1 of 23 Sustainable Earthen and Straw Bale Construction in North American Buildings: Codes and Practice A. Jenkins Swan, 1 A. Rteil, 2 G. Lovegrove, 3 Abstract: The building industry accounts for up to 40% of the earth’s energy usage from material extraction through building operation, with housing comprising roughly 30% of energy use in North America. Owners and consumers are looking for more efficient building systems that would decrease this use of energy. The material chosen to construct the structure of a building has the potential to reduce the building’s initial environmental impact as well as its life cycle energy use. However, this is rarely considered during conceptual design. Sustainable construction materials that have low embodied energy include earthen construction and straw bale construction. However, these materials are not widely accepted alternatives in North America because they are included only in select building codes in North America and around the world. In this paper, an extensive review of the current construction practice of sustainable construction materials is summarized. Durability concerns and limitations of the methods of construction are discussed and areas of future research are identified. Subject headings: Construction Materials, Construction Methods, Standards and Codes, Sustainable Development Introduction Sustainability is “the maintenance of ecosystem components and functions for future generations” while sustainable development is “development that meets the needs of the present 1 PhD graduate student in Civil Engineering, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected]2 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected], Tel: 250-807-9626 3 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected], Tel: 250-807-8717
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Sustainable Earthen and Straw Bale Construction in North American Buildings: Codes and Practice
A. Jenkins Swan,1 A. Rteil,2 G. Lovegrove,3
Abstract: The building industry accounts for up to 40% of the earth’s energy usage from material
extraction through building operation, with housing comprising roughly 30% of energy use in
North America. Owners and consumers are looking for more efficient building systems that
would decrease this use of energy. The material chosen to construct the structure of a building
has the potential to reduce the building’s initial environmental impact as well as its life cycle
energy use. However, this is rarely considered during conceptual design. Sustainable
construction materials that have low embodied energy include earthen construction and straw bale
construction. However, these materials are not widely accepted alternatives in North America
because they are included only in select building codes in North America and around the world.
In this paper, an extensive review of the current construction practice of sustainable construction
materials is summarized. Durability concerns and limitations of the methods of construction are
discussed and areas of future research are identified.
Subject headings: Construction Materials, Construction Methods, Standards and Codes,
Sustainable Development
Introduction
Sustainability is “the maintenance of ecosystem components and functions for future
generations” while sustainable development is “development that meets the needs of the present
1 PhD graduate student in Civil Engineering, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected] 2 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected], Tel: 250-807-9626 3 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: [email protected], Tel: 250-807-8717
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without compromising the ability of future generations to meet their own needs” (Meadows
2004). From these definitions, a possible definition of sustainable building material is - a
material that is harvested, produced and/or manipulated to a usable building form in such a way
as to have no negative impact on future generations during the material's life cycle and disposal.
With the increase in general and political interest in environmental matters such as peak oil
and climate change the public is asking for more environmental accountability in all matters,
including building construction and maintenance (Stern 2007; Lippiatt 1999). The building and
construction industry accounts for up to 40% of the world’s energy (Lippiatt 1999) combined
with approximately 40% of its raw material usage (Meadows 2004; Pulselli 2007). Within these
numbers, it has been reported that the structural system accounts for 25% of the building’s
environmental impact (Webster 2005). Most of a building’s overall impact is during its operation
(Zhang et al. 2006) therefore, if a structural system could also influence this portion of the
building’s life cycle it would be of even greater sustainable significance. There are some
professionals and builders who are trying to meet the public demand and even help create it in
constructing “green” buildings. These include builders using materials that have been all but
forgotten in the last 50 years due to a variety of factors. Such materials include straw bale and
earthen construction including cob, adobe, rammed earth and compressed earth block. Building
codes and engineering guidelines play an important role in supporting this shift to alternate
construction materials, as does appropriate and recognized testing.
Steel, concrete and timber have been tested and approved as the mainstay materials for the
building and construction industry of today. However, each of these materials must be extracted
or harvested at one, or several, site(s), transported to a different location for processing, and
transported again to the construction site for installation. The amount of energy required for these
operations and the material’s disposal is called embodied energy. For each of these steps, energy
is used and waste is produced, albeit varying levels depending on which material is harvested.
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Timber is generally recognized as the materials with the least embodied energy of the three.
More sustainable building materials such as straw bale and earth have substantially less embodied
energy than processed materials. Straw is a waste material from grain harvest, and earthen
construction can be comprised of soils available within a building’s footprint. This creates a
much different material production path than that of steel, concrete or timber.
