Strawbale construction is a
simple, yet revolutionary technology that is the focus of a growing
movement in alternative construction. The first strawbale buildings
were constructed in the Sandhills of Western Nebraska nearly one
hundred years ago. These quiet, efficient, and inexpensive homes
are now long-standing examples of this beautifully appropriate
shelter technology.
A strawbale construction revival started in the SW United States
in the 1980's which has now spread over North America and throughout
the World. Nearly all the states in the United States and provinces
in Canada now boast strawbale buildings. The movement is fueled
by the growing awareness of limited material and energy resources,
increasing development impacts, and adverse health effects from
buildings made with "toxic" materials.
TYPES OF STRAWBALE CONSTRUCTION
There are two primary forms
of strawbale construction, loadbearing and non-loadbearing. In
loadbearing structures the weight of the roof and lateral shear
pressures are actually carried by the bales and the plaster which
encases them. This is an easy load for strawbales each of which
can typically withstand up to 15,000 pounds of vertical pressure
when laid flat. In non-loadbearing strawbale construction the
bales serve primarily as in-fill insulation, although their function
as a mesh to directly hold plaster is another significant benefit.
Two string bales, found more commonly in the central and eastern
United States, are best suited for non-load bearing applications.
Three string bales, found more commonly in the Western United
States and Canada, are wider and thus more appropriate for load
bearing applications. Both types of bales have been used in both
forms of construction with satisfactory results.
Basements can be put under strawbale houses, although most structures
do not have basements. Standard width basement walls can be used
and the bales stacked on top of the first floor decking. Bales
are not generally used below ground.
WHY BUILD WITH STRAWBALE
While strawbale homes are beautiful
and comfortable, they are also ecologically sound and energy efficient.
Many of the people who show an interest in straw bale construction
have been enamored of this ecological perfection. Bale construction
allows owners to reduce the amount of energy used to heat and
cool their homes. It allows the creation of homes that are safe
from toxic products used in conventional frame construction and
to use an annually renewal material as a major component of their
home. Strawbale walls are also incredibly fire resistant and sound
proof. Construction with straw can also eliminate the use of non-ecological
insulation materials and reduce the amount of wood used in a structure
dramatically, lessening cost and environmental impact.
Strawbale homes can be low cost and easy to construct. Since straw
bales are relatively inexpensive compared to other building materials
and the skills needed to build with them are very basic, the cost
of raising walls can be far lower than with frame houses. A wall
raising can be a community event which involves everybody, the
young and old, the experienced and inexperienced alike. Strawbale
construction can be an accessible way to introduce the construction
trades to young people and those wanting to learn construction
skills. Super-insulated bale walls dramatically reduce energy
costs with R values of between R-35 for two string bales and R-50
for three string bales.
FINISHING BALE WALLS
A strawbale wall can be finished
with a variety of materials common to more conventional construction.
Exterior stucco and interior plaster have been the most common
materials used but wood siding and cement shingles have been used
also. There are also a number of prefinished concrete and stucco
panels available or in development for the exterior finish. Conventional
interior drywall can be readily used in non-loadbearing applications
but not as easily in loadbearing applications.
Wet applied stucco and plaster have been the most commonly used
finishes since straw bales provide an excellent surface for adhering
plaster and stucco. Reinforcement in the form of chicken wire
or stucco netting is generally recommended to provide a durable
and long lasting finish. The reinforcing effects of the stucco
and plaster skins are the most important in loadbearing applications.
The down side of stucco and plaster is that it is very labor intensive,
and if hired out to subcontractors, quite expensive. Also the
amount of stucco and plaster material used can be much greater
than that of conventional stucco finishes because of the natural
undulations that even the most straight bale wall has.
PLUMBING AND ELECTRICAL
In straw bale walls, pipes
which could leak or sweat are carried in sleeves as a precaution.
Electrical boxes and fixtures can be attached in a variety of
ways. Flexible and rigid conduit may be used, and romex, particularly
UG romes, is often laid between courses of bales. Electrical chases
can be constructed at the bottom of walls and wires run up the
back side of posts (in non-loadbearing applications) to avoid
running wires in the bales. Electrical inspectors are generally
more tolerant of such wiring techniques.
