STRAW BALE CONSTRUCTION, An Overview
Thomas A. Fisher, AIA Environmental Design Collaborative
1050 Allendale Drive Charlottesville, VA 22901 (804) 977-1910
email: tfisher@cstone.net
ABSTRACT
Straw bale construction is experiencing a renaissance of interest in semi arid states of the American Southwest, Australia and New Zealand. It is now gaining increasing attention in other parts of the US, Canada, Europe and China as many projects using straw bales have been built in these regions as well. A high thermal resistance (R-Value), low cost, and a non-toxic by-product of grain agriculture make straw bale an ideal Green or environmentally responsible building material. It is also an aesthetically pleasing material lending itself to soft curvature in building form, finishes and deep window recesses. Common concerns involving pest infestation, rot, mildew, fire and structural stability have been responsibly addressed by both construction and academic professionals. Specific concerns in Northern moist regions involve cost effective foundation systems, moisture vapor migration, condensation and building detailing for wet climate.
1. HISTORY
While straw for roofing and insulation is an ancient practice, the known use of straw bales as a wall system in the US dates to the early 1900's. Straw bale homes built in 1903 are still in use in Nebraska and South Dakota. In the 30's a straw bale mansion was built in Arkansas and is in use today as a museum.
2. MATERIAL QUALITIES
Stability: Contrary to the stigma bequeathed by the first little pig, straw bale exterior wall systems have the mass, cohesion and density to ensure a long lasting solid assembly. Detailing issues include the proper spacing and depth of "pinning" or the vertical reinforcing that provides shear strength to walls. Also, the density of the bales must be construction quality and the baling string must be polypropylene twine or wire.
Thermal Resistance of straw bale walls ranges between R-42.8 for a typical 18" wide two string bale, to R54.7 for a 24" wide three string bale1 . Model Energy Code requirement for an insulated 6" stud wall is R19.
Embodied Energy: Straw has low embodied energy compared to other insulating materials. 18" x 36" x 14" straw bales delivered direct from harvest to a building site have an embodied energy of 662 BTU's per square foot of wall area.2 The same insulating value in expanded polystyrene will contain approximately 92,200 BTU per square foot of wall area. Embodied energy for one square foot of R42 in fiberglas batts will be approximately 23,300 BTU's3 . Straw also has a negligible waste factor for deconstruction.
3. CONSTRUCTION Structural Systems: Straw bale buildings have two construction styles. A system which uses the bale walls to support the weight of roof loads is called a "Nebraska" or load bearing style. An infill style relies on a post and beam frame to carry roof and floor loads. Straw bales are built around this frame to form walls.
Construction: Straw bale foundation systems require a footing or base adequate for supporting the 18" to 24" wall thickness. Slab on grade systems are commonly used. In milder climates a "rubble trench" foundation may be used where a rock and gravel footing extends down below frost level. A thickened slab at the perimeter then bears the bale walls. Bales are typically laid in a running bond pattern. Pins of bamboo or #3 or #4 reinforcing steel bars are simultaneously placed approximately 2 per bale to vertically tie the wall together. Door and Window openings are framed with "bucks" or rough opening assemblies that the door and window framing and units are built into. Exterior finish Systems are generally cement plaster laid in a typical 3 coat manner. Wire mesh reinforcing is applied to both inside and outside walls surfaces onto the walls.
4. COMMON CONCERNS
Pest Infestation: Straw bales are composed of straw, not hay. Straw is primarily cellulose and lignin which have little nutritive value and are not sought as a food source by pests. Correct detailing and finishing makes a straw bale wall no more attractive to pest infestation and nesting than conventional stud walls. Boric acid powder is occasionally used to further discourage the presence of insects.
Rot & Mildew: Rot and mildew are a result of sustained or repeated moisture penetration. Normal humidity levels do not appear to support mildew build up. Proper provision of vapor barriers will prevent the migration of interior moisture from migrating to a condensation point inside the bale wall.
Availability: Availability of straw bales is a regional issue. Grain crops which provide straw as a by product are grown in most states. Farmers are happy to find an additional market for straw which is frequently burned. Coordination is needed to obtain the correct density, moisture levels, dimensions and baling twine.
Fire Resistance: Straw bales burn like phone books. Unless surrounded by a sustained heat and flame source, surface fibers flash burn but do not sustain flame. Tests have determined that plastered bale walls are more resistant to fire than conventional frame construction. The ASTM E-119 test gives a plastered straw bale wall a two hour fire rating. Care, however, must be taken during construction as loose straw does present a fire hazard.
Code approval: "You want to build with what!?" is the amused response of many building code enforcement officials. Assurance of structural stability is the overriding issue. The author has also had to address issues such as fire resistance, thermal values and infill wall resistance to wind loads. Straw bales used as an infill material need only an engineered frame. Load bearing straw bale walls are a common practice as well and engineering tests are available to substantiate most one story designs.
5. MOIST CLIMATE CONCERNS
Moisture is a concern for all insulated building systems. A strategy for dealing with vapor migration and condensation is a common topic of concern in the straw bale construction community. To date, no definitive tests have ascertained the correct response to vapor borne moisture though several are underway and monitoring results should be available in the near future.
Studies conducted by the Canadian Mortgage and Housing Corporation (CMHC) but not published at this writing will sound a cautionary note about the ability of straw bale walls to dry out in moist climates after becoming wet.
The longevity of historic structures provide anecdotal evidence that moisture is not a major concern for most residential applications in warm and dry regions. Straw bale construction in cold moist climates must still be considered experimental .
6. AESTHETIC POSSIBILITIES
To date most straw bale homes have been simple owner built dwellings with a handful of complex architect designed projects as well. The most notable commercial example of the use of straw bale wall systems is recently completed Real Goods product showroom in Hopeland, California.
The soft rounded corners, and deep window and door openings of straw bale dwellings evoke an old world appearance . It is also an easily sculpted material lending itself to curved forms and more complex and expressive buildings. The realm of possibilities for this new/old material is just now being explored.
7. BIBLIOGRAPHY
The Straw Bale House by Steen, Steen & Bainbridge with David Eisenberg. Is the definitive in-depth publication on straw bale construction and is available at bookstores.
Build it with Bales by S.O. MacDonald and Matts Myhrman is a practical step by step guide to straw bale construction by two of the original discoverers of the material for construction.
8. RESOURCES
Out on Bale by Mail is a clearinghouse for straw bale construction information, education and resources including videos, workshops, test reports and books. 1037 E. Linden St. Tucson, AZ 85719 (520) 624-1673
The Last Straw is the straw bale community's quarterly new journal. P.O. Box 4000 Tucson, AZ 85733
World Wide Web links to several of the major straw bale information sites can be found at the above Environmental Design Collaborative Web Site.