5.8 Straw Bale
Straw has been used as a building material for centuries, for both thatch roofing and also mixed with earth in cob and wattle and daub walls. Straw bales were first used for building over a century ago by settlers in Nebraska, USA, shortly after the invention of baling machines.
Straw is derived from grasses and is regarded as a renewable building material since its primary energy input is solar and it can be grown and harvested.
Straw is the springy tubular stalk of grasses like wheat and rice that are high in tensile strength. It is not hay, which is used for feeding livestock and includes the grain head. Straw is composed of cellulose, hemicellulose, lignins, and silica. It breaks down in soil and waste straw can be used as mulch. Different grasses have slightly different qualities, for instance rice straw has a significant amount of silica, which adds density and resistance to decomposition. Straw bale walls are surprisingly resistant to fire, vermin and decay. Australian straw bales have two strings (American how-to books often show 3-string bales) and are typically 900mm long x 450mm wide and between 350 and 400mm high and weigh 16 to 20 kg.
Finished straw bale walls are invariably rendered with cement or earth so that the straw is not visible. The final appearance of rendered straw bale can be very smooth and almost indistinguishable from rendered blockwork, or it can be more expressive and textural. There is a project in London, England, for instance, that made straw bales visible in the completed construction by placing them behind corrugated acrylic cladding.
The structural capacity of straw bale construction is surprisingly good. In the load-bearing (‘Nebraska’ style) straw bale method, walls of up to three storeys have been constructed, but straw bale construction commonly uses a frame for the building structure. Most buildings require a frame of timber or steel to comply with current building codes. [See: 5.5 Construction Systems]
There are now several examples of multi-storey buildings in framed straw bale construction, including three houses with two storeys of straw bale wall in the City of Adelaide.
Straw bales themselves have very low thermal mass, being composed, by volume, mostly of air. However, the cement and earth renders typically used on straw bales results in finished walls having some appreciable thermal mass in the thin masonry ‘skins’ either side of the insulated straw core. With the use of earthen renders a thick render skin of up to 75mm can be achieved, providing significant thermal mass. [See: 4.9 Thermal Mass]
Straw bales demonstrate excellent insulative properties, in fact possibly the most cost effective thermal insulation available. [See: 4.7 Insulation]
Inch for inch, or centimetre for centimetre, straw has a similar insulation value to fibreglass batts. The insulation value of a straw bale wall greatly exceeds that of any conventional construction.
All straw bale buildings demonstrate excellent insulation characteristics and the design goal in any structure must be to complement the insulation performance with the performance of the rest of the building. Thus, it is essential to insulate roofs and windows to maintain the overall performance of a straw bale building. [See: 4.10 Glazing]
Straw bales also provide the most costeffective sound insulation available. Dollar for dollar, the overall insulation value of a straw bale wall greatly exceeds that of any conventional construction.
The effect of sound insulation contributes to the livability of this kind of construction and can be quite marked. Even walking into the space created by an unfinished straw bale structure, one can appreciate the quietness and hear the difference compared with conventional buildings. [See: 2.7 Noise Control]
Straw bales are tightly packed and covered with a skin of cement render. Fire can’t burn without oxygen, and the dense walls provide a nearly airless environment, so the fire resistance of compacted straw is very good. Conclusive evidence of its good fire resisting performance can be found in laboratory fire tests conducted at the Richmond Field Station in 1997 by students at University of California Berkeley.
These rated a straw-bale wall at two hours. Straw bale homes survived Californian bush fires that destroyed conventional structures.