Straw Bale

12/12/2014

STRAW

+ STRAW BALING MACHINE

= STRAW BALE

Straw as a building material STRAW: The dry stems (the part between the ro ot stem and the ear) of threshed thresh ed grain (wheat, barley, oats, millet) or fibrous plants (flax, hemp, rice). Most suitable for the production of bales for building is wheat, spelt and rye. Barley and oats are less stable. STRAW is not HAY. Consists cellulos e, lignin, silica. Silica content extremely slows down rotting of straw. It is similar to wood in cellulose content, and is therefore not digestible by animals. Only 15% of the straw is used for for agricultural purposes or as bedding for animals . The remainder is considered a waste, and is usually burned producing CO2. Possesses a waxy, water- repellent skin. Straw has therefore therefore been used to thatch thatch roofs for centuries. It has been used as an aggregate in loam – increases thermal insulation and reduces cracks.

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Straw as a building material STRAWBALE: SMALL SIZE BALE DIMENSIONS: 32 - 35cm X 50cm X 50 - 120cm DENSITY: 90 to 120 kg/cu.m (lower density not s uitable as building material) MID SIZE BALE DIMENSIONS: 50 cm X 80cm X 70 - 240cm JUMBO SIZE BALE DIMENSIONS: 70cm X 120 cm X 100 – 300cm DENSITY: 180 – 200 kg/ cu.m The medium and jumbo size bales are used for load- bearing structures. They have very large wall thickness – difficult to handle and may require lifting devices.

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Straw as a building material Points to remember: The MOISTURE CONTENT of the straw needs to be LES S THAN 15% to be used as building material. Weeds are less stable and will decay faster when damp. Hence WEEDS MUST BE ABSENT in the bale. Bales must have ACCURATE DIMENSIONS. Bales should be STORED IN A DRY ENVIRONMENT and should not touch moist ground directly and must be protected from rain. Damp bales must be spaced to allow quicker drying, and prevent damage by micro- organisms. Stored bales SHOULD SHOW NO SIGNS OF DECAY/ MOULD. The tying strings should be tight – POLYPROPYLENE is suitable.

ADVANTAGES STRAW is a RENEW ABLE BUILDING MATERIAL. It is RECYCLABLE and hence disp osal is not a problem. In fact, it can ea sily be separated from other materials upon demolition and may be us ed as mulch in the garden or in agriculture for de-comp acting soil. Relatively LESS ENERGY IS REQUIRED FOR THE PRODUCTION of straw bales  – 14 MJ/ cu.m of energy. TRANSPORTATION of straw bales is fairly LESS ENERGY- INTENSIVE. Straw is a “SUSTAINABLE” building material, which has minimal negative impact on the environment – even less than timber FIRE RATING of F90 allows straw bale structures to be us ed in single and double storey structures. Strawbale structures have a LONG LIFE- SPAN as shown by existing structures. Suitable for DO-IT-YOURSELF CONSTRUCTION = BUILDING COST SAVINGS

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Straw as a building material Common Concerns: FIRE: Loose straw easily catches fire. But straw bale walls that are plastered on both s ides achieve a fire rating of 90 mins. MICE: Mice do not feed on straw. Moreover, the 90kg/ cu.m density strongly resists rodents. Cavities in the bales may be problematic. Plastering strawbales further discourage rodents. TERMITES: Termites don’t feast on straw. Though some species might be able to digest straw, they seem to rather prefer wood. MOULD: Mould cannot develop on dry straw (moisture content less than 15%). This can further be ensured by the installation of a vapor barrier. However, a thick layer of plaster can moisten the straw and pre vent the drying out of straw. Hence the next layer of plaster should be app lied only after the earlier one dries. Also, organic aggregates in plaster that slow the drying proc ess should be less.

PHYSICAL ASPECTS HEAT STORAGE & CONDUCTIVITY: Straw bales have LOW THERMAL MASS, and therefore have POOR HEAT STORAGE CAPACITY. Temperature balance is however improved by plastering material such as 3 – 6 cm thick EARTH PLASTERING with a high content of sand and fine gravel (of higher density = 1900-2100 kg/ cu.m). THERMAL INSULATION: Strawbale provides EXCELLENT THERMAL INSULATION. The insulation depends on the density of the bales, the position of the stalks (better insulation is provided if the stalk is perpendicular to the heat trans ition), and the humidity content of the straw.

As an agricultural co-product, inconsistent properties (eg dimensions, density and moisture content) can be problematic during construction 

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PHYSICAL ASPECTS MOISTURE PROTECTION: Exterior walls have to be protected against humidity/ water from all sides. The following methods can be used to achieve this: Splash guard: Walls must have a splash guard up to a height of at least 30 cm. Ideally the first layer of straw bales should start above this height. The risk of splash water can be reduced significantly by a gravel or crush stone bed or a dense low vegetation at the perimeter. Hard floors in front of the wall will have the reverse effect.

PHYSICAL ASPECTS Weather protection: A straw bale wall has to be protected from rain, hail and wind. This can be achieved by deep overhangs & a weather- proof plaster coat. Fire protection: A non- load bearing straw bale wall with interior & exterior lime plastering has a F90 fire rating. Fire resistance can b e attributed to the plaster itself as well as to the high com pression of the bales not leaving enough oxygen for the combustion of straw. Sound Insulation: Sound insulation of straw bales with double- sided plastering is higher than single- layered elements of the same weight. This can be attributed to a certain vibration of straw bales.

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STRAW BALE WALLS

STRAW BALE WALLS

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STRAW BALE WALLS There are 2 basic types of strawbale construction: LOADBEARING and NON-LOADBEARING (or infill). LOADBEARING CONSTRUCTION, utilizes straw bales as large bricks that support all of the building loads. NON-LOADBEARING CONSTRUCTION utilizes an independent structural system with straw bales filling in between.

PLASTERING STRAW BALE WALLS Interior render: Plastering HARDENS AND SMOOTHES THE BALE SURFACE and PROVIDES AIR- TIGHTNESS AND FIRE PROTECTION. Suitable materials for plastering: Earth, stucco, lime, cement and lime-cement Clay plaster: Absorbs more moisture than other materials. Usually, plaster is applied in three layers: Undercoat – smoothes the bale surface and immerses sticking- out stalks (it is best sprayed using a high- pressure pump). High clay content ensures strong adhesion to the straw stalks. Second coat – leaner mix (with added sand & fine aggregate) to reduce cracking in the drying process. Addition of dust or straw c haff is preferable. Finishing coat with coarse sand aggregate A lime top coat may be added to improve the strength of the clay plaster with a chicken mesh as underlay.

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PLASTERING STRAW BALE WALLS Cement plaster is fairly brittle compared to clay plaster, and h as a tendency to develop cracks as a result of movements of the sub- structure, wind loads etc. Reinforcement with glass fibre or metal meshes is advis able. Exterior plaster: Should prevent moisture from entering the straw bales. It should also be vapour permeable to allow condensate to diffuse to the outside. This makes CLAY PLASTER mos t suitable for exterior use if suitably protected from rain. A paint coat could also increas e water resistance. However, paint coats like linseed oil and varnish act as a vapor barrier and may lead to collection of condensation water within the bales. A LIME PLASTER OVERCOAT on clay plaster is preferred over an cement overcoat. Cement-based surface finishes are NOT recommended for use o n strawbale walls IN WET OR HUMID CLIMATES. Cements are BRITTLE and eventually form c racks that allow water to penetrate. Also, cements are NOT BREATHABLE, and d o not allow this moisture to transpire back out at the s ame rate.

PLASTERING STRAW BALE WALLS

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STRAW BALE WALLS

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