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This article was originally published in the July/August 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

 

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Home Energy Magazine Online July/August 1996


New Pioneering in Straw Bale Building

Two hundred million tons of straw are wasted every year in the United States. Today, innovative builders are turning this waste into a versatile, cheap, superinsulating wall material. 
straw bale house under construction

 


A Nebraska style wall-raising in progress. In Nebraska style straw bale buildings, the bales are loadbearing, or structural, supporting the roof load. Note the absence of framing; rebar pegs (right) hold the bales together. 

by Alex Jaccaci and Steven Bodzin

Alex Jaccaci is president of Archisun Associates in Durango, Colorado, designers and builders of ecological homes. Steven Bodzin is Home Energy's assistant editor.

 


No, they're not living in haylofts. The horses don't knock the houses down for bedding. And the big bad wolf doesn't chase the residents off in search of bricks and mortar. Instead, people living in straw bale homes enjoy excellent soundproofing, superinsulated walls, and structural stability.

Bale houses, which have been around for almost a century, are becoming more common as people recognize the benefits of using field waste as a building material. Straw bale house building helps offset the exploitation of natural resources, and it's sustainable, energy-efficient, and affordable. These homes have been built at very low cost by resourceful owner-builders. And though contractor-built straw bale homes are close to the price of conventional homes, there are no conventional wall systems in the same price range with insulation values upwards of R-50, excellent fire and pest resistance, and minimal resource use.

There are various methods of building straw bale homes, but all involve constructing walls by stacking large bales of straw and then plastering the exposed surfaces with cement, lime, mud, or stucco. Straw, the dry stalks of grain plants such as rice and wheat, is different from hay, which is moister grain stalks with seeds still attached. Bales cost between 50 cents and $3.50 each in different parts of the United States.

A wall made from bales is between 14 and 23 inches thick, depending on the size of the bales and the direction in which they are laid. Bale walls are moisture permeable, but with proper design and construction, they can be protected from damaging levels of moisture. The only major disadvantages to building with straw are that the thickness of the walls can take up valuable space and may require a larger foundation.

Energy Efficiency

Straw bale homes are extolled for their sound insulation, strength, beauty, and low cost. Energy efficiency is an equally important attribute. Energy simulations on straw homes have found that they outperform all other wall types, including adobe, fibrous concrete panel, and standard wood frame.
Joseph McCabe, at the University of Arizona, documented impressive insulating values for straw bale homes. He found that the insulation value differs depending on whether the bales are measured with or against the grain. (Straw is lined up within the bales, allowing air and heat to pass through more easily in one direction than the other.) The wheat straw bales he measured had R-values averaging 2.4 per inch with the grain and 3.2 per inch against the grain. A three-string bale is about 23 inches x 16 inches x 42 inches, so the walls are either 23 inches thick with the grain or 16 inches thick against the grain. Lighter, easier-to-manage two-string bales are about 18 inches x 14 inches x 36 inches. The three-string bale walls are between R-51 and R-55, and even the smaller two-string bales are R-42. Real R-values can vary as much as 25% between one bale and another, but even at the low end of that range, 23-inch bale walls have R-values upwards of R-43. With the addition of stucco or plaster, the R-value of the wall could be even higher. Stucco and plaster also add thermal mass to help passive solar homes retain heat. 
straw bale house interior

 


While the thickness of straw bale walls may be a disadvantage when available space is tight, it also makes for deep window wells, which can be both functional and decorative. 

In another study, Joe Huang, of Lawrence Berkeley National Laboratory (LBNL), ran DOE-2 simulations on straw bale houses proposed for the Navajo Nation in Arizona. He compared straw bale with other wall materials, including adobe, wood frame, and insulating concrete forms. None of the other materials measured up to straw bales when it came to thermal characteristics (see Table 1).

Huang also compared the effects of various energy conservation strategies on a wood-framed structure. Only passive solar design reduced heating and cooling loads as much as converting wood frame walls into straw bale walls (see Figure 1).
 
 


Figure 1.
Building energy researchers modeled several options for reducing heating and cooling loads in Albuquerque, New Mexico. The computer simulation progressively added features to a base case house that had a slab floor with 1-inch perimeter insulation, an R-30 roof, R-19 wood-frame walls, double-glazed windows, and medium infiltration levels. The greatest energy savings came from passive solar design and straw bale walls.

According to the 1993 Plastered Straw Bale Conference's Working Group Reports, the annual energy cost for heating and cooling of a straw bale home is half that of a conventional one. And a 1995 report from the Department of Energy (DOE), House of Straw, adds that the construction cost of a structural straw bale wall is about one fourth that of a comparably insulated conventional wall.

Straw bales can also be used in retrofit situations. According to Paul Lacinsky of the Northeast Sustainable Energy Association, bales can be wrapped around concrete or steel buildings for superinsulation. It is also possible to wrap some types of wood-framed structures.

