performance. Nonetheless, some alkyd and acrylic products can slow flame spread.
Ultimately, the finish decision needs to be based on whether the wood is being used in an interior or exterior application. The primary differences are that exterior coatings must have a fungicide to mitigate mildew, they almost always contain visible and invisible pigments since pigments are the primary UV inhibitor, and they must be more highly elastic to compensate for expansion and contraction of wood during temperature and moisture content fluctuation. Exterior coatings are generally single or multiple coats of the same formula to increase the coating thickness and durability since oil finishes erode over time (instead of flaking like paints). Interior coatings are usually a stain coat followed by a hard protective topcoat. Exterior coatings commonly contain elastic urethanes while interior coatings contain rigid polyurethanes.
There are several types of exterior coatings that range from solid color (longer lasting) to transparent color (faster-fading formulas). Acrylics are solid coatings with the best color longevity, but they completely hide the wood color. Alkyds are semi-solid coatings that can be applied in multiple coats to increase opacity. Next in durability are tung oils and some linseeds, which are semi-transparent and show more wood grain. Finally, transparent linseed oil formulas act as a lens that enhances the wood grain; however, they allow the wood to mellow or bleach from UV exposure. Designers can, of course, select the level of opacity or transparency to match an exterior design, but that means a decision is also being made at the same time on color longevity. Less color usually means more frequent maintenance and re-finishing. Totally clear coatings are not recommended on exteriors since they don’t typically meet all of the requirements and need very frequent (12 months or less) reapplication.
Wood Plank Details
Beyond the pattern and appearance, there are two other fundamental design decisions to be made related to the wood. The first is the grade of the wood to be used. The standard grade will contain some knots visible in the grain which may be visible if a wire-brushed product is selected with an unfinished, transparent, or semi-transparent finish. That may be quite desirable since the look can be dramatic, showing and enhancing the contours of the grain. In other cases, a premium grade may be preferred with no knots and a smoother, more contemporary appearance. The premium grade comes with a corresponding premium price, of course, so it would not make sense to use that for an unbrushed charred surface but solid stains, like acrylics and alkyds often look quite good with premium clear grade wood since the texture is so noticeable.
The second choice is to be clear on where shiplap-edged boards are used and where square-edge boards are to be used. Commonly, the siding boards rely on the shiplap edges to create the joints between them, however, 9/16-inch square edge boards are also used, particularly in rainscreen applications where the boards are separated slightly. Trim pieces at siding edges, around openings, etc. are commonly 5/4 stock square- edged material. The distinction between these should be made on design drawings and followed through in project specifications.
In order to put all of the above in an overall design context, William Belek, President of Nakamoto Forestry, North America points out what he often discusses with architects. “People are so focused on the coating, but the lumber and the millwork are the real essence of the products. That’s the real secret—the quality of the lumber and the quality of the milling should be used to identify the quality of the finished product.” He also points out a caution when comparing products or samples indicating “Finishes are certainly used, although not needed with quality products. However, heavy coatings can be used by some producers to hide low-grade millwork.”
PERFORMANCE CHARACTERISTICS
Based on the foregoing, it is easy to see why yakisugi is a rather sophisticated manufactured material with heat processing or charring much more than just a finish. When properly produced, authentic yakisugi combusts away the hemicellulose in the wood fibers, making the material much more resistant to the weather including airborne rain and moisture and ultraviolet rays (UV) from the sun. The lack of hemicellulose along with the high mineral content and fast drying times of the finished boards means that bugs, termites, and fungi have nothing to feed on so are not attracted to it. There is also less likelihood that rot and mold will set in. All of these factors add up to a product that has increased durability and can last longer than many other siding options.
Photo courtesy of Nakamoto Forestry North America
Using different orientations of siding and different finish treatments can provide dramatic design results.
In terms of fire resistance, the charred, hardened, lignite surface also means that it takes longer for fire to affect the wood. Attesting to this fact, ASTM E-84 testing standards have been used resulting in Class A fire resistance ratings available for many yakisugi products. If required for a particular project, yakisugi can also be treated with a flame retardant—consultation with a manufacturer is suggested if that is the case.