In addition, the materials themselves are more energy efficient within the building
envelope. The thermal resistance of a wall material in North America is represented by the R-
value. A higher R-value indicates a more insulating wall material. As an example, Pierquet
(1998) built two houses, one with straw bale walls and the other with wood-frame walls. The
roof system for the study was consistent for each wall type: a gable-style roof with 2x4 trusses at
600 mm (24 in.) on center, 12.5 mm (1/2 in.) gypsum wall board ceiling and R-44 blown
cellulose insulation. It was found that to enclose a floor area of 71.3 m2 (768 ft2), 2.4 m (8 ft.)
high walls constructed of straw bale with stucco on both faces have an embodied energy of
26,484 MJ and an R-value of 44.8 ft2Fh/Btu (RSI-value of 7.90 Km2/W) compared to standard
2x6 wood-frame residential exterior walls clad in vinyl siding with embodied energy of 33,314
MJ and R-value of 22.2 ft2Fh/Btu (RSI-value of 3.91 Km2/W) (Pierquet 1998). A stabilized
compressed earth block, mortar and stucco wall built to create the same wall area as above have a
total embodied energy of 13,213 MJ (Venkatarama Reddy, 2003; Shukla 2008) – roughly 50%
that of straw bale and 40% the embodied energy of wood-frame.
The comparison of the embodied energy and the insulating properties presented above clearly
shows that the sustainable construction materials are preferable to wood-frame and by extension
to concrete or steel. Uncertainty of the strength of these materials in resisting lateral loads (wind,
earthquake), which was likely compounded by informal construction methods, combined with the
reputation perpetuated by poor maintenance (Fitzmaurice 1958) led to a decline in the use of
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these materials in the last century. However, centuries old structures built of earth and straw have
withstood windstorms and earthquakes. In this paper, a review of sustainable construction
materials including straw bale and earthen construction will be presented and current construction
practice will be summarized as per available North American prescriptive codes. As with any
material, durability concerns and potential limitations exist for straw bale and earthen
construction and these are discussed followed by identified research needs.
Sustainable Construction Materials
Straw bale construction and earthen construction including cob, adobe, rammed earth and
compressed earth block are examples of building materials that are experiencing a resurgence of
use in the sustainability context due to the availability of these materials, their ease of
construction and insulating properties.
Straw Bale Construction
Straw bales can be thought of as building blocks of compressed cereal grain stalks. During
the harvest of wheat, barley, or rice the heads of the grain are harvested and the stalk left in the
field to dry. The straw is a waste product that is cleared from the field so that the next year’s crop
can grow unhindered. Once dry, the stalks are collected with a harvester and made into
compressed bales. Bale size varies, however those used in wall construction are approximately
381mm x 584mm x 1220 mm (15 in. x 23 in. x 48 in.) and weighing about 18-36 kg (40-80 lbs.).
The size and weight of the bale are dependent on the type of harvesting machine used and are set
by the farmer. Straw has typically been used for livestock pen bedding, produce and plant winter
protection, and other miscellaneous agricultural insulation uses. When there is more straw than
needed it is often burned as waste material (California 2009). Traditionally burning took place in
the field; however, recently it has started moving to the biomass generator (Kadam 2000).
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Straw bale construction originated on the Nebraska plains in the late 1800’s (Henderson
2007). As there was little timber, the settlers used what was available to them and invented load
bearing straw bale construction. The method of construction gained popularity due to its
reputation for having a cool interior in the summer while alternately remaining warm in the
winter. The oldest straw bale structure still standing is the Burke house in Alliance, Nebraska
built in 1903 (King et al. 2006).
Earthen Construction
Earth is the oldest and most traditional building material in the world. Up to 30% of the
world’s population continues to live in earthen construction (Binici et al. 2007). Such
constructions include cob, adobe, rammed earth, and compressed earth block. Earthen
construction is based on a clayey sand or clayey silt mix with varying levels of water. An
additional binder such as cement may be included to stabilize the mix; as well a local
reinforcement such as straw may be added. For cob, a clayey sand mixed with straw is laid up by
hand continuously in a thick wall, often up to 610 mm (24 in.) in thickness. This method is
common to the British Isles. In adobe construction, earthen blocks are made from a clayey sandy
silt mix with or without straw or cement, however, the durability of the blocks is greatly increased
with the presence of straw and cement (Bouhicha et al. 2005). The blocks are cast in moulds and
left to dry in the sun. Once dry the blocks are built into walls in masonry type construction with
mortar. Rammed earth construction incorporates formwork similar to concrete cast-in-place wall
construction. The dry (less than 4% water by weight) clayey mix is compressed in lifts within the
forms allowing the creation of horizontal colorations that often attract people to this type of
construction. Compressed earth block (CEB) is similar to adobe, however, the blocks are created
under pressure, expelling the excess water and eliminating the need to sun dry the blocks thus
resulting in a higher strength block with less curing time (Morel et al. 2007).
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The “Manual on Stabilized Soil Construction for Housing” (Fitzmaurice 1958) sponsored by
the United Nations discusses the poor reputation earthen structures has received due to poor
maintenance of non-stabilized soil structures and the need to separate stabilized from non-
stabilized structures. The manual discussed the properties and testing of soils, production of
stabilized soil for walling, surface coatings, design data and economics of stabilized soil for
building. The psychological factors affecting development of stabilized soil is also discussed in
the manual with the conclusion that prejudice to soil construction is irrational. Regardless of this,
it seems earthen construction’s reputation within the North American general and engineering
communities was tarnished or lost in the last 50 years. However, due to the resurgence of
sustainable methods earthen construction requires a second look.