RESEARCH, TESTING, AND PERMITTING
Over the last 15 years, a growing
number of modern pioneers have been refining strawbale construction
technology. A number of studies have been conducted related to
structural, moisture, and fire safety issues. The results of this
study and testing has shown that strawbale construction is a viable
alternative to stick frame construction and has also shown where
its limitations are. In January of 1996, the Tucson/Pima County
zoning department of Arizona became the first jurisdiction in
the United States to approve an amendment to the building code
itself, allowing for both load bearing and non-load bearing strawbale
construction.
Several states have adopted strawbale specific permitting guidelines
which make it easier to use this building technique. In nearly
all jurisdictions, it is possible to build using strawbales (under
the experimental portions of the building code) although building
code approval will take longer and may require the stamp of a
registered engineer or architect to be approved. Non-loadbearing
buildings are much easier to get through code approval since bales
are not used in a structural manner. Many states have code approved
non-loadbearing structures and a growing number of states (especially
in the Western US) have code approved loadbearing structures.
INSURANCE AND FINANCING
There is at least one national
insurance company that will insure strawbale homes and many people
have had success with their local insurance agent. The fire testing
that has been done is generally enough to convince most insurance
companies to insure strawbale structures. Financing is less straight
forward but many people have obtained mortgages on strawbale homes.
Often a larger downpayment is required or other restrictions are
put on the loan. Small, local banks are often a good place to
start since they are more likely to develop a personal relationship
with the people they loan to.
FIRE SAFETY
Straw-bale construction is
exceptionally resistant to fire. Unlike stud frame construction,
in which a series of chimneys (stud cavities) form the wall, bale
walls are dense and difficult to burn. If ignited, strawbales
tend to smolder and burn slowly, greatly lessening the risk to
life. However, they are difficult to extinguish, as embers tend
to slowly tunnel through the bales. However, since plaster applied
to the uneven bale surfaces tends to be thicker than normally
found on buildings, the bales can be said to carry an extra layer
of protection. Loose straw which is sometimes used to fill cavities
is much more vulnerable to fire and should be dealt with carefully
by sealing with plaster or treating with fire retardants.
The National Research Council of Canada carried out fire safety
tests of plastered strawbales and found them to be more fire resistant
than most conventional building materials. In their tests, the
surface coating withstood temperatures of up to 1850 degrees for
two hours before a small crack developed. Standardized tests for
fire-resistance completed in New Mexico also proved that a strawbale
infill wall assembly is a far greater fire resistive assembly
than a wood frame wall assembly using the same finishes.
HUMIDITY AND MOISTURE
A common concern about strawbale
walls is the possibility of rot. Fungus (dry rot) can occur in
straw at moisture contents above 20%. In order for significant
damage to occur, these moisture levels must be maintained over
a period of time or the fungus will die. Experience and test results
suggest that the best way to avoid sustained high moisture concentrations
lies in making certain that the bales are able to transpire any
accumulated moisture back into the environment. Canadian and other
studies showed that the humidity levels in bale walls do remain
low despite fluctuations in humidity in the environment around
the walls.
Because of the excessive amount of moisture which occurs along
the splash line at the bottom of a wall, it is recommended to
cover the lower course or courses of the walls with a vapor permeable
paper such as Tyvek. Because moisture in the bales tends to migrate
down to the lower courses of the bales, a capillary break (gravel)
rather than a waterproof membrane underneath the bales is preferred.
PREVENTING
ALLERGIC REACTIONS TO STRAW IN WALLS
Some concerns have been raised
about potential adverse reactions to straw in walls. Depending
upon climate, there are myriad strawbale wall designs to respond
to various specific user and climate needs. Clean, dry bales,
the only kind to use, contain few molds or pests. Walls should
be designed to prevent moisture entry or accumulation using either
a breathing or non-breathing wall approach. Wall breathing is
controlled with vapor barriers, vents, wall plaster formula, and
home air exchange systems. Some strawbale advocates prefer to
support the organic agriculture movement by purchasing the straw
from organically grown grains only.