Yakisugi is time-tested in Japan which is a long country (north to south latitudes) with climates ranging from semi-tropical to Siberian. Due to its durability yakisugi has been used primarily along the coast and in the humid south for hundreds of years. This suggests that yakisugi will probably outlast any other softwood in any climate. It does need to be installed properly in a wall assembly, though.
Exterior Wall Design
Exterior siding or cladding of any type is not intended to be a waterproof barrier, rather it is meant to shed water and protect the wall assembly behind it, especially the primary water barrier, from UV. Water will always get behind any siding and one of the critical aspects of the long-term durability of all wood siding is that the cladding allows water to weep away and dries quickly. The common way to achieve this is to use a rainscreen wall design.
For purposes of residential or low-rise commercial buildings, a rainscreen is an exterior skin system made up of siding/ cladding installed so that it is separated from the structural wall with an air gap that allows for drainage and ventilation, both of which allow for drying. The important surface is the weather-resistive barrier (WRB) that is installed continuously over the exterior face of the structural wall (e.g., wall studs with OSB or plywood sheathing). In the case of wood siding, the air gap is often created using furring strips nailed through the WRB and substrate. The siding is then installed onto the furring strips creating the space for ventilation and drainage resulting in drying. The air gap also allows any water vapor that’s expelled from the structure’s interior to escape instead of condensing and becoming trapped between the WRB and siding, creating an environment where fungi can grow.
Photo courtesy of Nakamoto Forestry North America
The use of rainscreen construction is regarded as a best practice for all types of cladding on exterior walls.
It is worth differentiating between furring used for drying (ventilation and drainage) compared to furring used only for siding attachment. Horizontal siding layouts work well since vertical furring provides both drying and siding attachment with fasteners penetrating into the substrate. However, vertical siding layouts need more attention. One layer of horizontal perforated engineered furring can be used that allows drying and fasteners that penetrate into the substrate. Alternatively, two layers of wood furring can be used—first a vertical layer for drying, and a second horizontal layer with sufficient siding fastener pull-out strength. Properly installed, furring does not need to be treated with a rot preventative since it generally stays dry and will dry out very quickly if wet.
Directly related to a rainscreen wall design is the shape of the siding and the manner in which it is secured. Some people believe that tongue and groove (T&G) boards are best because they keep water out best and should be blind nailed for best appearance. The fact is that T&G profiles create more water tension at the board joints which weeps and dries slower than shiplap or square planks. That means T&G will wick water and rot on an exterior unless it is covered by a roof. Proper millwork and heat treatment correctly executed for yakisugi produce a dimensionally stable material, so a locking-type T&G pattern is not necessary. Additionally, a thin T&G profile can’t be burned deeply enough to heat treat. The square edges on T&G (or channel lap) will melt if thoroughly burnt and wavy parallel lines simply do not look good. Based on all of the above, the best choice for a siding profile is shiplap.
Relatedly, exterior wood cladding, particularly yakisugi, cannot be blind nailed since hidden fasteners cannot be tightened during future maintenance. The wood will always move over time due to moisture fluctuation and latent instability in the planks. Face-nailed siding will move less and look better over time since headed fasteners have higher strength and can be easily tightened up periodically with a hammer. Also, due to the high-temperature heat treatment process, yakisugi is by nature a thin-stock material too thin for hidden fasteners.
The recommend face-nailing fasteners include headed, ring-shank stainless nails or screws at least 1-3/4” long. Some custom- ordered black factory-painted stainless nails and screws are also available from manufacturers to blend in with black or dark-finished boards. Other nail colors such as brite, brown, white, or gray are commonly available depending on the design intent. For interior applications construction adhesive and finish nails work well. case study #2
Maintenance Considerations
Historically in Japan, no maintenance is done at all to yakisugi until it starts to fall apart after about a hundred years. At that point it is patched or replaced, provided that the building is still in good usable condition. During the useful service life of the siding, weathering from the elements leaves a patina, or a “wabi-sabi” aesthetic—the beauty from variations or imperfections as it is expressed over time and change.
