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Building method that uses bales of straw

Straw bale construction is a building method that uses bales of straw (usually wheat 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 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 or wood (internal to the bales or on their faces), or with surface wire meshes, and then stuccoed or plastered, either with a 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. 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 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. The University of Bath has completed a research programme which used ‘ModCell’ panels—prefabricated panels consisting of a wooden structural frame infilled with straw bales and rendered with a breathable lime based system—to build 'BaleHaus', a straw bale construction on the university's campus. Monitoring work of the structure carried out by architectural researchers at the university has found that as well as reducing the environmental footprint, the construction offers other benefits, including healthier living through higher levels of thermal insulation and regulation of humidity levels. The group has published a number of research papers on its findings. High density pre compressed bales (straw blocks) can bear higher loads than traditional field bales (bales created with baling machines on farms). While field bales support around of wall length, high density bales can bear at least
Bale buildings can also be constructed of non straw bales—such as those made from recycled material such as tires, cardboard, paper, plastic, and carpeting—and even bags containing "bales" of wood chips or rice hulls. in Austria which meets the Passivhaus energy standard. In South Africa, a five star lodge made from 10,000 strawbales has housed world leaders 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 assembly consisting of bales, lath and plaster. The technique was used successfully for strawbale housing in rural China. Straw bale domes along the Syrio African rift at Kibbutz Lotan have an interior geodesic frame of steel pipes. Another method to reap the benefits of straw is to incorporate straw bale walls into a pre existing structure. Straw bales are widely used to insulate walls, but they may also be used to insulate roofs and sub floors.

[[File:Matama Straw Bale Library Interior IMG 1443. JPG|thumb|upright|Interior view of straw bale library given that the bales are over a foot thick, the R value per inch is lower than most other commercial insulation types including batts (3–4) and foamboard (~5). 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. In common with most building materials, there is a degree of uncertainty in the thermal conductivity due to the influences of temperature, moisture content and density. However, from evaluation of a range of literature and experimental data, a value of 0.064 W/m·K is regarded as a representative design value for straw bales at the densities typically used in building construction. Compressed and plastered straw bale walls are also resistant to fire. According to research, the thermal conductivity does not differ significantly depending on the type of straw. Samples with densities between 63 and 350 kg/m3 have been analysed., Reif et al. and Cascone et al. For different temperatures and densities, Vjelien studied four variations of the same kind of straw: two variations concerned the direction of the fibres in relation to the heat flow: perpendicular and parallel, and the other two concerned the macrostructure chopped straw and defibrated straw. The thermal conductivity of the defibrated straw was lower than that of the chopped straw.

The use of straw bales as thermal insulation in buildings has been studied by many authors. Liuzzi et al.

Two significant problems related to straw bale construction are moisture and mold. During the construction phase, buildings need to be protected from rain and from water leakages into the body of the walls. If exposed to water, compressed straw may expand due to absorption of moisture. In turn, this can cause more cracking through which more moisture can infiltrate. Further damage to the wall can be caused by mold releasing potentially toxic spores into the wall cavities and into the air. In hot climates, where walls may have become internally dampened, internal temperatures may rise (due to decomposition of affected straw). Rats and mice can infiltrate straw bale homes during construction, so care must be taken to keep such animals out of the material. Other problems relate to straw dust which may cause breathing difficulties among people with allergies to straw or hay. Several companies have developed prefabricated straw bale walls. A passive ecological house can easily be assembled with those panels.