TERMITES AND PESTS
Compared with wood, there are
few termites which like straw. In at least one building, termites
entered the structure, left the straw alone, and ate the wood
windows. Consequently, the normal precautions used with wood construction
are sufficient. For pests, bales provide fewer spaces than conventional
uninsulated wood framing, provided the stucco is intact. However,
care should be taken to prevent rodents from entering bales before
the walls are plastered. Should rodents enter a wall at a break
in the plaster coating, they would be likely to make a place to
stay. Fortunately, unlike hay, straw contains very little nutritional
substance and will not, in itself, support a pest population.
CULTURE OF THE MOVEMENT
The strawbale movement is peopled
with a an intriguing mix of visionaries, innovators, and just
plain folks. The form itself seems to attract do-it-yourselfers,
eco-designers, "natural living" types, and the curious. In this
"pre-code adoption" stage, big bucks are not yet involved, so
the work is carried by enthusiasts who are always experimenting,
some succeeding some failing, till these figures gain legendary
status like Paul Bunyon.
"The Last Straw", which touts itself as the quarterly journal
of the strawbale construction revival, is put out by pioneers
Judy Knox and Matts Myhrman, of the southwest US strawbale movement.
Matts and Judy have also done much historic research on buildings
in Nebraska which have informed modern building practices. There
are now also books on strawbale construction, electronic discussions,
and gatherings. Much of the teaching/training that occurs still
takes the form of strawbale construction workshops. It is not
yet included in the curriculum of vo-tech or trade schools, and
there are no guilds or unions guiding apprenticeship. In Minnesota,
the Community Eco-design Network, is organizing an Eco-Building
Guild which will research, set, and teach a quality standard of
strawbale construction techniques.
FUTURE GROWTH OF THE MOVEMENT
There is little doubt that
straw will become a significant element of the building industry
by the end of the 1990's. After New Mexico granted unlimited experimental
permits for strawbale construction, demand was so great that builders
couldn't keep up. New Mexico now has a Strawbale Construction
Association, the membership of which is over half women. Besides
strawbale construction there is also the emerging straw board
industry. If the over 140 million tons of straw produced annually
were put into compressed panels for homes, there would be enough
for over 1 million new 2000 square foot homes each year!
Besides the challenges & opportunities to existing building materials
producers, strawbale construction opens ways to change the role
that home contractors have in new construction. Since strawbale
construction can be a low-cost way to build, it can make home
ownership possible for a wider section of income groups. Thus
more opportunities are made available for firms that have learned,
and are offering this type of home construction. Since load bearing
strawbale construction is so easy to master, do-it-yourselfers
are provided with another opportunity to lower labor costs.
Strawbale
Resources
BOOKS AND
PAMPHLETS
Bainbridge, D.A., A. & B. Steen, & D. Eisenberg. "The Straw Bale House", 1994.
White River Junction, VT: Chelsea Green Publishing Co.
This is the most comprehensive book yet on strawbale construction.
Eisenberg, David. "Summary of Results of a Structural Straw Bale Testing Program."
1993,
Tucson, AZ: Community Information Resource Center.
MacDonald, S.O. "A Straw Bale Primer", 1991.
MacDonald, S.O. and Matts Myhrman, "Build It With Bales - A Step-by-Step Guide to Straw-bale
Construction." Version Two, May 1997,
Tucson, AZ: Out On Bale.
Strang, G. "Straw-bale studio"
Fine Homebuilding 12/84-1/85:70-72.
Wilson, Alex. "Straw: The Next Great Building Material?"
Environmental Building News, May/June 1995,
4(3):1,11-17
PERIODICALS
"The Last Straw"
The Last Straw
HC 66, Box 119
Hillsboro, New Mexico, USA; 88042.
Subscriptions are $28/year.
The only "magazine" devoted to strawbale construction.
Website: http://www2.strawhomes.com/tls/