In North America, building owners may or may not be accepting of a color and appearance change over time. Either the color change can be accepted as it progresses, or an oil stain can be applied to restore or change the appearance. While in Japan yakisugi is rarely re-oiled, in the West, fresh color via re- oiling is the cultural norm. Color longevity is usually the main object, but regular re-oiling will also allow the siding to last longer.
The only other recommended maintenance is to tighten up fasteners after a few years and then every decade or two over time. The substrate and siding will naturally move due to weather and pull out the fasten- ers little by little. So gently tapping them back in with a hammer will return the siding to a flat plane again. This is usually optional and often unnecessary.
Cost Considerations
Since this is a vernacular material, affordability has always been associated with yakisugi in Japan. It tends to maximize millwork tradecraft, so making it is more knowledge-based than expense-based. That means it should have good value compared to other cladding options, although a particular vendor’s price might reflect custom millwork, high overhead, or pricing strategy. In reality, the Japanese heat treatment process does not add much production cost to the wood product so yakisugi should not be much more expensive, if at all, compared to other stain-grade wood siding options. Of course, when long term durability and little need for maintenance is considered, long term cost performance is usually quite favorable.
GREEN BUILDING CONTRIBUTIONS
When yakisugi is incorporated into a building design or renovation, there are several contributions that may be made based on green building standards due to their inherent material make-up, the way that they are used, and general characteristics. These can include the following.
Materials and Resources
Life cycle assessments show that wood cladding in general is better for the environment compared to other building material choices—1/15th the carbon of cement board, for example. In terms of carbon capture, wood planking, including yakisugi, can be carbon neutral or even carbon negative with up to 2 kg of carbon captured for every 1 kg of material shipped. Yakisugi by nature is less carbon-intensive to manufacture since it is air-dried before being subjected to a short- duration heat treatment in a recirculating type of kiln. Other wooden siding products, especially chemically treated wood or thermally modified wood by the Finnish process, are subjected to a days-long kiln drying process. The amount of energy consumed to modify yakisugi is therefore lower by nature than modern wood modification treatments. The long life and durability of yakisugi also support a favorable life cycle assessment since replacement is not needed for many years into the future.
Since authentic yakisugi cannot be made without the traditional manufacturing processes described, it is never made from reclaimed wood, driftwood, or chemically treated wood. Nonetheless, it can be sustainably harvested in Japan. While the Forest Stewardship Council (FSC) is recognized in the U.S., internationally, PEFC, the Programme for the Endorsement of Forest Certification is more commonly referenced. This non-profit, non-governmental organization is a leading global alliance of national forest certification systems. Manufacturers who adhere to their standards and programs produce a chain of custody certification for sustainably grown and harvested wood, including sugi cedar.
When looking at product delivery, it is notable that finished products are more efficient to ship than raw materials. Further, ocean freight has a much lower carbon footprint than regional trucking or even transcontinental rail freight in terms of pounds moved per mile (the typical ratio is ocean = 1, trucking = 7.1, rail = 4.9). Hence, it is not a given that a locally produced product is more sustainable than one that is shipped from a distance.
Photo courtesy of Nakamoto Forestry North America
Japanese or sugi cedar can be sustainably harvested from forests that are managed and certified following international protocols.
Indoor Environmental Quality
When used for interior applications, it is significant that yakisugi is a low-emitting material. Coatings are generally used by manufacturers to seal in the soot, but low- VOC plant-based coatings can be specified. Uncoated yakisugi is also common for ceiling applications. Relatedly, a growing interior design trend that has been directly related to human health and welfare is the use of biophilia in buildings. Defined literally as a love of living things or a love of nature, biophilic design is a concept focused on increasing occupant connectivity to the natural environment in creative ways. Theorists argue that this approach taps into the hard wiring of human beings who, over the span of history, have developed an affinity for the life-supporting aspects of the natural world. Independent researchers have long been aware of the health and wellness benefits of such features in many different building environments. Natural or heat-treated wood used on the interior of a building supports this biophilic design approach. It is also notable that cedar lumber has some positive effects on indoor air quality. First, it is prone to off-gas beneficial chemicals such as terpenes that improve emotional health by providing a relaxing effect and easing pain. It reportedly can also filter pollutants from the air as the wood is exposed to them—yakisugi has historically been applied to ceilings as an air purifier in interior applications.
