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Straw-bale construction is a building method that uses straw bales as structural elements, insulation, or both.
Straw-bale construction was pioneered in Nebraska in the early 20th century, in response to the lack of significant amounts of lumber with which to build housing. Often, the straw-bale house was seen as a make-shift structure, to provide temporary lodging until enough funds were available for a "real" house. However, the houses were found to be comfortable, durable, and affordable, and soon became regarded as permanent housing.
Straw-bale construction experienced a re-emergence in the late 1970s, after the 1973 energy crisis brought issues of sustainability to the forefront. Because it is simple, inexpensive to build and own, made from a renewable resource, and involves few synthetic chemicals, it continues to grow in popularity, especially with do-it-yourselfers and proponents of sustainability.
A conventional timber frame may be used to support the roof, and infilled with straw bales for insulation. This type of structure, which can easily be made to conform to a building code or analyzed for stresses, gives up a large part of the cost advantage that would be obtained by using bales as structural elements.
Alternatively, the bales themselves may be used as load-bearing walls. Sometimes a combination of both techniques is used, where outside walls have load-bearing straw bales and inside walls are timber-framed.
In the case of load-bearing walls, the bales are laid in running bond like bricks. They may also be staked to increase stability.
Straw-bale walls are typically finished with stucco, sometimes in creative colours or textures. Structural analysis has shown that the straw-bale/stucco assembly behaves much like a sandwich panel, with the stucco skins bearing some of the load and adding considerable strength to the wall. Plaster for straw-bale walls normally contains a high percentage of lime (often 50 to 100%) because lime allows water vapour to escape from the walls.
Due to the limited structural strength of straw bales, dug foundations are not usually used if the bale walls will be load-bearing. Instead, a slab-on-grade foundation or rubble trench foundation is often used. Straw bales have been used to insulate the floor from the slab but this practise has fallen out of favour due to the difficulty of isolating the bales from moisture.
Many different types of roofs are appropriate for straw-bale building. Some small structures use an igloo or dome-like structure to create a roof out of straw-bale. More commonly, the roof is constructed from wood or other materials, and attached to the top of the straw walls using a top-plate. Even in this case, however, the roof may be insulated with straw bales.
The thick walls (typically 18") result in deeply recessed windows. As well, since the bales are not rigid, when plastered they tend to adopt smooth rounded curves. These features give straw-bale buildings a characteristic 'curvy' aesthetic.
Straw-bale houses tend to retain heat very well due to the exceptional insulative value of straw. The theoretical R-value (thermal resistivity) for a 16.5" straw bale was calculated by Joseph McCabe as 52. This is compared with a theoretical R-value for 3.5" of fiberglass (the conventional insulation material used in home construction) of 13.
Some lab tests of straw-bale assemblies have found significantly lower R-values in practise. However, the more conservative of these results still suggests an R-value of 28, which is a significant improvement over the R-14 of an energy-efficient insulated 2x6 wall. Straw-bale experts suggest that it is possible to approach theoretical R-values by giving more attention to detailing.
A straw-bale building, with its dense, thick walls (typically 18") also provides excellent thermal mass. This reduces the thermal swings due to daytime warming and nighttime cooling, and lessens the need for fuel or electricity to regulate temperature. This is more important at high desert altitudes where a clear sky contributes to both warm days and cool nights. Straw bale construction distributes the thermal mass throughout the wall.
Straw-bale building requires little specialized equipment. It has often been successfully used by inexperienced builders working on their own homes.
Straw is widely available, renewable, and generally inexpensive. Because it is a plentiful byproduct of grain harvesting, farmers are often willing to provide it at low or no cost. Unlike conventional building which requires co-operation between many fields of specialized knowledge, much of the labour of straw-bale building is fairly simple (hauling bales, applying stucco). This can provide a significant cost savings to the builder/owner who is willing to perform that work themselves.
Straw bales are thick and dense enough to keep out many kinds of pests. As well, the outer layer of plaster makes them unattractive or impenetrable to animals and insects. Finally, because straw contains no nutrient value, it does not attract pests.
Although loose straw is quite flammable, once packed into a bale it is too dense to allow enough air for combustion. By analogy, it is easy to light a single piece of paper on fire, but difficult and time consuming to burn an entire phone book.
The outside layer of plaster over the straw-bale wall increases its fire retardant properties.
Load-bearing straw-bale walls are typically used only in single storey or ocasionally double-storey structures. A dug foundation (basement) is uncommon.
Straw-bale construction is still considered experimental in many jurisdictions. Building codes may not include it, local authorities may not recognize it, and most contractors will probably not be experienced in its use.
Straw-bale buildings must be carefully designed to eliminate the possibility of moisture entering the walls, especially from above. Successful designs often incorporate roof overhangs that are wider than normal and roof shapes and detailing that minimize the risk of water splashing against walls.
Because straw-bale walls are much thicker than normal walls, there is sometimes a compromise between the size of the building's footprint and the amount of living space.
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