Spring 1993 (v5n3)
Bainbridge, David A.
Article written
for Sustainable Agriculture Technical Reviews.
The field burning of waste
straw in California produces more carbon monoxide and particulates than
all of the electric power generating plants in the state combined. These
byproducts of burning have a significant effect on the quality of air
in certain regions at certain times of the year.
During the past decade, many
grain farmers in California have been looking for other ways to manage
crop residues. In fact there are a number of alternatives to field burning.
One option is to incorporate straw back into the soil. Another option
is to bale and remove the straw for some other purpose. This article summarizes
an innovative construction technique that utilizes straw bales.
Straw bale construction is
one of the best building systems known. Straw bale construction began
in the late 1800s in the Nebraska Sandhills, a vast grassland area with
few trees. This method of building proved to be durable and more comfortable
than either wood frame or sod houses. A straw bale renaissance is currently
underway, led by builders and pioneers who have rediscovered the benefits
of this low cost, durable, and super energy-efficient building material.
Straw bales have been used
to build houses, apartments, farm buildings, schools, hotels, government
buildings, and churches. Many buildings more than 50 years old are still
in "like-new" condition, loved by their occupants, and unrecognized
by passers-by. These include two churches, a post office, and a county
courthouse. This building method is particularly attractive for farm structures
in areas where straw is a waste material available at low cost (this includes
the Sacramento Valley, Willamette Valley, and the area around Couer de
Lane, Idaho). Bale prices as low as $1 per bale are found in rural areas
- but even at suburban prices it makes sense to build with bales.
This is perhaps the only
ecologically and economically sound building system. Homes can be "grown"
in sustainable agroecosystems without pillaging the old growth forests,
at owner-built prices competitive with mobile homes (now the home of approximately
16 million Americans.)
The Canada Mortgage and Housing
Corporation (the equivalent of the U.S. Dept. of Housing and Urban Development)
sponsored a technical report that concluded, "The basics of this
technique can be learned in a few days. A two week practical training
is sufficient to teach all intricacies related to this construction method.
These factors make it a well suited self-help building method for people
with limited resources."
These Canadian tests also
verified the fire safety and structural integrity of plastered straw bales.
"The straw bales/mortar structure wall has proven to be exceptionally
resistant to fire. The straw bales hold enough air to provide good insulation
value but because they are compacted firmly they don't hold enough air
to permit combustion."
Material costs represent
less than one-fifth of the cost of the wall system so owner-builders can
realize greater savings by providing their own labor. These structures
also lend them selves to work parties because unskilled labor is needed.
Several houses have been built for less than $10 per square foot by combining
several other cost saving techniques with the low cost super-insulated
bale walls.
Building Options
Structural Use of Bales.
Bales should be compact for structural use. Dry straw bales are preferred
(rice, wheat, rye, oats, etc.). Three-wire bales at about nine to ten
pounds per cubic foot are good. If bales are bought directly from the
baler both full and half bales should be ordered. The bales are laid flat
(like bricks) and pinned with hardwood dowel or rebar pins set at an angle
and vertical in the corners. Joints are staggered to provide additional
strength.
Non-Structural Infill.
Bales can be used as insulated infill in timber frame or pole structures
to meet building code structural requirements. However, building a timber
or wood frame and then cutting and fitting bales to fit around the frame
adds a considerable amount to the work and cost. Most bales today are
quite strong and the timber frame is generally an unneeded and unwarranted
expense.
Finishes. Temporary
buildings can be left exposed (an uncoated bale wall had held up fine
for many years in northern New Mexico and agricultural buildings often
last 20 years if the straw is protected with snow fence or mesh). One
farmer in Alberta has regularly used unplastered bale buildings for livestock.
But in most case a fire-resistant and water-proof finish is desirable.
Stucco and plaster are commonly used. Gunite or shotcrete (sprayed concrete)
has worked very well on straw bale walls and was used to finish the State
of Texas Demonstration Farm in Laredo, Texas. This finish goes on quickly
and the pressure of the sprayer promotes an excellent bond with the bales.
This is not a do-it-yourself operation but it is very fast. Asphalt stabilized
earth plaster has been very effective in tests in Arizona. This mix is
very inexpensive (a gallon of asphalt per 30 shovels of adobe mix) and
strong. An evaluation of alternative coatings that can be sprayed on quickly
and economically is needed.
The plaster coats are normally
reinforced with wire mesh. The reinforcing wire can be tied to the wall
with wire laced between the bales, wire laid up in the wall, or nailed
to stakes pounded into the bales. Expanded metal lath at corners and around
windows and doors provides additional strength and makes it easier to
plaster.
Wiring and Plumbing.
Wiring and plumbing can be done as they are with cement block or adobe
buildings. Utilities can be laid up in the walls as they are built or
run in moldings, interior walls, under the floor, or in the attic. Polypans
(used for infiltration control) can be used to back up the electrical
boxes. A chain saw or wood drill can be used to make room for wire and
pipe runs. Many builders have simply used ROMEX like a normal stud-framed
building.
Further Reading
A more detailed description
of this construction technique is presented in Plastered Straw Bale
Construction by David Bambridge, and Bill and Athena Steen. The book
is 44 pages and contains more than 85 references about or related to straw
bale building. It is available for $10 ppd. from agAccess, Box 2008, Davis,
CA 95617, (916) 756-7177. For information about workshops on straw bale
construction contact: Matts Myhrman, 1037 E. Linden Street, Tucson, AZ
85719.
(DEC.457)
Contributed by David Bainbridge
[ Back | Search | Feedback ]