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Straw bale construction

Peter Lloyd, a recent NBS Technical intern, and Charles Stirling, NBS Technical author, welcome straw bale walling to the mainstream.

Now that the rather satirical huffing and puffing references to the ‘Three little pigs’ have come and gone, straw bale construction has come of age. Projects in the UK involving straw bale have evolved from the smaller utility and out-buildings to large dwellings and local amenity buildings.

In response NBS has introduced a new work section, F42 Straw bale walling systems. Although straw bale walling is rightly regarded as a unique form of construction, relevant standards and codes have been referred to where appropriate, as with other work sections.

This new section provides guidance on the materials and process of straw bale construction in a clear and concise manner. Many of the processes within the work section will be familiar, such as the application of plaster and render, although the use of lime and natural clays may be less familiar. Hazel sticks used to pin bale courses, and the need to allow for settlement over openings in unframed walling systems, are peculiar to straw bale and specific guidance should be sought.

Guidance, drawings and sketches given in publications such as the Information guide to straw bale building, produced by Amazonails [www.strawbalefutures.org.uk], are extremely useful and should be referred to in conjunction with the work section.

Straw bale construction originated on the plains of Nebraska over 100 years ago following the introduction of mechanical baling techniques. The original structures, initially intended as temporary, used the straw bales in a ‘Lego’ building block fashion, where the bales are loadbearing, and this form of construction is still referred to as the ‘Nebraska’ style. We refer to it as unframed. Framed systems are also used – these incorporate an integral framed structure, normally timber, which carries the primary loads.

Within the UK, straw bales are available in a range of sizes and agricultural suppliers will supply bales to specific sizes and bale density. During construction some bales must be reduced in length to accommodate site variations and to facilitate course bonding. Techniques are available for cutting and re-binding bales to specific lengths to suit construction needs.

Baling twine performs a vital role in determining the long-term performance of the straw bale wall, particularly where the bales have been cut and re-bound, and tough and durable materials should be specified.

Performance

Initial concerns over durability in the wetter, more temperate, climate of the UK have been dispelled. With effective detailing at ground level and wall heads, and with weatherproof vapour permeable renders applied externally, straw bale walling can have an effective service life equivalent to its masonry counterparts. Care needs to be taken in specifying membranes or finishes with high vapour resistance within and on the straw bale walling as this will impede the natural drying of the bales. Cement-based renders and gypsum plasters are not recommended for direct application onto straw. Instead coatings incorporating lime and natural clay based materials should be used. These are breathable, and accommodate movement better.

Equally, concerns over fire performance are unfounded. Straw bales, when well compacted and built into an integral unit, tend to behave as solid timber. This is particularly so where the bales are under compression, e.g. in unframed construction. In a fire, the straw bales char on the outside, providing protection from further burning. For this to be fully effective the straw bales must be tightly packed and the linings to openings protected.

Straw bale construction provides a means of thermally insulating buildings at low cost and low embodied energy. The good thermal performance of straw bales is due to the high proportion of entrapped air within the straws and the bale matrix. Straw bales around 450 mm thick will have a U-value of 0.13 W/m²·K.

Straw bale construction has impeccable green credentials. The primary material, straw, is a waste product resulting from seed production. Straw has no animal feed value and, although used as bedding material, cannot now be disposed of by burning, and must instead be returned to the ground by mulching. The environmental purist may, however, be uncomfortable with the fertilizers and pesticides used during growing and the fuel burning equipment used in harvesting, but these costs are primarily incurred for seed production.

There are no UK standards or codes of practice which relate specifically to straw bale construction. There are a number of publications which provide detailed guidance relevant to practice in the UK and elsewhere (see for example www.cc-w.co.uk/SB_Resources.htm). Given the specialist nature of straw bale construction, it is recommended that detailed discussions are held with the contractor to ensure that proposed materials, techniques and finishes are available locally and can be achieved. This is especially the case for specifiers inexperienced in straw bale construction. Of course, this assumes that the contractor is experienced. If there will be any doubt about this, then method statements could be requested at tender.

Straw bale construction is an inherently simple form of construction, which can be erected by (supervised) unskilled labour and provide a cost-effective, environmentally-friendly building which has a unique feel and resonance when completed.

Written October 2007

 

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