Straw Bale Construction
Straw, a type of high-mass building material, can provide insulation. Straw bale construction can yield about R-2.4 per inch of thickness. Straw panels are also available. These panels are usually 2- to 4-inches thick and faced with heavyweight Kraft paper on each side. Although manufacturer's claims vary, R-values realistically range from about R-1.4 to R-2 per inch. They also make effective sound-absorbing panels for interior partitions. Some manufacturers have developed Structural Insulated Panels from multiple-layered, compressed-straw panels.
Despite its introduction around 100 years ago, straw bale construction remains somewhat unconventional. However, combined with modern techniques, straw bale buildings feature well-insulated walls, simple construction, and low costs. Straw is also a renewable resource.
Most straw bale houses being built today employ some kind of post-and-beam structure to support the weight of the roof, with the straw bales serving as infill. Straw bales come in all shapes and sizes. The medium-sized rectangular three-string bales are preferred for building construction. Three-string bales are better structurally, have higher R-value, and are often more compact. A typical medium-sized, three-wire bale may be 23 " X 16 " X 42 " and may weigh from 75 to 85 pounds. The smaller two-wire bales, which are easier to handle, are roughly 18" X 14" X 36" and weigh 50 to 60 pounds. Also, some manufacturers have developed Structural Insulated Panels from multiple-layered, compressed-straw panels.
Americans want comfortable, attractive, functional, and durable housing. Yet, many increasingly find high quality housing beyond their means. Conventional building methods rely on plentiful resources. With some of these resources dwindling, housing costs are sky rocketing. The cost of a home includes materials, construction, financing, taxes, energy consumption, and insurance. This booklet explores recent attempts to reduce those costs. Construction techniques discussed in this booklet focus on building resource-efficient and energy-conserving homes, without sacrificing affordability or quality.
In a cooperative demonstration project between the U.S. Department of Energy (DOE), the U.S. Department of Housing and Urban Development (HUD), and the Navajo Nation, current home designs on the Navajo reservation were evaluated and recommendations were made to improve quality and lower the costs. The resulting design utilized straw-bale wall construction.
Straw-bale building is a practical and perhaps under utilized construction method. Initiated in the United States at the turn of the century, straw-bale building is showing new merit in today's marketplace. Walls of straw, easily constructed and structurally sound, promise to take some of the pressure off of limited forest resources.
Straw is a viable building alternative, plentiful and inexpensive. Straw-bale buildings boast super insulated walls (R-50), simple construction, low costs, and the conversion of an agricultural byproduct into a valued building material. Properly constructed and maintained, the straw-bale walls, stucco exterior and plaster interior remain water proof, fire resistant, and pest free. Because only limited skill is required, a community house-raising effort can build most of a straw-bale house in a single day. This effort yields a low-cost, elegant, and energy efficient living space for the owners, a graceful addition to the community, and a desirable boost to local farm income. This booklet offers an in-depth look at one such community house-raising, in addition to a general overview of straw-bale construction.
Figure 86: A demonstration project using affordable, energy efficient construction techniques with an emphasis on materials produced near the building site and erected by local labor resources.
History of Straw Bale Construction: People have built homes using straw, grass, or reed throughout history. These materials were used because they were reliable and easy to obtain. European houses built of straw or reed are now over two hundred years old. In the United States, too, people turned to straw houses, particularly after the hay/straw baler entered common usage in the 1890s. Homesteaders in the northwestern Nebraska "Sandhills" area, for example, turned to baled-hay construction, in response to a shortage of trees for lumber. Bale construction was used for homes, farm buildings, churches, schools, offices, and grocery stores.
Straw: A Renewable Resource: Straw, the stalks remaining after the harvest of grain, is a renewable resource, grown annually. Each year, 200 million tons of straw are under utilized or just wasted in this country alone. Wheat, oats, barley, rice, rye, and flax are all desirable straws for bale walls. Even though the early bale homes used hay for the bales, hay is not recommended because it is leafy and easily eaten by creatures great and small. Straw, tough and fibrous, lasts far longer. Straw-bale expert Matts Myhrman estimates that straw from the harvest of the United States major grains could be used to construct five million, 2,000 square-foot houses every year! More conservative figures from the U.S. Department of Agriculture indicate that America's farmers annually harvest enough straw to build about four million, 2,000 square-foot homes each year, nearly four times the houses currently constructed.
Building a straw-bale house is relatively simple. A basic 2,000 square-foot house requires about 300 standard three-wire bales of straw (costing approximately $1,000). Placed on a foundation, the bales are skewered on rebar pins like giant shiskabobs. After wiring and plumbing, the walls are sealed and finished. Because grains are grown in almost every region of the country, straw bales are readily available, with minimal transportation costs. Lumber from trees, in addition to becoming more scarce and expensive, must be transported over longer distances.
Straw is a form of cellulose that has reasonably good insulating properties; and because a bale can be up to two feet thick, a straw-bale wall has extremely high thermal resistance. Recent tests following ASTM procedures resulted in bale R-values between R-2.4 and R-3.0 per inch, depending on the direction of the straw, and showed that thermal resistance is affected by moisture and density of the pack (Joseph McCabe, January 1993). Matts Myhrman, another straw-bale expert, suggests that R-2.4 per inch is representative of straw-bale thermal resistance in the field. Therefore, straw-bale homes should have lower heating and cooling costs than conventional homes.