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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: amy.jenkins.swan@alumni.ubc.ca 2 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: ahmad.rteil@ubc.ca, Tel: 250-807-9626 3 Assistant Professor, School of Engineering, University of British Columbia Okanagan, Kelowna, BC Canada V1V 1V7, E-mail: gord.lovegrove@ubc.ca, 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

sides to resist 5.25 kN/m (360 plf) lateral loads (wind and/or seismic).

Tests specified in codes and standards

Straw bales to be used in straw bale construction must be tested for moisture content and dry

density with five bales randomly selected and tested (Pima County 2007a). To determine the

moisture content one of two methods may be used: (1) the field method where a moisture meter

specifically designed for straw or hay is inserted to the center of the bale; (2) the laboratory

method which determines the moisture content by dividing the initial weight of the bale less the

dry weight of the bale by the dry weight. The maximum allowable moisture content is 20%

(Pima County 2007a).

The dry density must be laboratory tested and is the weight after removing moisture content

divided by the volume of the bale. The minimum dry density of the bales to be used in

construction is 112.1 kg/m3 (7 pcf) (California 2009).

Earthen Construction

Current Codes and Standards

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Due to the prevalence and history of earthen construction, its presence in existing building

codes is more predominant than that of straw bale construction. Yet at the local level in North

America, the US southwest is again the leading region for code inclusion with Pima County

(2007b) and New Mexico (2008a) setting the pace. The International Building Code (IBC) has

incorporated adobe construction into the masonry chapter, 2109.8 (ICC 2006). The current local

building code practice is to adopt the IBC and amend sections to local conditions or regulations

where appropriate. Other national level codes include the Turkish Adobe Code and Standard (TS

2514) (Binici et al. 2007), Australian Standards HB 195 (Walker et al. 2002), New Zealand

Standards NZS 4297, 4298 and 4299 (1998) and the Peruvian Adobe Seismic Code (MTC 2000)

(Vargas et al. 2006). Within the African Regional Organization for Standardization (ARSO)

there are compressed earth block standards and also compressed earth block testing procedures

(CDE 2000).

Construction Methods

In general, earthen materials may be mixed on site or produced by licensed producers but

must contain not more than 0.2% soluble salts (Pima County 2007b). Within local code

amendments stabilized soils are those that contain a minimum of 6% portland cement by weight

or a soil that has been qualified by passing the specified tests (New Mexico 2008a; Pima County

2007b). For adobe construction, IBC (2006) restricts mortar to that complying with IBC Chapter

21 or to adobe soil complying with material requirements for stabilized adobe, with mortar for

unstabilized adobe restricted to portland cement mortar. Compressed earth blocks are to contain

mineral soil with aggregate less than 25.4mm (1 in.) diameter and soluble salts less than 2% (New

Mexico 2008a). Finally, rammed earth mix must not contain rock greater than 38mm (1-1/2 in.)

diameter, nor clay lumps greater than 12.5mm (1/2 in.) diameter. The mix must be free of

organic matter and contain not more than 2% soluble salts (New Mexico 2008a).

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Mix design and optimum water content is determined during material testing and must be

maintained during production and construction. Periodic testing is required during construction

to ensure materials used are represented by the tests (Pima County 2007b). Adobe and

compressed earth blocks are left to cure while rammed earth must be mixed and placed in the

formwork within sixty minutes. Rammed earth forms can be stripped immediately provided

adjacent wall ramming does not affect the structural integrity of the completed walls (New

Mexico 2008a).

Unstabilized exterior earthen walls must be adequately finished in stabilized stucco or

equivalent (New Mexico 2008a; Pima County 2007b). Moisture protection and finishing

requirements are detailed in IBC (2006) and local amendments. Stabilized materials must be

used within 100mm (4 in.) of the foundation, the top 100mm (4 in.) of parapet, and around roof

drains (New Mexico 2008a). Outside these jurisdictions the exterior face of unstabilized adobe

walls must be covered with a minimum of two-coats of portland cement plaster having a

minimum thickness of 75mm (3/4 in.) with lathing (ICC 2006).

