Many areas of Japan have hot, humid summers with relatively small diurnal temperature swings. Traditional Japanese and southeast Asian architecture deals with summer heat through cross ventilation. Open floor plans and large window and door openings encourage air movement. Moreover, evaporation from thatch roofs and adjacent gardens with water elements also help to cool interior spaces. In contrast, the American southwest, the home of the modern straw bale revival, experiences relatively large diurnal temperature swings in summer and relies on isolating the indoor environment from the hot outdoors during the day, and then letting in cool air during the night. Utilizing diurnal temperature swings is possible in some areas of Japan. Where this is not possible, straw bale buildings require air conditioning in summer. In these cases, one could argue that the level of insulation provided by straw bales is unnecessary.
Given the complications involving straw bale building in Japan, namely, (one) the lack of availability and high cost of bales in Japan, (two) the relative incompatibility of straw bales and conventional Japanese timber framing and (three) the need for super insulation in only cold regions, one could argue that straw bale building might not be the best form of straw building in Japan. The following methods, light straw clay and old straw tatami mat infill hold significant promise.
Light Straw Clay
Since 2010, Nihon University’s Architectural and Regional Ecological Design Studio has been researching the potential for light earth construction in Japan. Numerous samples using various light weight aggregates, including straw, rice hulls and wood chips, have been made and the thermal conductance measured.
Further details regarding these results will be presented in a future article on earth building in Japan.
With the author’s assistance, in 2013, the first full-fledged light straw clay home was built in Japan. The building was designed by Tadashi Ryoukawa and built by master carpenter Asao Tamai.
The thickness of the light-straw-clay insulation is 120mm, which is the depth of the posts. In order to improve seismic stability, horizontal wood lath was applied to the exterior and interior of the light straw clay walls. In some cases, the lath doubled as permanent formwork, but we found that the lath tended to bow outwards when the wall cavity was packed with light straw clay.
A base coat of earth plaster was applied to the exterior and interior of all light straw clay walls. In order to protect the first story walls from rain erosion and moisture damage, wood siding (rain screen) was used. The second story walls are finished with Shikkui lime plaster.
With passive solar design, the masonry rocket stove only needs firing for roughly 30 minutes a day to keep interior temperatures comfortable in winter.
Although some aspects require further development, light straw clay holds tremendous promise in Japan.
Old Tatami Mat Infill Insulation
Tatami mats are traditionally used as a flooring material in living rooms and bed rooms in traditional Japanese homes. They vary in dimensions, but are generally 1820mm x 910mm x 60mm. They consist of a straw core with thin soft rush finish.
It costs a home owner or tatami manufacturer roughly 1000JPY (approximately $10USD) to dispose of a tatami mat. Old tatami are often salvaged, shredded, mixed with earth and fermented by dorokonya, the businesses which produce earthen plaster for walls and tile roofs.
Otherwise, home owners sometimes use old tatami as mulch under fruit trees or simply allow them to decompose in some back corner of the garden. In either case, the brocade edging and strings can be problematic.
The use of old tatami mats as infill insulation continues to spread in Japan. It is difficult to say who first thought of the idea, and there are no organizations in Japan promoting the reuse of old tatami mats as insulation. The following are just a few examples of the use of tatami mats as insulation.
In 2008, Permaculturist Phil Cashman renovated a home in Hayama, Kanagawa Prefecture with old tatami mats and finished the walls with earthen plaster.
In 2012, Akira Yamaguchi built a small structure of bamboo, tatami and earth in Kimino-cho, Wakayama Prefecture.
In 2014, the author led the finish plastering of a home renovation using tatami as insulation in Ayakawa-cho, Kagawa Prefecture. The designer and head carpenter was Yoshiro Benjamin Iida.
Two methods were used. In one method, the tatami functioned as the base for a Shikkui lime plaster, similar to a drywall or plywood panel. Shikkui was chosen because of its strong adhesive qualities. In this case, tatami mats were attached to the posts and horizontal 36mmx36mm framework with washers and screws. The joints were first treated by applying a thin coat of Shikkui, pressing in a strip of burlap tape, and then following with a second coat of Shikkui. After several months, there has been one small crack, and the author would recommend gypsum when preparing joints for greater strength.
In the second method, tatami were cut to fit between posts and horizontal wood lath was attached to the posts, covering the tatami. A base coat of earthen plaster was applied.