Straw Bale Construction/Materials
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Straw sources, bale dimensions and compression
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. They are usually used in "stacked bond", with the straws running vertically for greatest strength and tied with "re-mesh" on both sides before stucco application.
Choosing your bales
- straw source
- twine tightness
Keeping your bales dry before building
Keeping your bales dry is extremely important. This can be a challenge, especially when building load bearing. A dry barn on site is a big advantage. Or try to negotiate to leave a trailer with bales on site for a month.
If neither of those is an option, some hints:
- Have your bales delivered only when ready to build
- Put a double layer of pallets under your bale pile.
- Make multiple local piles of bales. Put those near the places where you will need bales. This will speed up construction, and thus limit the times bales are exposed.
- Stack the bales in pyramid shape. Cover them in tarps. The pyramid shape will help rain water drain off the pile.
- You don't expect windy rain? Then leave the bottom part of the tarp exposed. Wind will help moisture (condensation, ...) that did get to the bales dry up.