Allowable Dimensions

Exterior walls in one-storey construction must be at least 254mm (10 in.) thick and have a

maximum unsupported lateral length of 7.32m (24 ft.); interior walls can be 200mm (8 in.) thick

(ICC 2006). Wall height must be less than ten times the wall thickness (ICC 2006). Adobe is not

permitted for load-bearing isolated piers or columns; therefore walls must be at least 600mm (24

in.) long (ICC 2006).

New Mexico Standards (2008a) allow two storey constructions. Earthen walls of the lower

storey must have a minimum thickness of 356mm (14 in.) while the upper storey walls must have

a minimum wall thickness of 254mm (10 in.). Wall height to thickness ratio and lateral support

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spacing requirements remain as per single storey construction. Bond and tie beams must be

constructed of either concrete or wood (New Mexico 2008a).

Connections with other building elements

Bond beams must be provided at floor and roof bearing. Concrete bond beams must be

150mmx254mm (6 in. x 10 in.), reinforced with two 10M (#4) bars, with concrete minimum

compressive strength of 18 MPa (2500 psi). Wood bond beams can be made up of multiple plys

with minimum dimensions of 150mmx254mm (6 in. x 10 in.). Joints in wood bond beams must

have a minimum 150mm (6 in.) splice with no splices allowed within 300mm (12 in.) of an

opening. Wood must be naturally decay resistant or pressure treated (ICC 2006). For a 16mm

(5/8 in.) diameter bolt embedded 300mm (12 in.) in adobe masonry the allowable shear is 890N

(200 lbs.) (ICC 2006).

At intersecting walls, whether they are intersecting earthen construction or abutting framed

walls, the design must allow for the difference in material properties. New Mexico (2008a)

details requirements when intersecting a rammed earth wall. When intersecting with another

rammed earth wall, such as one providing lateral support, a 75mm (3 in.) deep by 150mm (6 in.)

long vertical keyway is required. When adobe wall is to abut the rammed earth wall, a keyway

into the rammed earth less than one-third (1/3) the rammed earth wall thickness can be provided

or reinforcing steel ladder rammed into the wall and bent into the forms can be provided every

four courses. Wood or steel frame walls may be attached to the rammed earth wall in one of two

ways: (1) provide 12.5mm (1/2 in.) diameter anchor bolts with a 100mm (4 in.) hook embedded a

minimum of 300mm (12 in.) into the rammed earth wall and spaced at 600mm (24 in.), with

plates and nuts to secure the anchor bolt to the vertical stud; (2) provide an 18 gauge galvanized

steel strap-tie grouted into the concrete bond beam and nailed to the top plate of the stud wall.

The vertical stud must also be fastened to the rammed earth wall with 30D nails or screws

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embedded a minimum of 75mm (3 in.) into the rammed earth wall spaced at 200mm (8 in.) (New

Mexico 2008a).

Mechanical and other engineering properties

The earthen construction must have a minimum average compressive strength of 2000 kPa

(300 psi), average modulus of rupture of 340 kPa (50 psi), moisture content less than 4% and

water absorption less than 2.5% (ICC 2006). No unit can have more than three shrinkage cracks

and with no crack longer than 75mm (3 in.) or greater than 3mm (1/8 in.) wide (ICC 2006). Five

samples taken at random from each 5,000 units are to be tested. IBC (2006) details the

requirements for each test. Table 1 summarizes the strength requirements and the allowable

design stresses as listed in the available codes. Note that IBC (2006) covers only adobe block

material, whereas the New Mexico, Pima County and New Zealand Standards include rammed

earth and compressed earth block as well.

Tests specified in codes and standards

Compression test requirements are detailed in IBC (2006) and the local code amendments, as

is the modulus of rupture test. Absorption tests for adobe are explained in the New Mexico and

Pima County standards (New Mexico 2008a; Pima County 2007b) as well as the IBC (2006).

A soil is considered stabilized if it contains a minimum of 6% portland cement by weight or

has passed a wet compression test. The wet compression test is performed after the earthen

cylinder has been completely submerged in water for a minimum of four hours. The absorption

test for stabilized earth blocks involves placing a 100mm (4 in.) cube cut from a sample unit on a

constantly water-saturated porous surface for 7 days. The amount of water, by weight, that is

absorbed determines the type of adobe mix (New Mexico 2008a; Pima County 2007b).

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For the compression strength and modulus of rupture tests, a sample of five units per 25,000

bricks (or each 2,500 square feet of wall) is required for adobe and compressed earth block. The

samples block length should be at least twice its width. Cylinders of rammed earth must pass the

compression test after curing for seven days (Pima County 2007b).

The African Regional Organization for Standardization (ARSO) testing procedures for

compressed earth block include raw material identification tests such as water content, particle

size distribution: wet sieving, sedimentation, consistency limits, methylene blue test, and organic

matter. Manufacturing tests include the testing method for optimum water content. Finished

product tests cover dry and wet compressive strength, dry tensile strength, abrasive strength,

capillary absorption, dimensions, mass and apparent density (CDE 2000).

Durability Concerns

In the North American climate a construction material must withstand a variety of

environmental conditions including rain, temperature fluctuations, and fire to name a few. As

with most structural building elements, the sustainable construction materials discussed here are

meant to be protected from the exterior elements and built into a wall assembly. As such, any

testing and research on the durability of the materials must take this into consideration.

Earthen construction must be properly protected from weather similar to wood-frame

construction. If unstabilized and unprotected it will return to its natural state over time, returning

to the earth and completing the most basic material life cycle. In areas which experience up to

635mm (25 in.) of rain per year unstabilized earthen walls left unprotected are likely to erode at a

rate of 25.4mm (1 in.) in 20 years on vertical surfaces, and 50-75mm (2-3 in.) per year on

horizontal surfaces (ASTM E 2392-05). This process is favorable from a life cycle perspective

and at the end of a building’s life; however, it also led to the poor reputation that was documented

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by Fitzmaurice (1958), and which is unfounded when sound engineering design and construction

practices are followed.

The Canadian Mortgage and Housing Corporation (CMHC) has sponsored straw bale

research with respect to moisture and energy use. Straub (2000a) found 40 mm (1-1/2 in.) thick

cement: sand stucco coated in oil paint will meet code requirements for a vapor barrier.

Alternatively, the permeability of earth plasters is in the order of most building papers and

housewraps (Straub 2000b). Straw bale houses require 20% less energy for space heating than

conventional wood-frame houses (Gonzales 2002) based on actual energy bills from straw bale

homes and equivalent simulated 2x6 wood-framed homes built to British Columbia building

code. These results are consistent with those from the Pierquet et al. (1998) analysis.

Straw bale and earthen construction have demonstrated fire ratings. “Bale walls, when

covered with plaster, drywall or stucco are considered to have the equivalent fire resistive rating

as wood frame construction with the same wall-finishing system” (Austin 1997). Recently the

Ecobuild Network (a non-profit association of builders, engineers, architects and academics)

sponsored an independent fire rating test according to ASTM E119-05a standards. A 3m x 3m (10

ft. x 10 ft.) wheat straw bale assembly with 17 GA stucco netting and 25.4mm (1 in.) of cement

stucco successfully met and passed the ASTM E119-05a two hour fire rating test. The same test

performed on a 3.65m x 4.27m (12 ft. x 14 ft.) non-load bearing wheat straw bale assembly clad

in 25.4mm (1 in.) of earthen plaster resulted in one hour fire rating (Lawrence 2009). Earthen

building walls of minimum 254mm (10 in.) thickness have a fire resistance rating of two hours

(Pima County 2007b).

