Straw bales, a by-product of the mechanical harvesting and threshing of grain, are produced in large quantities in mechanised agricultural countries. The traditional rectangular bales, which are cheap and can be manhandled individually, are appropriate for building. The large cylindrical and very large rectangular bales, which require mechanical lifting, are less useful in construction and are not considered here. Standard bales (typically 330 X 530 X 1050 mm) are produced within the baler by compressing quantities of straw into flakes about 100 mm thick. These layers are built up along the length of the bale, which is then automatically tied, usually with two polypropylene strings. There is inevitably some variation in length, and the ends are slightly rounded. For construction, the bales should be well compressed in manufacture, dry (maximum 20% moisture) to prevent the growth of moulds and fungi, and with the minimum amount of remaining grain, which might attract rodents.
In building construction, bales are stacked, large faces down, making the orientation of the straw fibres
predominantly horizontal. At ground level, straw bales must be protected from rising damp and from any risk of saturation from surface water. Additionally steel mesh protection from rodents is necessary. Adjacent bales must be firmly packed together to ensure stability and to reduce settling under load both during and after construction. Bales are normally secured with metal spikes or hazel rods from coppiced timber and may be sprayed with insecticide for added protection. Externally, lime render on wire mesh is appropriate as it is flexible, self-healing, and will breathe to prevent the build-up of trapped moisture. Alternatively a rainscreen, separated from the external face of the bales, may be used. Internally, straw bales are usually finished with gypsum plaster on wire mesh. Openings in straw bale construction may be formed with timber framing, but careful detailing is required to prevent water penetration at these locations. Roofs are normally set onto a timber wall plate fixed through the top bales for stability.
An alternative approach to using load-bearing straw bales is timber- or steel-frame construction with straw bales as the insulating infill (Fig. 15.2). While fire is a risk during straw bale construction, the non-combustible internal and external finishes and the compact nature of the straw make the completed construction resistant to fire. (The thermal conductivity of baled straw is approximately 0.050 W/m K.)
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