Straw-bale construction

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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

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History

While use of grass-family plant fibers has long been a part of building methods worldwide, dating far back into prehistory, actual straw-bale construction was pioneered in Nebraska in the United States, in the late 19th/early 20th century, in response the then-new availability of baling machines and the lack of significant amounts of lumber or buildable sod needed to build barns and housing in the Sandhills region. Under the Homestead Act of 1862 and the Kinkaid Act of 1904, the "sod-busters" were required to develop and live on their new property for five years in order to maintain ownership; building housing was a legal requirement. The straw-bale house was first seen simply as a make-shift structure, to provide temporary lodging, until enough funds were available to pay for the shipping in of timbers, to build a "real" house. However, these homes quickly proved to be comfortable, durable, and affordable, and so became regarded as permanent housing. Over the past century they have indeed outlived many neighbouring timber-frame buildings, and a number are in continuing use today and beginning their second century.

After World War II a scattering of U.S. veterans turned to straw-bale for shelter, but modern straw-bale construction experienced a re-emergence in the late 1970s, after the 1973 energy crisis helped bring issues of real sustainability to the forefront, with first examples built primarily in the southwestern United States. Now, they are being built the world around, from northern Canada, Mongolia and post-Chernobyl Russia, to Mexico, Australia and New Zealand. Because it is based on an inexpensive and renewable so-called "agricultural waste product," with a technique relatively simple for beginners to implement, involving few synthetic chemicals and providing effective energy-conserving insulation, it continues to grow in popularity, especially with do-it-yourself-ers "owner-designer-builders" and other proponents of sustainability.

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Materials

Straw-bales can be made from a range of plant fibers, not only grass-family species like wheat, rye, barley, blue-grass and rice, but also flax, hemp, etc. (Bales of recycled materials like paper, pasteboard, waxed cardboard, crushed plastics, whole tires and used carpeting have also all been used or are currently being explored for building.)

Basic straw-bales are produced on farms and referred to as "field-bales". These come in a range of sizes, from small "two-string" ones 18 in (460 mm) wide, by either 14 or 16 in (350 to 400 mm) high, and 32 to 48 in (0.8 to 1.2 m) long, to three-string "commercial bales" 21 in wide, by 16 in high, by 3 to 4 ft long. These sizes range from 40 to as much as 100 pounds (18 to 45 kg).

Even larger "bulk" bales are now becoming common, 3 by 3 ft (1 by 1 m), or 3 x 4 ft (1 m by 1.2 m) by 6 ft (2 m) long and even 4 x 4 x 8 ft (1.2 by 1.2 by 2.4 m) long, weighing up to a ton, plus rolled round bales 4 to 5 ft (1.2 to 1.5 m) in diameter. All of these "economy-size" units also offer unique potential for imaginative designers.

A newer trend is the use of high-density recompressed bales, sometimes called strawblocks, offering far higher compression strength. These bales, "remade" from field bales, in massive stationary presses producing up to 1 million pounds of force (4 MN), were originally developed for cargo-container transport to over-seas markets.

But innovators soon discovered that where a wall of "conventional field bales" is able to support a roof load of 600 pounds per foot (900 kg/m), the high-density bales can support up to 3,000 to 4,500 pounds per foot (4,500 to 7,000 kg/m). This makes them particularly suited to load-bearing multi-storey or "living-roofed" designs, and they may be faced with siding, gyp-board or paneling and have cabinetry hung directly from them with long sheet-rock screws.

They are available in a range of sizes from different companies' presses but 2' long by 2' high by 18" wide might be considered "typical"; because they are bound with horizontally ties or straps, at 3" or 4" intervals vertically, they may be recut with a chain-saw at a range of heights. And they usually used in "stacked bond", with the straws running vertically for greatest strength and tied with "re-mesh" both sides, before stuccoing.

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Techniques

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Walls

The original "Nebraska" straw-bale building technique was one in which walls of straw-bales actually provided the support for the roof-structure above, so these are now referred to as load-bearing, and straw-bale homes of this style continue to be built and permitted.

An alternative method of construction uses a post and beam framing system to carry roof, wind and seismic loads. Once that structure is in place, the walls are then infilled with straw bales for insulation. This type of structure is popular because it allows bale placement to be accomplished with the roof already in place, "in the dry", and can easily be demonstrated to conform to building codes, using conventional engineering techniques or a pre-engineered pole-structure design.

Some projects best lend themselves to a combination of both techniques, with load-bearing perimeter walls and pole or stick-frame support at the interior or ridge; this is termed a "hybrid" structural system.

The building code in the State of New Mexico (1994 ed.) required that all straw-bale homes there be built with rigid structural frames, while other state or regional building codes lack this restriction (see codes for California, Pima County Arizona, etc.) In other jurisdictions without specific "straw-bale codes", strawbale construction is often approved under the building code provisions for alternate methods and materials. Plans are commonly required to be stamped by a licenced structural engineer.

Field bales are often laid in running bond like bricks. They are easily retied to make half or custom sized bales. They may also be easily "pinned" internally or on both surfaces (with bamboo, reed, rebar or wood), or can be "caged" on both faces with pre-welded or woven mesh, to increase pre-stuccoed wall stability.

Bale stacking is often done in community "bale raisings", where family and friends pitch in together to raise the walls in a weekend or two. Novice owner/builders and their friends can continue the work through lathing and plastering of the bales, giving the house their own special imprint, and achieving savings in construction costs, as well.

