Earth

Straw-bale construction

Provided by Craig White; Director of White Design

 

What, Why, When, How, Extras

 

What is it?

Straw bales have been used to construct buildings since the middle of the 19th century with the advent of the baling machine. Prior to this straw had been used for centuries as a fibrous binder in clays and mud to create materials such as cob. A systemised technique of building with straw bales, known as the Nebraska method, was developed in America. Availability of timber for construction was limited and straw was readily available. The ability to create homes that used a by-product of growing wheat using an adapted brick laying technique caught on quickly. The excellent thermal performance of straw made these simples homes very comfortable.

Why use it?

Today, straw bale is used in modern construction because of its excellent thermal properties and its low environmental impact. Straw is described as a renewable material. A renewable material is one that can be replaced by natural processes at a rate comparable or faster than its rate of consumption by humans, rather than a finite resource that is depleted through use. For example, farming in the UK produces 3 million tonnes of straw as a by-product of growing wheat and barley. A typical 3-bedroom house uses 5 tonnes of straw. There is enough straw available in the UK to build 600,000 homes year on year, more than 5 times the size of the current new build housing market.

The Energy Performance in Buildings Directive (EPBD) has set challenging targets to reduce Carbon Dioxide emissions. Straw’s excellent thermal properties reduce emissions generated through the heating and cooling of buildings. U Values typically fall between 0.13 and 0.19, depending on the overall method of construction, and high levels of air-tightness can also be achieved.

As a renewable material, straw has what is known as a carbon positive footprint. As plants grow they absorb CO2 molecules from the atmosphere. Through the process of photosynthesis, plants can separate the two-oxygen atoms form the single carbon atom. They return the oxygen to atmosphere, and keep the carbon to make complex sugars such as cellulose, the building blocks of plants. Carbon positive means that there is more CO2 equivalent banked in the form of carbon in straw than is emitted through the process of planting, harvesting, baling and building a building using straw.

When to use it?

Straw can be used in most types of construction. Increasing legislative demands to reduce energy use and consequent CO2 emissions, combined with straw’s thermal performance means it can be considered for most building applications.

Where the embodied carbon of materials is a key consideration.

The new building regulations on ventilation in buildings will insist on mechanical ventilation being used if the air-tightness of a building is better than 5 m3/m2/ph @ 50 Pa. This has been done because the moisture build up in conventional buildings would become to high. Because rendered straw bale construction is able to breathe ie its vapour permeability allows moisture to translate through the wall, mechanical ventilation can be reduced or avoided altogether.

Staw bale construction works well in acoustically sensitive environments. While the over all decibel reduction is reasonable (50db), it is effective at cutting out certain frequencies.

There are four methods for building with straw:

  • The traditional method, known as the Nebraska method, is a load-bearing system. It uses straw bales as blocks laid in stretcher bond and pinned together using sharpened hazel sticks. The wall is stabilised using render on the internal and external faces resulting in a composite action being achieved.
  • Timber framed studwork. Straw bales are used as the insulating infill between timber studs built insitu.
  • Pre-fabricated timber studwork cassettes built off-site.
  • Pre-fabricated rendered panels.

 

How to use it

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. The use of straw bale should be confirmed as a material choice at the end of RIBA Stage B. Stage D is too late.

 

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 storey building a wide overhang offers no shelter to the wall lower down.

 

Related strategies

Stack ventilation Cross ventilation Night ventilation

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