The Potential for Light Straw Clay Construction in Japan: An Examination of the Building
Method and Thermal Performance

Kyle Holzhueter*1 and Koji Itonaga2
1Research Associate, Graduate School of Bioresource Sciences, Nihon University, Japan
2Professor, College of Bioresource Sciences, Nihon University, Japan

Abstract
Light straw clay is a mixture of straw and clay used as an infill in wood framed construction to create nonload bearing walls. Light straw clay has numerous advantages: Consisting of straw and clay, the raw materials
are locally available throughout most of Japan. Light straw clay's relatively low thermal conductivity provides
good thermal insulation, which reduces energy use and CO2 emissions due to heating and cooling. The tools andtechnology needed to construct light straw clay walls are relatively simple and commonly available. And lastly,
upon deconstruction, the walls can safely decompose without becoming landfill. There are relatively few noncommercial insulation materials consisting of renewable materials. The purpose of the present study is to evaluate
the potential for light straw clay in Japan, including an investigation of the building method, thermal conductivity
and overall heat transfer. The building method was found to take more time than conventional construction, which
in turn increases labor costs. The mean thermal conductivity of the actual light straw clay insulation was 0.067W/mK. The heat transfer coefficient for a cross section of the insulated wall was 0.531W/m2K.
Keywords: light straw clay; thermal conductivity; heat transfer

1. Introduction
Light straw clay is a mixture of straw and clay used
as an infill in wood framed construction to create nonload bearing walls. Earth and straw have been used asinfill in half-timber buildings throughout the world. In
recent years light straw clay has gained international
attention as an ecological building material (BakerLaporte and Laporte, 2015). Light straw clay has
numerous advantages: Consisting of straw and clay,
the raw materials are locally available throughout mostof Japan. Light straw clays' relatively low thermal
conductivity provides good thermal insulation, which
reduces energy use and CO2 emissions due to heating
and cooling. The tools and technology needed toconstruct light straw clay walls are relatively simpleand commonly available. When designed and built
correctly, protecting from moisture damage, lightstraw clay has proven to be durable and resistant todecay (Baker-Laporte and Laporte, 2015). And lastly,
upon deconstruction, the walls can safely decompose
without becoming landfill.
*Contact Author: Kyle Holzhueter, Research Associate,
Graduate School of Bioresource Sciences, Nihon University,
1866 Kameino Fujisawa Kanagawa 252-8510 Japan
Tel: +81-466-84-3364 Fax: +81-466-84-4464
E-mail: nihondaigaku.kairu@gmail.com( Received April 3, 2016 ; accepted December 1, 2016 )DOI http://doi.org/10.3130/jaabe.16.startpage

Currently, Japan has no mandatory thermal insulation
standards for homes. According to the Ministryof Land, Infrastructure, Transport and Tourism,
commercial and residential buildings in Japan are
responsible for 33.8% of all final energy consumption(Mizutani, 2015), and account for 34% of Japan's
CO2 emissions (2007). In 2005, Residential homesalone produced 174 million tons of CO2, of whichapproximately 25% was due to heating and cooling.
The Ministry of Land, Infrastructure, Transport andTourism estimates that energy efficiency improvements(not including water heating, lighting and appliances)such as improvements in thermal insulation could
reduce 8.5 million tons of CO2 emissions per year
(2007).
In 2020, energy efficiency standards will be requiredof all new construction in Japan. There are relatively
few insulation materials consisting of natural andrenewable materials. Light straw clay is a natural,
low embodied energy insulation material capable ofmeeting energy efficiency requirements.
With the author's assistance, in 2013, Japan's first
fully-fledged light straw clay home was built in
Nagano Prefecture (Fig.1.). The building was designed
by Tadashi Ryoukawa and built by master carpenter
Asao Tamai.

Journal of Asian Architecture and Building Engineering/January 2017/221217

in a vertical axis mortar mixer to create clay slip.
Approximately 300 liters of water was added to 1.2
tons of delivered earthen plaster. The clay slip was thenmixed with straw (Fig.3). The clay slip and straw weremixed like salad and salad dressing, turning the straw
with a pitchfork.

Fig.1. Japan's First Light Straw Clay HomeThe purpose of the present study is to evaluate
the potential for light straw clay in Japan, including
an investigation of the building method and thermal
insulation.
2. Materials and Methods
2.1 An Examination of the Building Method
The building process of light straw clay insulation
will be investigated, determining quantities ofmaterials needed and time and labor requirements forconstruction.
The earth used for the project was sourced from anearthen plaster producer in Nagano City. The earth wasoriginally produced as base coat plaster (Jp: arakabetsuchi) (Fig.2.). During the initial production processin Nagano City, high clay content soil was mixed withchopped straw and allowed to ferment, breaking down
the straw into thin fibers and creating an ideal mix forplastering.

Fig.3. Light Straw Clay Mixing Station

Light straw clay was used as thermal insulationin the walls, floor and roof (Fig.4.). In the walls, thethickness of the light-straw-clay insulation is 120mm.
The thickness in the floor and roof is 100mm.