Construction Practice
Current construction practices for sustainable construction materials are summarized in this
section based on the available North American codes and standards. It must be noted that these
are the minimum required prescriptive building standards in the US for buildings requiring
building permits. It is recognized that, in many parts of the world, straw bale and earthen
constructions are built without permitting and may not be built to these standards or may be built
using other methods under the Alternative Materials Section of the local code.
Straw Bale Construction
Current Codes and Standards
Although straw bale construction has its root in the US mid-west it was in the southwest US
where the construction method was first formalized into local building codes. To be initially
accepted in the New Mexico code the fire rating was demonstrated to local officials (Henderson,
2007). Acceptance into the Pima County, AZ code (Pima County 2007a) as a load-bearing
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element was based on historical evidence and testing that was conducted at the University of
Arizona in 1993 by Bou-Ali (Henderson 2007). Individual bales were tested followed by 2.44m
high x 3.66m long (8 ft. x 12 ft.) wall assemblies. The wall assemblies were pinned with bond
beams at roof level, but were not plastered or otherwise finished. The gravity load capacity of the
wall was 19.2 kN/m (King 2003). This led to load bearing straw bale wall code inclusion
gradually growing throughout the southwest US over the last 25 years including Austin City,
Texas (Texas 1997); City of Boulder, Colorado, and the state of California (California 2009). It
has since spread globally with Belarus adopting the “Compressed Straw Construction Bales (Heat
Insulating) – Technical Condition”. In countries such as the Netherlands, UK, Australia and
South Africa the method of construction may be used if designed by an engineer under the
“Alternate Methods” section of the local building code (King et al. 2006).
There are differences between the codes in terms of the extent and the detail of the
guidelines; however the basic information of testing techniques, expected capacities and
construction methods is relatively consistent. The following summary on straw bale construction
is based on local amendments to the IBC (2006); the codes are prescriptive in nature and
therefore do not typically require design by an engineer. Construction with straw bale outside
jurisdictions with local amendments must be permitted and built through the Alternative
Materials Section of the applicable code. It should also be noted that there are alternate methods
to construct with straw bale, however, use of these methods also require permitting and building
through the Alternate Materials Section.
Construction Method
The stacking, reinforcing and anchoring requirements for a typical straw bale construction,
per available prescriptive codes, are illustrated in Figures 1 and 2. Bale reinforcing is comprised
of 10M (#4) rebar driven from the top of the bales with a minimum of 2 bars per bale and 200mm
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(8 in.) from the edge of the bale. The rebar pins are required at the fourth course and above. The
roof bond beam assembly must be anchored to the foundation in at least two locations per wall
and at a maximum spacing of 1.82m (6 ft.). Anchoring is completed with a 12.5mm (1/2 in.)
diameter threaded rod coupled to a 12.5mm (1/2 in.) diameter anchor bolt embedded at least
178mm (7 in.) into the concrete foundation (Fig. 1) (Pima County 2007a). Interior and exterior
walls must be finished with either cement stucco with wire mesh, or plastered with earthen or
lime plaster applied directly to the bales. If earthen plaster is chosen the exterior exposure must
be stabilized (Austin 1997; Pima County 2007a).
Allowable Dimensions
Straw bale construction is limited to one or two-family dwellings and utility/accessory
structures. Single storey walls must be a minimum 356mm (14 in.) thick. Wall height to width
ratio is limited to 5.6:1 (i.e. a height of 3.25m (10 ft. 8 in.) for a 584mm (23 in.) wall thickness)
and the wall unsupported length to thickness ratio of 15.7:1 (Pima County 2007a) (i.e. a 9.14m
(30 ft.) unsupported length for a 584mm (23 in.) wall thickness). The area of openings in a straw
bale wall is limited to 50% of the total interior wall area if the wall is resisting lateral load.
Connections with other building elements
To attach intersecting wood stud walls to bale walls there are three approved methods. The
first method is to drive a 16mm (5/8 in.) diameter wooden dowel 300mm (12 in.) into the bale
through bore holes in the abutting stud and spaced to provide one dowel per bale. The second
option uses wooden stakes at least 300mm (300 in.) in length and 38mm x 89mm (1-1/2 in. x 3-
1/2 in.) at the exposed end into each course of bales as anchorage points for the abutting stud.
The third method incorporates bolted or threaded rod connections at the abutting wall, through
the bale wall to a steel nut and 5mm (3/16 in.) thick steel or 12.5mm (1/2 in.) thick plywood
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washer at least 150mm x 150mm (6 in. x 6 in.) square in a minimum of three places (Pima
County 2007a).
Mechanical and other engineering properties
The allowable vertical load (from dead and live loads) with the resultant at the center of the
wall is 17.2 kPa (360 psf) per Arizona Standards (1996), 19.1 kPa (400 psf) per Austin Standards
(2002), and 11.7 kN/m (800 plf), according to California Building Standards Code (2009). In
addition, California Building Standards Code (2009) allows straw bale walls plastered on both