Straw-bale construction is a building method that uses bales of straw (commonly wheat, rice, rye and oats straw) as structural elements, building insulation, or both. This construction method is commonly used in natural building or "green" construction projects.
Advantages of Straw-bale construction over conventional building systems include the renewable nature of straw, cost, easy availability, and high insulation value. Disadvantages include difficulty in designing for high wind, susceptibility to rot, potential danger from flammability and high space requirements for the straw itself.
Grasses and straw have been in use in many ways for building since prehistory. The incorporation in machine-manufactured modular bales seems to date back to the early 20th century in the midwestern United States, particularly the sand-hills of Nebraska, where grass was plentiful and other building materials (even quality sods) were not.
Rick West, the director of the Smithsonian Institution's National Museum of the American Indian, has stated that “Straw bale construction is at once an American invention and a sustainable answer to housing needs on and off the reservation.”
Straw bale building typically consists of stacking rows of bales (often in running-bond) on a raised footing or foundation, with a moisture barrier or capilary break between the bales and their supporting platform. Bale walls can be tied together with pins of bamboo, rebar, or wood (internal to the bales or on their faces), or with surface wire meshes, and then stuccoed or plastered, either with a cement-based mix, lime-based formulation, or earth/clay render. The bales may actually provide the structural support for the building ("load-bearing" or "Nebraska-style" technique), as was the case in the original examples from the turn of the last century.
Alternately, bale buildings can have a structural frame of other materials, usually lumber or timber-frame, with bales simply serving as insulation and plaster substrate, ("infill" or "non-loadbearing" technique), which is most often required in northern regions and/or in wet climates. In northern regions, the potential snow-loading can exceed the strength of the bale walls. In wet climates, the imperative for applying a vapor-permeable finish precludes the use of cement-based stuccos commonly used on load-bearing bale walls. Additionally, the inclusion of a skeletal framework of wood or metal allows the erection of a roof prior to raising the bales, which can protect the bale wall during construction, when it is the most vulnerable to water damage in all but the most dependably arid climates. A combination of framing and load-bearing techniques may also be employed, referred to as "hybrid" straw bale construction.
Straw bales can also be used as part of a Spar and Membrane Structure (SMS) wall system in which lightly reinforced 2" - 3" [5 cm - 8 cm] gunite or shotcrete skins are interconnected with extended “X” shaped light rebar in the head joints of the bales. In this wall system the concrete skins provide structure, seismic reinforcing, and fireproofing, while the bales are used as leave-in formwork and insulation.
Typically "field-bales", bales created on farms with baling machines have been used, but recently higher-density "precompressed" bales (or "straw-blocks") are increasing the loads that may be supported. Field bales might support around 600 pounds per linear foot of wall, but the high density bales bear up to 4,000 lb./lin.ft., and more. The basic bale-building method is now increasingly being extended to bound modules of other oft-recycled materials, including tire-bales, cardboard, paper, plastic, and used carpeting. The technique has also been extended to bags containing "bales" of wood chips or rice hulls.
References and notesEdit
- ↑ S-House writeup indicates that “facade was built as a structure made of wooden boards and straw balls that are pressed and mounted free of thermal bridging.”
- ↑ Canada Mortgage and Housing Corporation. "Energy Use In Straw Bale Houses". Retrieved on 4 September 2008.
- ↑ 3.0 3.1 3.2 Steen, Steen & Bainbridge (1994). The Straw Bale House. Chelsey Green Publishing Co.. ISBN 0-930031-71-7.
- ↑ 4.0 4.1 Magwood & Mark (2000). Straw Bale Building. New Society Publishers. ISBN 0-86571-403-7.
- ↑ "Huff as hard as you like - you can’t blow a straw house down". The Times, May 20, 2010. http://www.timesonline.co.uk/tol/news/environment/article7130995.ece.
- ↑ The first recorded straw bale construction using steam-powered balers was in Nebraska
- ↑ <(Myhrman, Matts; S.O. MacDonald (1994). Build it with Bales. Out on Bale. ISBN 0-9642821-1-9.
- ↑ Black, Gary, and Mannik, Henri, “Spar and Membrane Structure” The Last Straw journal, #17, Winter 1997
- Design of Straw Bale Buildings. Bruce King. Chelsea Green Publishing, 2007.
- More Straw Bale Building: A Complete Guide to Designing and Building with Straw. Chris Magwood. New Society Publishers, 2005.
- Straw Bale House, The. Steen, Steen, Bainbridge & Eisenberg. White River Junction, Vermont: Chelsea Green, 1994.
- Building a Straw Bale House. Nathaniel Corum. Princeton Architectural Press, 2005.
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