The energy properties of straw walls are remarkable, but walls alone do not make a house. Roofs, windows, and doors are all potential energy drains. To get the most benefits from the superinsulating walls, designers and builders must pay extra attention to these nonstraw parts of the house. The reduced heating and cooling loads of a fully superinsulated house may allow installation of smaller heating and cooling systems, saving more energy and money.

post-and-beam home under construction

 


In this post-and-beam home under construction near Santa Fe, New Mexico, straw bales are being used as insulating infill. Because these bales are not supporting roof loads, they do not need to be compressed before the roof is built.

Fire and Moisture

Images of stacked hay make people worry about moisture, mold, fire, and unwanted varmints, but these concerns are mostly unwarranted. The hollow, woody stalks that make up straw have a relatively low moisture content and no seed heads. Compared to hay, straw provides little material to support biological activity. House of Straw indicates that plastered straw bales provide even fewer opportunities to house insects and vermin than conventional wood framing, since there are fewer natural voids in the walls. 

Tests of fire resistance conducted by the National Research Council of Canada and in the state of New Mexico found that the stuccoed straw bale walls perform better than most conventional building materials. Tests in New Mexico following the ASTM E-119 Two-Hour Test procedures subjected a bale wall to 1,942 degrees Fahrenheit heat for two hours. The wall was plastered on the heated side and stuccoed on the other. Some plaster cracked on the heated side. Where the cracking occurred, the bales were charred only to a depth of 2 inches, demonstrating exceptional fire resistance.

A straw bale wall can be damaged by moisture, but proper construction techniques make bale construction feasible in almost any climate. According to David Eisenberg, president of Development Center for Appropriate Technology in Tucson, Arizona, straw bale walls must be treated similarly to wood frame walls. Since any wall will allow some moisture in at some time, at least one side of the wall must be breathable to allow the wall to dry out. Eisenberg says that moisture-sealing techniques will vary depending on climate. In colder climates, it is critical (as with other types of construction) to ensure that there is no air leakage into the walls from the interior of the building. Generally, he does not recommend a moisture seal on the outside of the wall. In fact, he points to a straw bale house in eastern Washington that has no exterior finish and has experienced no moisture degradation in 11 years.

Either way, proper construction protects the walls from direct moisture. Where driving rain is likely, a roof overhang is useful. In wet climates, the foundation should keep the bottom bales at least 6 inches above grade to protect them from standing water and rain splashes. A moisture barrier is usually placed between the foundation and the first course of bales.

Straw bale homes have stood without signs of deterioration for many years in wet areas such as Huntsville, Alabama (60 years) and France (75 years). Eisenberg cites moisture monitoring in straw bale buildings in Quebec and Nova Scotia--cold, damp maritime climates. Even there, the walls remained below the threshold of moisture that would cause concern.
 

Carrying the Weight

Early straw bale homes on the Nebraska plains used bales to support the weight of the roof. This use of structural bale walls has come to be known as the Nebraska style. Today, there are various straw bale construction techniques ranging from the Nebraska style to framed structures using the bales for infill between the studs, or a hybrid of the two. Any of these methods can provide a strong, structurally sound home, if it is well-designed and well-built.

Load-bearing bale walls in Nebraska style homes must be built carefully to prevent collapse. Because the weight of the roof and live loads such as snow have the potential to deform the bales, the walls must be compressed by roof loads for three to ten weeks before the roof is put up or plaster coatings are applied.

The roof's weight should be evenly distributed, so many Nebraska style homes are one story high and have hipped roofs. In locations with heavy snow loads, shorter roof spans, hybrid designs, or post-and-beam construction may be advisable. A short roof span lessens the snow load. California architect Bob Theis repeatedly loaded a rice bale wall, and found that typical 10-week wall compression prevented the wall's deflection under snow loads one-and-a-half times larger than the loads anticipated at his northern California site.

Non-load bearing bale walls require the roof and live loads to be supported by framing. The bales act as infill and have far fewer structural requirements. Typically, the non-load-bearing bale wall will have little or no deformation and can be surfaced at any time. There is greater design freedom with this method, since the engineering is limited only by the framing material. It is usually easy to form the bale walls to fill in the frame of the building. Because the bales need only support their own weight, less dense bales can be used, and bales can be laid on edge for thinner walls.

Hybrid solutions, where the bales carry some of the load and other structural elements carry the rest, require careful forethought and a good understanding of structures. These houses must be designed and built to accommodate the potential difference in compression and movement of the wood framing and the structural straw bales without failing. 

Straw bale house built in 1925

 


The Martin/Monhart house in Arthur, Nebraska, was built of baled meadow hay in 1925--an example of the strength and longevity of this sustainable, energy-efficient building material. 

 

A new method, recently tested in Canada, prestresses the bales, creating the strongest structural bale walls yet. The walls are built as panels, which are prestressed with a powerful binding machine. The resulting panels were very strong in structural tests conducted by architect Linda Chapman and engineer Bob Platts, in Ottawa, Ontario. They are now exploring the possibility of prestressed straw bale roofs.