Fig.4. Light Straw Clay Thermal Insulation in Walls, Floor andRoof

Fig.2. Clayey Soil Sourced from Nagano CityHowever, there is no need to ferment clay for lightstraw clay. The higher the clay content, the less clay
is needed to stabilize the light straw clay. The lower
the clay content, the less dense the light straw clay
and the lower the thermal conductivity, which, in turn,provides greater thermal resistance.
Additional water was added to the plaster and mixed

218 JAABE vol.16 no.1 January 2017

In order to improve seismic stability, two additional
measures were used:1) 90mm x 45mm boards were toenailed at every
600mm of wall height as seen in Fig.5. Not only does
this prevent the light straw clay infill from settling, italso prevents an entire wall system from failing at thesame time by breaking the wall system into smaller
compartments, similar to half-timber construction(Langenbach, 2010). Since the 90mm x 45mm boards
are toenailed parallel to interior and exterior finishes
and do not transverse the wall system, they do not act
as thermal bridges.
2) Horizontal wood lath was applied to the exterior
and interior of the light straw clay walls (Fig.6.). In
order to save time, in some cases, the lath doubled aspermanent formwork, but the authors found that thelath tended to bow outwards when the wall cavity waspacked with light straw clay. In retrospect, we might
consider not packing the cavity as densely.

Kyle Holzhueter

2.2 Evaluation of Thermal Insulation
Since 2010, Nihon University's Architectural
and Regional Ecological Design Studio has been
researching the potential for light earth construction inJapan. Numerous samples using various lightweightaggregates, including straw, rice hulls and woodchips, have been made and the thermal conductivity
measured.
The thermal conductivity of the in situ light straw
clay wall is compared with laboratory samples. Twolaboratory samples were made using the same lightclay straw mix (clay slip and straw) as the actual wallin situ (Fig.8.).
Fig.5. 90mm x 45mm Boards Toenailed at Every 600mm

Fig.8. Laboratory Samples
Fig.6. Wood Lath over Light Straw Clay Walls

A base coat of earth plaster was applied to theexterior and interior of all light straw clay walls. Inorder to protect the first story walls from rain erosion
and moisture damage, wood siding (rain screen) wasused (Fig.7.). The second story walls are finished withShikkui lime plaster.

Fig.9. depicts the five testing locations of the in situ
light straw clay wall. When testing in situ, locations 1and 2 were finished with an earthen plaster. Locations
3-5 had not yet been plastered.

Fig.9. Testing in situ

Fig.7. Wood Siding over Earthen Plastered Light Straw Clay
Infill Walls

Through conversations with the architect andbuilder, the labor requirements of this building method
are investigated.

JAABE vol.16 no.1 January 2017

An AINEX KD2 Pro Thermal Properties Analyzer
was used to measure thermal conductivity of laboratorysamples and the actual wall. The KD2 Pro Thermal
Properties Analyzer is accurate to ±0.01W/mK
between 0.02-0.2W/mK. The thermal conductivity ofthe laboratory samples will be compared to the in situ
wall.
Moreover, the U-value and average heat transfer
coefficient (W/m2K) of the insulation layer will
be calculated and compared to Japan's new energy
efficiency standards.
In 2020, the 2013 Energy Efficiency Standards will
become mandatory for all new construction in Japan.
The 2013 Energy Efficiency Standards divide Japan
into eight climate zones (Fig.10.).

Kyle Holzhueter

219

Table 2. Light Straw Clay Materials and Quantities
MaterialQuantity (m3)Straw44.8
Earth10.5

The volume of light straw clay used in walls, floorand roof is listed in Table 3.
Table 3. Volume of Light Straw Clay
Location
Walls
Floor
Roof

Fig.10. Eight Japanese Climate Zones [from Mizutani (2015)]

The 2013 energy efficiency standard's overall heattransfer coefficient for each climate zone is listed inTable 1. These values represent the envelope averagethermal transmittance (UA=W/m2K), in other words,
the area-weighted average U-values for the building
envelope including every component. These values
are calculated by averaging the thermal conductivityof all building components such as walls, roof, floor,windows and doors.
Table 1. 2013 Energy Efficiency Standard's Overall Heat
Transfer Coefficient for Each Climate Zone
Climate Zone 1234
5
678Overall Heat
Transfer
0.46 0.46 0.56 0.75 0.87 0.87 0.87 na
Coefficient2(W/m k)In order to calculate the overall heat transfer
coefficient for a wall section, the thermal conductivity
and thickness of each element is needed. Since
the present study does not examine the thermal
conductivity of earthen plaster, the authors refer to theliterature for such data. According to Horst Schroeder's
Sustainable Building with Earth (2015), the thermal
conductivity of earthen plaster is 0.65W/mK.
Lastly, through conversations with the homeowner,
the indoor thermal environment is subjectivelyevaluated.
3. Results and Discussion
3.1 The Building Method
Table 2. lists the materials and quantities used toconstruct the light straw clay walls, roof and floor.
For every 1m3 of earth, approximately 4.3m3 ofstraw was used.