Limitations

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To date the available building codes for sustainable construction materials have limited

construction to two-storey structures. Typically residential and outbuilding structures may be

built to the prescriptive codes, while additional structures are permitted within the Alternative

Materials section of the code or when designed by an engineer. Adobe earthen construction has

been included in the IBC (2006), however; straw bale has only been included within local

amendments and is not part of the more far reaching IBC (2006). Due to the limited code

acceptance of these materials, the building and engineering community is slow to accept these

materials as legitimate structural materials. As well, perceptions of building officials, as

evidenced during the development of the New Mexico and Pima County straw bale codes, can

stall acceptance, code inclusion, and construction (Henderson 2007).

Within the IBC (2006), the use of adobe construction is limited to low to moderate seismic

regions. However, Peru has adopted the Adobe Seismic Code due to the high seismicity of the

region and the high incidence of earthen construction (Vargas et al. 2006) illustrating that earthen

materials may be used in such regions with proper engineering design and judgment.

The materials have not been widely included in major North American building codes due to

lack of available information on material properties and wall assembly behavior combined with

the challenges of duability perception. The materials are readily available and can be

manipulated to usable form across the continent which is a blessing and a hindrance. There is no

critical mass of specialized material producers to lobby the code governing bodies, nor do the

grassroots advocates appear adequately organized or able to provide the technical information

required for inclusion. However, there are groups of professionals, such as the Ecological

Building Network, that are actively working to this end with some success (the straw bale code

inclusion in California, for example). To suggest the materials have limited application due to

fire, durability or water penetration issues ignores that the materials are part of a larger assembly,

similar to wood-frame construction.

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A potential limitation on the use of sustainable construction materials is the climate in which

it is constructed. The majority of the jurisdictions that have adopted sustainable construction

material building codes to date have long dry seasons and relatively mild winters. However,

straw bale originated to provide protection from harsh mid-west winters, and cob earthen

structures are prevalent in the UK. The historical evidence indicates that with proper roof

detailing, site grading and potentially the inclusion of an insulation layer, climatic considerations

can be overcome.

Research Needs

Based on the literature review conducted to date and the information presented, the following

research needs were identified.

Few experimental engineering studies have been conducted in North America. Therefore,

a comprehensive experimental program is needed to characterize the mechanical

properties and the structural performance of these materials.

Studies of the long term durability performance of the materials with data collected from

accelerated lab tests and monitoring of real structures.

Research is needed on improvements to the standard mix designs. Potential research

options include alternative stabilising agents, or reinforcing options.

A theoretical study (analytical and numerical) to facilitate the design process, and to

allow the inclusion of these materials in building codes and engineering design standards

is required.

Cost/benefit analyses are needed, including life cycle analysis of construction assemblies.

Conclusions

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Structures built from sustainable construction materials such as earthen and straw bale

construction have stood the test of time for hundreds of years. The materials have enjoyed a

recent resurgence in popularity in the North American market due to the materials’ low embodied

energy and green marketability. However, for a building material to be viable it must also be

durable and structurally sound.

Earthen construction and straw bale construction have been included in several North

American building codes within the last 20 years and earthen construction is included in several

national level building codes including Peru, Turkey, Australia, New Zealand and the IBC. In this

paper a review of the North American prescriptive codes is presented. Both types of construction

are accepted under the Alternative Methods section of the respective code for permitting in many

countries. Despite this, university level research in North America is limited thus affecting the

adoption of these materials. General acceptance and use of these materials in North America

outside the jurisdictions with available codes is restricted. Further research will aid in developing

appropriate engineering design standards and educating building officials, builders and potential

owners.

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Figure and Table Legend

Figure 1: Straw Bale Wall Section

Figure 2: Straw Bale Wall Elevation

Table 1: Earthen Construction Strength Requirements