Straw-bale Construction

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 construction projects. Research has shown that straw-bale construction is a sustainable method of 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.

History
Straw houses have been built on the African plains since the Paleolithic Era. Straw bales were used in construction 400 years ago in Germany; and straw-thatched roofs have long been used in northern Europe and Asia. When European Settlers came to North America, teepees were insulated in winter with loose straw between the inner lining and outer cover.

Straw-bale construction was greatly facilitated by the invention of the mechanical hay baler in the 1850s widespread usage 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 and began using bales of straw as a substitute. The first documented use of hay bales in construction in Nebraska was a schoolhouse built in 1896 or 1897. 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 1990, nine 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 1990s straw-bale construction has been substantially revived, particularly in North America, Europe, and Australia. This revival is likely attributed to greater environmental awareness and the material’s natural, non-toxic qualities, low embodied energy, and relative affordability. Although straw-bale construction has encountered issues regarding building codes in several countries, in the USA the introduction of Appendices S and R in the 2015 International Residential Code has helped to legitimize and improve understanding of straw-bale construction.

Straw Bale Construction Methods
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.

Thermal Properties
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 R21–R55 depending on the study, compared to R-values for a conventional wall in the range of R13-19. 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.

Straw-bale Limitations
Straw-bale construction is highly susceptible to moisture and mold. Proper construction of the straw-bale wall is therefor important in keeping moisture levels down, just as is the case with the construction of any type of building. During the construction phase, buildings need to be protected from rain and from water leakages into the body of the walls. In terms of design and detailing, brand eaves are essential to keeping excess rainwater at bay. Similarly, selection of the proper plastering system (e.g. lime plaster vs. conventional stucco), the detailing of the application, and potentially the additional of fluid waterproof coating as additional insurance are measures utilized to mitigate waterproofing concerns.

For Further Reading:
• CASBA, the California Straw Building Association, is an excellent resource for those who would are interested in strawbale technology. The website is: https://www.strawbuilding.org/
• An excellent article discussing both the methods and the pros/cons of this technology can be found at TreeHugger Magazine, https://www.treehugger.com/straw-bale-house-5199775
• “The Case for Straw-Bale Houses” is an article discussing the technology’s merits. Published by Fine Homebuilding Magazine, the article can be found at: https://www.finehomebuilding.com/2018/10/10/houses-design-case-straw-bale-houses

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