Straw Bale Design
Buildings often require insulation to either keep in the heat or prevent overheating. Conventional insulation materials are synthetically produced and contain non-renewable, toxic, and often non-recyclable materials. Ironically, we burn enough straw in agriculture each year to comfortably insulate most if not all buildings. And in doing so, turn a waste product into a resource!
It is estimated that enough straw is incinerated each year in the U.S. to build 5 million 2000 square foot homes.
rapidly renewable, agricultural waste, burning, structural, insulation
Straw bale post & beam construction with straw bales as infill insulation. Straw bales used as insulation or as support in a building is becoming more than just a rare occurence, especially in agricultural regions and in colder climates where extra insulation is ideal.
History of Straw Bale
Agricultural materials have been used in construction for millennia. The earliest straw bale structure in North America was likely built in Bayard Nebraska in 1896. It was a ‘Nebraska-style’ load-bearing schoolhouse with a sod roof. These straw bale buildings were meant to be temporary shelter but once people lived in them they realized that the straw bale design afforded superior insulation and protection from the elements.
The first post and beam strawbale home was built in Alabama in 1936. Straw bale popularity waned however after World War II due mainly to the availability of other building materials such as timber and manufactured synthetic insulation products. The energy crisis of the 1970’s revived interest in straw bale and led to Roger Welsch’s 1974 seminal article, ‘Baled Hay’ in the journal Shelter.
The first permitted (bank-financed and insured as well) straw bale structure in the United States was built in 1991 in Tesuque, New Mexico by Virginia Carabelli. ‘The Last Straw’ newsletter began circulation this year as well by pioneer straw builders Matts Myhrman and Judy Knox. A permitted load-bearing straw bale structure followed in 1993. This was the same year of the first straw bale conference, ‘Roots of Revival’ in Arthur, Nebraska. The event attracted more than 50 design and construction enthusiasts.
Strawbale structures appeared in Canada as well, with significant work by Francois Tanguay in Quebec during the early 1980’s. He along with Michel Bergeron and the Canada Mortgage and Housing Corporation conducted strength and heat conduction research that allowed for further funding and wider adoption by the building industry. They also experimented with straw fibres in place of steel rebar in concrete. Their work led to the formation of ArchiBio (Architecture Bioclimatique). They later taught workshops in France and were involved in a number of straw bale buildings in Europe. However they weren’t the first as France’s first straw bale building was commissioned in 1979. In the early 1990’s a Fin designed and built a straw bale home without the knowledge that straw bale buildings existed.
In western Canada, a straw bale church built in the 1950’s remains as a testament to the early Alberta pioneers of this building archetype. Jorg and Helen Ostrowski have led the straw bale initiative in Alberta with the design of a number of straw bale buildings including what maybe the first straw bale commercial building in North America (2000).
Straw bales have been used to make buildings in the Steppes of Russia since at least 1994, in Mexico since the early 1990’s and in Guatemala in 1994. The technology has been spreading in Latin America with the aid of a variety of organizations including the ‘Farmer to Farmer’ program of the University of Arizona.
Benefits of Straw Bale Buildings
Straw bales are almost the ideal building material. Typically straw is treated as an agricultural waste product as it is slow to decay unlike nitrogen-rich hay. As a result, farmers often burn straw on the field as a means to remove it. This creates significant air pollution. Sacramento California suffers for a month each year as a million tons of straw are burnt in the valley releasing carbon monoxide and particulates that cause respiratory problems and cancer. This pollution is equal to the total amount produced annually by the state’s electrical generation facilities combined. It is estimated that enough straw is incinerated each year in the U.S. to build 5 million 2000 square foot homes.
As a resource instead of a waste product, straw bales can be sustainably grown (ideally as perennial crop; intercropped in China) in low quality soil and are a biodegradable all natural material. Bales are durable, breathable, and provide significant thermal mass and sound attenuation. The R-value of straw bales is 3/inch (2.4 with grain) as compared to wood 1/inch, brick 0.2/inch, and fibre glass batt insulation 3/inch. The embodied energy of straw bales is approximately 1/50th of concrete (1 ton straw = 112,500 BTUs, 1 ton concrete = 5,800,000 BTUs). A typical straw bale wall has 1/30th the embodied energy of a timber frame wall. Straw bale construction requires non-specialized labour (basic skills can be learned in a 2-day workshop), minimal tools, and is a catalyst for social interaction and community involvement.
Details of Straw Bale Construction
Straw bales can be made from rice, wheat, rye, flax, barley, and oats. Halophytes (plants that grow in salt water) and recycled paper fibre may also be used. Bales that are bound with string (jute or polypropylene) are easier to work with than wire.
Straw bale walls are effective as load-bearing members and typically sustain 10,000 lbs/sq.ft. when bales are laying flat. Bales that are arranged on edge (see Figure below) are less effective as load-bearing members but are better insulation and thus preferred for straw bale infill insulation. Straw bale design is effective at absorbing seismic loads. With 8 out of 10 buildings worldwide constructed of earth, adobe, and stone, straw bale could significantly reduce the damage caused by earthquakes around the world.
Straw bales have exceptional fire resistance due to the lack of air circulation that penetrates the bales. Rodents are less a concern than once thought. More space is available for rodents in other building methods, and the seed of the straw, which attracts rodents has been removed. Termites prefer wood and can be deterred by termite shields, sand barriers, vapour barriers, diatomaceous earth and borax (also a good fire retardent).
Load-bearing designs are more difficult to attain building code approval (where required) and thus a variety of infill designs are often used.
If you were located closer to agricultural areas you could consider straw bale as a building material.
Straw Bale Cost Analysis – Residential
- In agricultural regions where straw is burned or otherwise considered a waste product, consider utilizing straw as an insulation alternative.