Straw Bale Building in Japan: History and Current State
In Japan, rice straw is the most common type of straw, though wheat is also grown in some areas. Bales of rice straw are the most common type of bale, while bales of wheat straw, dried meadow grasses, susuki (Miscanthus sinensis), and bamboo grass (Bambusoideae luerss) are also available in some areas of Japan.
According to the now defunct Japan Straw Bale House Association, the first straw bale home in Japan was completed in January 2001. Thirteen years later, there are over 50 straw bale structures throughout Japan, from sub-tropical Okinawa to cold climate Hokkaido.
The NPO Japan Straw Bale House Association lead by Kiyohiko Umagami folded in 2013. Presently, there are four organizations in Japan designing and building straw bale structures: Slow Design Research Group directed by Goichi Oiwa, the Straw Bale Project headed by Hideto Oshima, the Toyama Straw Bale House Association lead by Hiroaki Yoshimoto, and the Hokkaido Straw Bale Building Network organized by Takeshi Jinnouchi. In addition, there are numerous individuals and DIY homeowners building with straw bales.
Working with all of the afore mentioned organizations, the author has been involved in the construction of over 20 straw bale buildings in Japan, including both load-bearing and non-load-bearing straw bale walls.
The non-load-bearing straw bale walls have been combined with framing in a number of ways. Of structural wood buildings in Japan, conventional Japanese timber framing represents the greatest portion of new wood construction. In contrast to traditional Japanese timber framing, characteristics of conventional timber framing are the use of bolts to secure joints and large diagonal bracing for seismic stability. Building officials and professional carpenters are accustomed to the conventional norm and straw bale construction often finds itself within this context. The robust and ubiquitous diagonal bracing of conventional Japanese timber frames tends to push bale walls to either the exterior or interior side of the timber frame.
There are three principle methods combining straw bale walls and timber frames:
Stack bales entirely on the exterior side of conventional Japanese timber framing. In this case, the exterior finish consists of plaster applied directly to the bale walls. Due to fire, thermal performance, and moisture concerns, the interior side of the bales should also be plastered. However, these concerns are often overlooked and the interior side of the bales may or may not be plastered. The interior walls are usually finished with wood, earthen plaster on a bamboo lattice, or drywall, providing a gap the width of the timber framing between the bale wall and interior finish. Although this gap provides a space for utilities such as wiring, it also provides a space for thermal convection. In order to prevent thermal convection, packed straw bundles have been used, and an infill of light straw clay or cellulose is also possible. In other cases, the interior side of the straw bales is plastered, exposing the posts and diagonal bracing.
Stack bales entirely on the interior side of framing. In this case, exterior siding is attached to the exterior side of the frame. In new construction, it is possible to plaster the exterior side of the bale wall before the exterior siding is attached. When using bales in the renovation of existing buildings, it is nearly impossible to plaster the exterior side of the bale wall unless the exterior siding is removed. In one instance, cellulose insulation was installed between the framing members. The interior side of the bales is finished with plaster.
The bales are notched with a chainsaw/grinder/hedge trimmer/brush cutter to fit in between and to the inside of a timber frame. In these cases, the use of a conventional Japanese timber frame is uncommon. For example, in one instance, the timber framing style used was reminiscent of North American or European timber farming with relatively small diagonal bracing (knee bracing). The exterior and interior side of the bales was plastered and functioned as the finish.