Lightweight Straw Loam

A mix of straw and clay is rammed directly into the wall, or produced as blocks (which can later be built up with a clay mortar) (Figure14.19).The straw loam is produced asfol-

lows:

1. All clays can be used.The clay is dried, crushed and poured into a tub of about 200300 litres, ten times as much water is added and mixed well in. A motorized mixer can be used or the work can be done by hand. About 2% sodium waterglass is added to reduce the surface tension, so that the water can more easily penetrate the clay particles. This reduces the amount of water required and shortens the drying time.The clay should lie in the water for two hours. If using wet clay, it should be laid in water so that it is just covered and left for 24 hours.

2.The mixture is tested: 1 dl of the mixed clay gruel is poured evenly onto a piece of glass.The diameter is measured. If it is much less than 15 cm, it needs more water. If it is much more, then it needs more clay.

3.The clay gruel is poured onto the straw until it is totally drenched. In order to increase thermal insulation, straws with rigid shoots are preferred, since they do not deform easily, and hence keep air trapped inside.

4.The mixture is put into moulds to form blocks, or rammed into simple movable shuttering on either side of a timber frame wall, 30-60 cm thick. Before ramming, the timber framework-the structural part of the wall-is covered with clay as a sort of impregnation. The mixture must not be rammed hard. The middle is pushed down with the foot, whilst the edges are given more pressure: they can be beaten down with a piece of wood.The more compressed the mixture is, the stronger the wall, but with a corresponding reduction in thermal insulation. The different layers need to overlap each other when rammed within the shuttering.The holes left after

removing the shuttering are filled with clay. The drying time during the summer is between sixand eight weeks, depending on the weather.

STRAW BALES

Bales of straw stacked on top of each other representan economical and effectivether-mal buffer. In smaller constructionsthe bales can be load bearing, but useasframe infill is more common. Two-string bales are usually 36 x 46 x 92 cm and weigh about 20 kg, whilst 3-string bales are 41 x 58 x 114 cm with a weight of over 30 kg. Both dimensions and weight can vary depending upon the straw type, the baling equipment and the pressure used.The bales must be properly dry (10-16%) with no sign of mould or rot. Additional rot resistance can be achieved by dipping the bales in a solution of 5% waterglass, thin runny clay or lime gruel.This also improves fire resistance. All fresh straw has a thin waxy surface layer that breaks down over time.This prevents rendering from adhering to the straw, so straw should be stored for ayear before use.

Bales are stacked up on each other and coursed like normal brickwork. Between courses, 70 cm long stakes can be inserted to hold them together. Extra reinforcement is used at corners and around openings. After two to four weeks the walls can be clad with a wire mesh and rendered, either with hydraulic lime plasterora clay-based plaster, usually in three to four layers. Rendered straw structures are non-flammable. On very exposed sites, rendered surfaces must be protected by an extra outer skin such as timber panelling.

Building with straw bales was very popular in the USA until after the Second World War. They were used for everything from schools to aircraft hangars. In the early 1980s, a 75-year-old school built of straw was demolished in Nebraska, and the straw was undamaged and fresh enough to be used as cow fodder. Such relatively unexpected experiences have led to a renaissance of straw bale building in Canada, USA and Europe (Figures14.20and14.21).

14.21

Straw baled kindergarten at Kongsberg Norway. Architects: Gaia Tj0me, 2007.

14.21

Straw baled kindergarten at Kongsberg Norway. Architects: Gaia Tj0me, 2007.

Matting and felt can be produced by binding fibres or stalks together with galvanized wire or by glueing, needle-punching or weaving together.

Hemp, flax and stinging nettles provide good fibres. The fibre fraction in mixed grasses produced on ordinary grassland is also very usable. With flax and hemp one normally uses the shorter fibres that are discarded during the production of textiles. The fibres are usually treated with flame retardants and fungicides. Hemp products have a natural resistance to fungal and are usually only treated with soda as a fire retardant (Figure 14.22).

14.22

Insulating with hemp matting. Source: Camilla Hoyem, Thermo-hanf.

114.23

Needlepunching of matting makes the use of glue unnecessary.

In needle-punching, fibres are filtered together into airy, elastic insulation mats (see Figure 14.23). Glued products are more common, and usually consist of polyolefin (e.g. polypropylene) or polyester (e.g. polyethylene terephthalate) fibres or fibres produced from maize starch (318% by weight) melted into the fibre mass (thermal bonding). Starch glue produced from potatoes or maize can also be used. The fibre mats are used in the same way as those of mineral wool or wood fibre.

Denser felt products and strips for sealing joints are usually produced by filtering or weaving the fibres together. These are often also used for sound insulation on floors.

Traditional reed mats are produced from stalks bound together with galvanized string or wire. Reed mats can be used as thermal insulation and as reinforcement in concrete walls. When rendered, the mats are also used as false ceilings.

Boards can be produced from straw in a mould with the stalks lying at right angles to the direction of the board. By exposure to pressure and heat, natural glueing agents in the straw are released and bind the board together. This process is stimulated by adding laccase. Porous boards have only moderate thermal insulating qualities, approximately equivalent to woodwool cement. Under damp conditions they will be exposed to attack by fungus. Straw boards can be produced with different densities (see page 344).

The first insulation boards made of straw were produced as early as the 1930s. They were made in thicknesses of 5-7 cm, under low pressure, reinforced with cross-wires and covered with paper.

Linseed oil putty is produced from the seeds of the flax plant. In addition to linseed oil, it consists of stone flour, such as chalk, heavy spar, powdered fired clay, powdered glass, etc.

Linseed oil putty is the only alternative to plastics-based mastics and window putties. It is environmentally much sounder than the alternatives, with no negative effects during production or use. As a waste product, it can be composted as long as no additives (such as lead) have been mixed in to improve its elasticity. The elastic qualities of the putty can be preserved for some time by painting with oil-based paint. Despite this, the putty will eventually harden and begin to crumble. Linseed oil-based putty must not be used in contact with damp lime or cement surfaces.

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