Straw-bale construction

From Academic Kids

Straw-bale construction is a building method that uses straw bales as structural elements, insulation, or both. It is commonly used in natural building.

Contents

History

Straw-bale construction was pioneered in Nebraska in the late 19th 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-yourself'ers and proponents of sustainability.

Techniques

Wall framing

One method of construction uses a post and beam framing system to support the roof. Conventional timber frames, steel frames, and concrete are all common depending on the preference of the builder. Once the structure is in place, the walls are then infilled with straw bales for insulation. This type of structure can easily be analysed for stresses and made to conform to a building code.

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.

The building code in the State of New Mexico (1994 ed.) requires that all straw-bale homes be built with rigid structural frames. No other state or regional building code has this restriction. See State of California, Pima County Arizona, etc.

The bales are often laid in running bond like bricks. They may also be staked to increase stability.

The primary cost advantage to building with straw bales is that many buildings are largely built by the homeowner and/or with community involvement through work parties or workshops. While the material to build a straw-bale wall is typically less than that to build a wood frame wall system, a glance at standard home construction cost distribution immediately shows that the cost of the perimeter wall systems replaced by the straw-bale wall system are typically between 5% and 12% of the overall cost of the building.

Plastering

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.

Foundation

Standard concrete foundations or slab-on-grade foundations are typical. With load-bearing straw-bale homes rubble trench foundations are often used. Straw bales have been used to insulate the floor from the slab but this practice has fallen out of favour due to the difficulty of isolating the bales from moisture.

Roofing

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.

Characteristics

The thick walls, typically 21 inch (530 mm) when plastered, result in deeply recessed windows and doors. Since the bales are not rigid, when plastered they tend to adopt smooth rounded curves. In fact, curved walls are popular as skilled installers can curve the stacked straw-bale walls very easily. These features give straw-bale buildings a characteristic 'curvy' aesthetic.

Advantages

Insulation

Straw-bale buildings tend to retain heat very well due to the exceptional insulative value of straw. The theoretical R-Value (thermal resistivity) for a 16.5 inch (420 mm) straw bale was calculated by Joseph McCabe as 52. This is compared with a theoretical R-Value for 3.5 inch (90 mm) of fibreglass (the conventional insulation material used in home construction) of 13. This means fibreglass has an R-Value of about 3.7 per inch and straw bales have about 3.2. Fibreglass commonly achieves R-Values up to 4.2 (R-15 3.5 in (90 mm) fibreglass insulation.

Some lab tests of straw-bale assemblies have found significantly lower R-Values in practice. However, the more conservative of these results still suggests an R-Value of 28[1] (http://www.cmhc.ca/publications/en/rh-pr/tech/tech02-115-e.html), which is a significant improvement over the R-14 of an energy-efficient insulated 2x6 wall[2] (http://www.cmhc.ca/publications/en/rh-pr/tech/tech02-115-e.html). Straw-bale experts suggest that it is possible to approach theoretical R-Values by giving more attention to detailing.

Thermal mass

The typical interior finish of a straw-bale wall is either cement or gypsum plaster, or a combination. This wall material provides excellent thermal mass. Thermal mass reduces the thermal swings due to daytime warming and night time 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 stores daytime heat and releases it gradually during the cold night. This cold soaked wall then keeps the interior cool during most of the day.

Simplicity

Straw-bale building requires little specialised equipment. It has often been successfully used by inexperienced builders working on their own homes.

Availability and cost

Straw is widely available, renewable, and generally inexpensive. Because it is a plentiful by-product 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 specialised 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.

Resistance to pests

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.

Resistance to fire

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. In construction it is critical to have, at a minimum, a parge coat of plaster on all surfaces of the wall.

Typical failure of straw-bale homes involves frame walls set against straw-bale walls without a parge coat. A spark from an electrical short or an error by a plumber ignites the hair-like fuzz on the exposed bale. The flame spreads upward and sets the wood framing on fire causing the wood framing to burn. The typical fire results in little fire damage to bales, but extensive water damage due to the fire suppression activities.

The ASTM E-119 fire resistance test for plastered straw-bale wall assemblies in 1993 passed for a 2 hour fire-wall assembly. In this test a gas flame blows on one side of the wall at approximately 2000 degree Fahrenheit (1100 degrees Celsius) while the temperature of the other side of the wall is continuously measured. The results of this test had no burn-through and a maximum temperature rise of 60 degrees Fahrenheit (15.5 degrees Celsius).

Disadvantages

Limits to structural strength

Load-bearing straw-bale walls are typically used only in single-story or occasionally double-story structures. A dug foundation (basement) is uncommon.

Post and beam straw-bale structures have been used for buildings as large as 14,000 square feet (1,300 m²) and even for a United States Post Office, in Corrales, NM. see: http://www.sarep.ucdavis.edu/NEWSLTR/v5n3/sa-10.htm

Design and construction challenges

Straw-bale construction is still considered experimental in many jurisdictions. Building codes may not include it, local authorities may not recognise 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 minimise 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.

See also

External links

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