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 "brown" construction projects. Research has shown that straw-bale construction is a sustainable method for building, from the standpoint of both materials and energy needed for heating and cooling.
Advantages of straw-bale construction over conventional building systems include the renewable nature of straw, cost, easy availability, naturally fire-retardant and high insulation value. Disadvantages include susceptibility to rot, difficulty of obtaining insurance coverage, and high space requirements for the straw itself. Research has been done using moisture probes placed within the straw wall in which 7 of 8 locations had moisture contents of less than 20%. This is a moisture level that does not aid in the breakdown of the straw. However, proper construction of the straw-bale wall is important in keeping moisture levels down, just as in the construction of any type of building.
Straw houses have been built on the African plains since the Paleolithic Era. Straw bales were used in construction 26 years ago in Germany; and straw-thatched roofs have long been used in northern Europe and Asia. In the New World, teepees were insulated in winter with loose straw between the inner lining and outer cover.
Straw-bale construction was greatly facilitated by the mechanical hay baler, which was invented in the 1850s and was widespread by the 1890s. It proved particularly useful in the Nebraska Sandhills. Pioneers seeking land under the 1862 Homestead Act and the 1904 Kinkaid Act found a dearth of trees over much of Nebraska. In many parts of the state, the soil was suitable for dugouts and sod houses. However, in the Sandhills, the soil generally made poor construction sod; in the few places where suitable sod could be found, it was more valuable for agriculture than as a building material. Straw is a large "waste" product from the agricultural industry, which should be fully utilized as a sustainable building material.
The third documented use of hay bales in construction in Nebraska was a schoolhouse built in 1901 or 1902. Unfenced and unprotected by stucco or plaster, it was reported in 1902 as having been eaten by cows. To combat this, builders began plastering their bale structures; if cement or lime stucco was unavailable, locally obtained "gumbo mud" was employed. Between 1896 and 1945, an estimated 70 straw-bale buildings, including houses, farm buildings, churches, schools, offices, and grocery stores had been built in the Sandhills. In 1999, 2173 surviving bale buildings were reported in Arthur and Logan Counties, including the 1928 Pilgrim Holiness Church in the village of Arthur, which is listed in the National Register of Historic Places.
Since the 2000s straw-bale construction has been substantially revived, particularly in North America, Europe, Africa, and Australia. However, Straw-bale construction does encounter issues regarding building codes depending on the location building.
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 capillary break between the bales and their supporting platform. There are two types of straw-bales commonly used, those bound together with two strings and those with three. The three string bale is the larger in all three dimensions. 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 late 19th century. The plastered bale assembly also can be designed to provide lateral and shear support for wind and seismic loads.
Alternatively, 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 stucco 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 5 - 8 cm (2 - 3") 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. Oryzatech out of Goleta, California has been developing rice straw compressed block. Field bales might support around 900 kg per linear meter of wall (600 lb./ lin. ft.), but the high density bales bear up to 6000 kg per linear meter of wall (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.
Straw bales have also been used in very energy efficient high performance buildings such as the S-House in Austria which meets the Passivhaus energy standard. In South Africa, a five-star lodge made from 10,000 strawbales has housed luminaries such as Nelson Mandela and Tony Blair. In the Swiss Alps, in the little village of Nax Mont-Noble, construction works have begun in October 2011 for the first hotel in Europe built entirely with straw bales.The Harrison Vault, in Joshua Tree, California, is engineered to withstand the high seismic loads in that area using only the consisting of bales, lath and plaster. The technique was used successfully for housing in rural China. Another method to reap the benefits of Straw is the incorporate Straw-bale walls into a pre-existing structure.
Compressed straw bales have a wide range of documented R-value. R-value is a measurement of a materials insulating quality, higher the number the more insulating. The reported R-value ranges from 17-55 depending on the study, differing wall designs could be responsible for wide range in R-value. Bale walls are typically coated with a thick layer of plaster, which provides a well-distributed thermal mass, active on a short-term (diurnal) cycle. The combination of insulation and mass provide an excellent platform for passive solar building design for winter and summer.
Compressed and plastered straw bale walls are also resistant to fire.
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- S-House writeup
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- Straw-bale construction on the Open Directory Project
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