220 JAABE vol.16 no.1 January 2017

Quantity (m3)
16.77
5.23
5.26

The total volume of light straw clay insulation
is 27.26m3. Thus, for every 1m3 of light straw clay
insulation, 1.64m3 of straw and 0.39m3 of clay wasneeded.
According to the builder, approximately 60% of
total building labor was used to construct the lightstraw clay walls, including finish plastering. For
every 3.3m2 of floor space (Tsubo in Japanese), in
total approximately 7-8 labor days were required forconstruction. Of those 7-8 days, 4 days were spent on
the light straw clay walls including plastering.
The walls specifically required 60 people-days to fillwith light straw clay. At a volume of 16.77m3, the rateof mixing and packing is 3.58m3/person/day.
The Tsubo (3.3m2) unit cost of the building increased
by 15,000JPY compared to commercial insulationconsisting of 85% wool.
3.2 Thermal Insulation
The thermal conductivity of the laboratory
samples (Table 4.) and actual in situ wall (Table 5.)
was measured using an AINEX KD2 Pro Thermal
Properties Analyzer.
The mean thermal conductivity of the actual lightstraw clay wall was 0.067W/mK. For comparison,
the thermal conductivity of concrete is 0.8 W/mK(Young, 1992). Fig.11. outlines a thermal analysis
of a cross section of the light straw clay wall. The
thermal resistance (R-value) at 120mm is 1.791m2K/
W. According to Schroeder (2015), the R-value of
30mm of earthen plaster is 0.046m2K/W. Thus, the heattransfer coefficient for a cross section of the light straw
clay wall, including insulation layer and interior andexterior finishes, is 0.531W/m2K. For comparison, theheat transfer coefficient for 180mm of concrete is 4.444
W/m2K.
Table 4. Density and Thermal Conductivity of Laboratory
Samples
Sample No.
Density
Thermal Conductivity(kg/m3)(W/mK)
1343.145
0.060
2369.395

0.083

Mean356.270

0.072

A complete thermal analysis of the home is beyond
the scope of the current study, and therefore only across section of the light straw clay wall is compared

Kyle Holzhueter

Table 5. Thermal Conductivity of the in situ WallLocation No.
Thermal Conductivity (W/mK)
10.07620.07230.08240.04250.061
Mean0.067

efficiency requirements in climate zones 3-8, but at
t=120mm, light straw clay insulation failed to meet therequirements for zones 1-2. In order to meet energy
efficiency standards for zones 1 and 2, the light straw
clay wall can be made (1) thicker than 120mm and/or (2) the density can be decreased which would also
decrease thermal conductivity.
Acknowledgments
The authors would like to thank architect Tadashi
Ryoukawa for assisting and enabling the study. Without
his help this study would not have been possible.
References

Fig.11. Thermal Analysis of a Cross Section of the Insulated
Wallto Japan's new energy efficiency standards. At 0.531
W/m2K, the cross section of the wall is permitted inclimate zones 3-8. In climate zones 1 and 2, additional
insulation is required.
In order to meet energy efficiency standards forzones 1 and 2, the light straw clay wall can be made(1) thicker than 120mm and/or (2) the density canbe decreased. If the density of the light straw clay
is decreased, which would in turn decrease thermalconductivity and increase thermal resistance, 120mmof light straw clay insulation may pass the heat transfer
coefficient for zones 1 and 2. In practical application,in order to decrease density, purer and stronger clay isneeded.
Lastly, through conversations with the homeowner,
the home seems to be performing well thermally. With
passive solar design, the masonry rocket stove heater
only needs firing for roughly 30 minutes a day to keepinterior temperatures comfortable in winter.

1) Baker-Laporte, P. and Laporte, R. (2015) The EcoNest Home:
Designing & Building a Light Straw Clay House. New Society
Publishers.
2) Langenbach, R. (2010) Rescuing the Baby from the Bathwater:
Traditional Masonry as Earthquake-Resistant Construction. 8thInternational Masonry Conference 2010 in Dresden.
3) Ministry of Land, Infrastructure, Transport and Tourism (2007)
住 宅・ 建 築 分 野 に お け る 省 エ ネ ル ギ ー 対 策 の 現 状 に ついて http://www.mlit.go.jp/singikai/infra/architecture/energy_
conservation/images/070823_3.pdf
4) Mizutani, S. (2015), How do Housing Energy Efficiency Standards
in Japan affect the choice of equipment? Jyukankyo ResearchInstitute Inc.
5) Schroeder, H. (2015) Sustainable Building with Earth, p.472.
6) Young, Hugh D. (1992) University Physics, 7th Ed., AddisonWesley.

4. Conclusion
In 2020, energy efficiency standards will be requiredof all new construction in Japan. There are relatively
few insulation materials consisting of natural andrenewable materials. Light straw clay is a natural,
low embodied energy insulation material capable ofmeeting energy efficiency requirements. The purposeof the present study was to evaluate the potential for
light straw clay in Japan, including an investigationof the building method and thermal insulation. The
building method was found to take more time thanconventional construction, which in turn increases
labor costs. The mean thermal conductivity of theactual light straw clay insulation was 0.067W/mK.
The heat transfer coefficient for a cross section of theinsulated wall was 0.531W/m2K, which met the energy

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221