The biggest differences between Nebraska style and framed bale walls are in construction time and cost. Straw bale builders have built very inexpensive Nebraska style homes by getting cheap or free materials and using family and friends as laborers. All the bale walls of a medium-size Nebraska style house are often raised in a day, even with a mostly inexperienced crew. Because non-load-bearing walls necessitate framing, they require more construction time, cost, and resources.

 


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Working with Building Codes

Across North America, building departments are starting to accept straw bale construction. Since Tucson granted the first building permit for a Nebraska style home in 1993, towns in California, Washington, Oregon, Maine, Colorado, and Florida have followed suit. At least 32 states have modern straw structures.

David Eisenberg of the Development Center for Appropriate Technology has been one of the key players in writing a set of prescriptive codes for southern Arizona's Pima County Building Department and the City of Tucson. These codes for load-bearing and non-load bearing straw bale structures were adopted in January 1996 and are now an appendix chapter to the local adoption of the 1994 Uniform Building Code (UBC). According to Eisenberg, This is not the universal straw bale code, but one specifically developed for this area and its particular design requirements.... This code provides a starting point for code discussions and processes in other regions.

California Assembly Bill 1314, signed into law October 15, 1995, established guidelines for local code jurisdictions to voluntarily adopt a version of the codes written for the Tucson area. Napa and Yolo Counties have adopted the code language in AB 1314 into their building codes. Also, the state of Nevada has enacted AB 171, which mandates the development of local building codes for straw bale construction.


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Amber Panels of Grain

The wizards in the straw industry are happy that straw bale exteriors are catching on. But aside from bales, straw can also be used to make panels and composite wood. Rigid straw panels have been made and used in Europe and Canada for over 40 years, but they've never been successfully commercialized in the United States. Now, mills from Texas to South Dakota are producing such panels for use as interior partition walls, cabinets, and even exterior structural walls. They are made by cooking straw under pressure, causing the fibers to bind to one another very securely.

Agriboard Industries of Fairfield, Iowa, is now gearing up to begin production of its straw panel system. Their panels are made of compressed wheat straw covered in a stressed skin of oriented strandboard (OSB). They boast that the panels offer more fire resistance, acoustical insulation, thermal performance, structural strength, and construction speed than conventional wood-framed walls, while cutting out 65% of the framing lumber that a comparably sized wood-framed house would need. The panels are as large as 8 ft x 24 ft and are 4 inches thick, including the two layers of OSB. They are heavy, at 4.53 lb/square foot; the biggest panels weigh 870 lb. The 8-inch thick exterior panels are rated R-28.4, and the combined ceiling/ roof system is rated R-42.8. Similar panels are used in hundreds of homes in climates from Jamaica to Canada. The structural straw panels are even used as roof decking in Europe. The company has no reports of problems with mildew or moisture in the OSB or in the straw; the panels are meant to be covered with exterior building materials to prevent moisture damage.

When the first mass-produced panels roll out in August, they will be for predesigned, panelized homes. Buyers will purchase the building shell as a kit from Agriboard, then finish the interior and exterior with materials of their choice. Soon panels will be available individually, and Agriboard is producing a design manual for architects to help them use the boards effectively.

Compressed straw boards as thin as 1/8 inch are available from Meadowood Industries of Albany, Oregon. This company makes Meadowood panels from rye straw and does not laminate them. Made for designers and cabinetmakers, these panels are made in a variety of shapes, including a corrugated wave. The material comes in different densities, from a lightweight corkboard to heavy, construction-grade material. Both Agriboard and Meadowood panels are effective sound dampers.

The first major U.S. manufacturer of agricultural fiber panels was Stramit USA of Perryton, Texas, which has recently ceased production. Despite very strong demand, they were unable to keep the business going, due to problems with the quality of their straw, with their machines, and with technical support. Agriboard's Bill Thompson believes his business will do better, since they have recruited people who have been working on straw panel fabrication for over 40 years.

 


Further Reading Bainbridge, David, Plastered Straw Bale Construction,The Canelo Project, 1992.

Eisenberg, David, Prescriptive Code for Load-Bearing and Non-Load-Bearing Straw Bale Construction, Tucson: Development Center for Appropriate Technology, 1996.

Eisenberg, David, Straw Bale Construction and the Building Codes, Tucson: Development Center for Appropriate Technology, 1996.

Guidelines for Residential NonLoad-Bearing Straw Bale Construction, State of New Mexico, 1995.

U.S. Department of Energy, House of Straw, Washington, DC: U.S. Government Printing Office, 1995.

MacDonald, S.O. and Matts Myhrman. Build It with Bales, Out On Bale (un)Ltd: Tucson, Arizona, and InHabitation Services: Gila, NM. 1994.

Steen, A., D. Steen, D.A. Bainbridge, and D. Eisenberg, The Straw Bale House, White River Junction, VT: Chelsea Green Publishing Co., 1994.

Working Group Reports, Plastered Straw Bale Conference: Roots and Revival, Arthur, Nebraska, 1993.

 


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