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Structural Capabilities of Bale Walls

The bale assembly can do a number of things, depending upon the structural design of the building:

  • Hold itself up, be self-supporting and resist tipping.
  • Keep out the wind; inhibiting air/moisture infiltration.
  • Resist heat transfer (insulation)
  • Reduce water intrusion and migration, store and transfer moisture within the wall.
  • Keep the assembly from buckling, under a compressive load.
  • Keep the assembly from deflecting in a strong wind, when pushed from the sides or end.
  • Keep the assembly from bursting apart in an earthquake, when pushed and pulled from all directions.
  • Hold the plaster at least while it’s curing.
  • Keep the plaster from cracking after it’s cured, from shrinkage or movement.
  • Support the plaster skins from buckling.
  • Transfer and absorb loads to and from the plaster.
  • Support the roof load (compression).
  • Reduce damage or failure from high winds (ductility).
  • Reduce damage or failure from earthquakes (ductility).
  • Stop bullets and/or flying debris.
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Finishes

Straw-bale walls are most typically encased with stucco outside and plaster inside, sometimes in creative colours or textures. Structural analysis has shown that the straw-bale/stucco assembly behaves much like a sandwich panel, with the rigid stucco skins initially bearing most of the load and adding considerable strength to the wall. Stucco for straw-bale walls is most commonly is cement/sand-based, although mixes containing earth or clay and/or with a high percentage of lime, replacing part or all of the cement (allowing higher water vapour permeability through the walls) are increasingly popular trends. (Advocates of sustainable construction are becoming increasingly concerned with the fact that for every ton of cement manufactured and used, another ton of climate-changing fossil CO2 is released into the atmosphere.)

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Foundation

Standard concrete footing/foundations or thickened-edge-slab-on-grade foundations have been typical. Bales can also be stacked over stem walls with joisted floors. With load-bearing straw-bale homes rubble trench foundations or Earthbag construction foundations are increasingly used, as an alternative to conventional footings. Some pioneer designers are even using rock-filled gabions or earth-filled "bastions" in lieu of concrete. Straw bales have been used to insulate the floor from the slab, or to provide subgrade perimeter insulation, but this must be done with care, due to the importance of isolating the bales from undue moisture. (Moisture levels higher than 18% support mold growth in both straw and wood.)

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Roofing

Many different types of roofs can be used for straw-bale buildings. Some building have been designed with a barrel-vaulted roof utilising straw bales' compressive strength and ductility as part of the structure. Most often a conventional roof structure is attached to a load-distributing plate or beam at the top of the straw walls. These conventional roof structures may be insulated with straw bales, taking advantage of their high insulation values and acoustic properties. Other alternative insulation includes rice-hulls, cotton or wool batts, soy-based foam and recycled cellulose.

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Characteristics

The thick walls (typically 21 to 26 inches (530 mm) when stuccoed/plastered), result in deeper window and door "reveals", similar to stone and adobe buildings. Since the bales are irregular and may be shaped easily, they are readily adaptable to curved designs, and when plastered, tend toward a relaxed, imperfect texture and shape. If flat, straight walls are desired, this can be achieved, as well, by the application of more plaster.

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Advantages

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Insulation

Straw-bale buildings have excellent thermal performance because of their combination of high insulative value and the well-distributed thermal mass provided by thick plaster coating.

The theoretical R-value (thermal resistivity) for a 16.5 inch (420 mm) straw bale was calculated by Joseph McCabe as 52 (RSI-9.2). 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 (RSI-2.3). This means fibreglass has an R-value of about 3.7 per inch (RSI-0.26 per centimeter) and straw bales have about 3.2 per inch (RSI-0.22 per centimeter).

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], which is a significant improvement over the R-14 of an energy-efficient insulated 2x6 wall[2]. Straw-bale experts suggest that it is possible to approach theoretical R-values by giving more attention to detailing.

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Thermal mass

Template:Main The typical interior finish of a straw-bale wall is either cement or gypsum plaster, or a combination. An appropriate thickness of this wall material can provide excellent thermal mass on a diurnal cycle.

Thermal mass reduces the thermal swings due to daytime warming and night time cooling, by absorbing and then gradually releasing heat. This can result in a direct reduction in the need for fuel or electricity to regulate temperature, and indirectly in savings through lifestyle adjustments: occupants of a moderate environment, with only gradual temperature swings, are less likely to use artificial heating and cooling. This is most easily achieved at high desert altitudes where a clear sky contributes to both warm days (solar gain) and cool nights (nighttime cooling), but the principle works well in other climates as well.

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Simplicity

Straw-bale building utilizes locally availble materials, and basic construction techniques that require little specialised or proprietary materials and equipment. It has often been successfully used by inexperienced builders working on their own homes.

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Availability and cost

Straw is an agricultural waste product, a by-product of grain harvesting. Many different kinds of straw are baled and can be used for construction. Straw is widely available, and is generally an abundant, renewable resource. Relatively little energy is consumed in harvesting, baling and transporting bales to a building site. In bulk, straw bales are generally sold for close to the cost of baling and transport. Farmers will sometimes sell bales for under cost in order to clear storage sheds prior to a harvest.

In most regions, straw is baled only once each year, and so must kept dry and stored for use at other times of the year. Straw production and demand is relatively constant, however high demand for bales used for erosion control following forest fires can create a temporary shortage of bales.

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Resistance to pests

Straw bales are both thick and dense enough to keep many kinds of pests out of the home. The outer layer of plaster makes the wall impenetrable to animals and insects. Straw contains little nutrient value to attract most pests.

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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. Parge coating the wall involves troweling on a thin coating of mortar and brushing it smooth.

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 (33.3 degrees Celsius).

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Disadvantages

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Limits to structural strength

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

An all-straw vaulted building was designed and built in Joshua Tree, California, and greatly exceeded the structural requirements for this highly active seismic zone.

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 [3].

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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.

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See also

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External links

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