According to many sources, including the U.S. Department of Energy, straw as a building material is environmentally sensitive, inexpensive, seismically sound, readily available, and very energy efficient.
The U.S. Department of Energy estimates that 200 million tons of straw - the stalks that remain after the harvest of wheat, oats, barley, rice, rye, and flax - are wasted in this country alone every year. U.S. Department of Agriculture figures indicate that American farmers harvest enough straw each year to construct 4 million 2,000-square-foot homes - almost four times the number currently built using conventional materials.
Inexpensive And Easy To ConstructA no-frills, 2,000-square-foot house requires about 300 standard three-string bales of straw ($3 to $4 each). The bales are placed on a foundation and then compressed by a load-bearing roof or become part of a post-and-beam structure in which the bales do not bear any of the weight. After wiring and plumbing systems are installed, the walls are sealed with stucco, lime, or earthen plaster and finished.
"Most people use stucco, but we use earthen plasters instead," said Athena Steen, co-owner of the Canelo Project, a nonprofit organization 70 miles southwest of Tucson whose workshops have given hands-on straw-bale training to more than 120 owner-builders each year for the past decade.
"Cement and lime finishes wick the bales," she explained. "If there's a leak somewhere, the moisture gets trapped. Mud, on the other hand, is breathable, so the water's not trapped. It has the ability to absorb a huge amount of moisture. When the humidity drops, the wall releases the moisture."
Earthen plaster is also a lot cheaper than stucco; builders can often use the soil in the home's backyard, "provided it has some degree of clay in it," Steen said.
A Tricky Load"Straw-bale walls are so darn thick," said Steen - typically 23 by 16 by 42 inches and 75 to 85 pounds for three-wire bales - that their thermal mass alone accounts for the energy savings (all other things, such as insulation, being equal).
During summer, the walls absorb heat throughout the day and slowly radiate it into the exterior and interior environments during the night. In the winter in a fairly mild climate, owners may only need heat on cloudy days, especially if windows are used to trap the sun's warmth in exterior and interior walls. Load calculations must be done carefully, taking these factors and more into consideration.
"I know of [straw] houses with furnaces, but the pilot light alone keeps them heated," said Steen.
"Without [mechanical] heating or cooling, the interior temperature stays between 64 degrees and 84 degrees F. In winter, when it gets as low as 9 degrees outside, it'll be 65 degrees inside - without heat."
Maurice Bennett, executive director of the California Straw Building Association, a nonprofit clearinghouse and referral service for straw-bale architects, contractors, designers, and owner-builders, calls himself "a retired Naval officer and wanna-be hippie."
He sold his 2,400-square-foot frame house in coastal Sonoma County and moved inland to Angels Camp in California's Gold Country, where he built a 2,000-square-foot straw-bale home. "My energy bills are a third of what they were," he said.
Sidebar: R-Values Don't Tell The Whole StoryR-values for straw-bale walls have varied widely from study to study - from R-55 for bales laid flat in a 1993 University of Arizona test to R-17 in a 1996 Oak Ridge National Laboratory (ORNL) study of two-string bales.
In 1997, the California Energy Commission (CEC) conducted its own tests using a state-of-the-art technique to measure an R-value of 26 for bales that were laid flat and 33 for those on edge. Such variations were partly attributable to poor test design, improper sealing of the walls, and inadequate drying of the straw prior to testing.
In 1998, ORNL researchers conducted a second study, this time with nationally recognized straw-bale builders providing oversight. The two-string bales were plastered on both sides - not just one, as in the earlier study - and allowed to dry to a moisture content of 13 percent. The resulting R-27, though half the value most commonly reported at the time, is twice the R-value of a 2-by 6-foot wood frame wall insulated with fiberglass batts, tested in the same facility.
As a result, the CEC "gave a blanket R-30 rating for straw bale," said David Eisenberg, director of the Development Center for Appropriate Technology. "Higher R-values than that in walls provide diminishing returns, except in extreme climates."
According to Eisenberg, "R-values are an inadequate metric to understand the thermal performance of a wall system. It's important to look at the relationship between thermal mass and insulation, and to consider radiant comfort effects.
"Bale walls have a layer of thermal mass on each surface," he explained, "separated by a lot of insulation. As a result, the interior wall surface tends to be at ambient air temperature, little affected by outside temperature changes. This means your body isn't gaining or losing heat to the wall and thus, air temperatures can be lower in winter or higher in summer with no loss of comfort - a bigger benefit than just the high R-value."
To fully appreciate the benefits of straw bale and other alternative construction methods, you have to consider a much bigger picture.
"Concrete," says Eisenberg, "is an energy-intensive material to produce. Portland cement accounts for 8 percent of the human contribution of CO2 production," which in turn contributes to global warming. "Cement is a wonderful material for all kinds of uses, and it's durable. But its environmental impact has been largely ignored.
"Environmentally responsible materials, on the other hand, should reduce the energy intensity and costs of building," Eisenberg said. "They're often minimally processed."
- By Heidi Nye
Publication date: 09/27/2004