Specifying different finishes, different fasteners, and different types of yakisugi products can produce notable designs and strong performance.
SPECIFYING YAKISUGI IN BUILDINGS
When specifying yakisugi it is advisable to first talk with manufacturers or suppliers of such products to understand and assess the available product offerings and possibilities. It is also helpful to review industry standards and model specifications that can be edited for specific projects, whether new construction or renovation. In a standard, three-part specification format following the CSI MasterFormat system, charred wood siding, and paneling are commonly found in Division 06 Wood usually under Section 06 26 00 or a subsection thereunder. Some of the relevant items to address are highlighted as follows.
Part 1: General
The scope or extent of specified work as well as any definitions should be called out along with references to other related work. Quality control can reference ISO 21930 and ISO 14025 products and manufacturers. Submittals and samples can be requested based on the project needs, including the request for any green building standard documentation if any relevant credits are being pursued. Project conditions and product storage should all be addressed in a typical fashion with a particular emphasis on temperature and moisture. It is also advisable to have the material acclimate at the project site for at least 14 days.
Part 2: Products
All of the different performance and material criteria for the different products specified need to be called out. This includes the sizes, treatments, material choices, and textures for rainscreen furring, exterior siding, interior paneling, fasteners, and adhesive. If factory- finished products are called for, then the details of finish material, color, and number of coats need to be specified and differentiated for different applications or locations as shown on drawings. Any related materials or accessories provided by others should either be specified in this spec section or be referenced in other relevant specification sections.
Part 3: Execution
As with any site-installed product, the instal- lation requires multiple steps that need to be clearly articulated in the specification in order to achieve the best results.
- Examination and Preparation: The importance of this step should always be stressed. The installer and general contractor or construction manager should review and examine the substrate for conditions that may affect the installation or the overall performance of the wood siding or paneling. Any issues will need to be corrected, likely by the general contractor, if they are found to be out of compliance with the stated requirements.
- Installation: Furring first needs to be installed for rainscreens followed by the yakisugi products in the pattern and layout designed. When the yakisugi needs to be cut, only a sharp, ultra-fine carbidetipped finishing saw should be used. All long joints should align (butt joints may be staggered) and be nailed to a solid bearing behind. Wood siding should be installed after any adjacent masonry is installed since masonry work can be rather dirty and find its way onto the wood. The recommendations and procedures of the product manufacturer as well as the applicable requirements of codes and standards should be followed in all cases.
- Cleaning and Repairing: Once installed and finished, the surface should likely be able to withstand use by the remaining construction personnel on the project. However, the general contractor should be instructed to cover and protect any surfaces that are vulnerable to damage or abrasion, particularly in high-traffic areas. Most products are factory-oiled to lock in the soot and minimize any transfer to adjacent surfaces, but handling and cutting the wood may release some. It is recommended that all yakisugi and adjacent walls should be washed down with a hose or wet rags. The final step is to touch up any blemishes from handling and installation by dabbing with touchup oil and blending with a rag.
When designed, specified, and installed correctly, yakisugi siding or paneling can readily provide the desired look, the general health and wellness benefits, and the long- term performance characteristics which are intended.
CONCLUSION
Yakisugi/shou sugi ban is a unique and organic wood product, with different characteristics and specifications from standard millwork. Form follows function with these products, and rigid millwork and drying protocols need to be followed for the planks to stay straight through the high-temperature heat treatment process. While grading and millwork produce consistent products, it is somewhat exotic, and sooty, and will patina over time for a variety of design outcomes. Architects should become completely comfortable with the product design and performance capabilities before specifying it for any project. Ultimately, it contributes to buildings that are beautiful, sustainable, and durable.
Peter J. Arsenault, FAIA, NCARB, LEED AP is a nationally known architect who has authored nearly 300 continuing education courses focused on advancing building performance through better design. www.pjaarch.com, www.linkedin.com/in/pjaarch
