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Strawbale Construction

– Posted in: Methods of Construction
An Extract Taken from The Royal Institute of British Architects

Key points:

  • Moisture content – Straw bale construction is able to breathe, meaning its moisture content will rise and fall naturally. Moisture content above 25% for any extended period of time is a risk.
  • The u-Value of straw is dynamic in use and will rise and fall in line with moisture content.
  • The u-Value of straw varies with density. The density of straw can be specified.
  • The dynamic relationship between density, moisture content and u-Value can work together to provide significant thermal mass, especially when rendered.
  • Straw bale construction can provide an optimum combination of super insulation and thermal mass.
  • Straw bale construction has been viewed by the commercial construction industry as quaint – more ‘little house on the prairie’ than mainstream. However, it can be used at scale, especially when pre-fabrication is considered.
  • It is important to consider which technique of straw bale construction to use. Straw will settle over time and this needs to be carefully considered when using the Nebraska method.
  • Where settlement can not be accommodated then studwork, pre-fabricated cassettes or pre-fabricated rendered systems will work.
  • Often, straw bale construction is considered too late in the design process.

Design Procedure:

  • Step 1: Start as early as possible in the RIBA Plan of Work, ideally at the end of Stage B. Any later and its successful deployment can be compromised.
  • Step 2: A decision should be made as early as possible on which method of straw bale construction is to be used – Traditional to pre-fabricated. Traditional is most commonly used in one-off housing and community projects, prefabricated systems for higher volume housing and commercial projects.
  • Step 3: Air-tightness requirement may determine whether traditional or pre-fabricated methods should be used.
  • Step 4: Designers can often become overly concerned about straw bale module sizes. Straw bales are (approximately) 1000 x 450 x 350. However, bales can be split easily to allow them to be brought onto setting out dimensions.
  • Step 5: Vertical dimensions are more significant than horizontal dimensions. Bales course at 350 mm high with an allowance of 5 mm per bale for settlement.
  • Step 6: Have a go at building a test panel to better understand how to design, work and detail straw bale construction.
  • Step 7: The National Building Specification has introduced a new work section, F42 Straw bale walling systems to help put together a specification that can form part of a tender package.
  • Step 8: Straw bale construction does not use vapour barriers, cavity trays, wall ties etc. It is a solid wall construction and needs to be considered very differently.
  • Step 9: The wall needs to be separated from the ground using a robust damp proof course and normally sits on a timber sole plate.
  • Step 10: The common phrase associated with straw bale walls is that it should have a ‘good pair of boots and a good hat’.
  • Step 11: Good Boots – walls should be raised off the ground to prevent unwanted splash back of rain, unless protective detailing is in place.
  • Step 12: Good hat – often mistakenly assumed to mean a wide overhang that provides a rain shadow. This is not necessarily its meaning, instead care should be taken to ensure water cannot enter the head of the wall behind the render or rain screen. On a two story building a wide overhang offers no shelter to the wall lower down.

U-Value Definition = A U value is a measure of heat loss in a building element such as a wall, floor or roof. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat.