The ultimate eco construction, straw bale buildings are no longer just the preserve of home builders and “green” enthusiasts. In the UK, public authorities are commissioning them via public tenders.
Straw bale buildings, which are usually built with a timber frame and breathable lime render, have a number of advantages: they are relatively cheap and easy to build, they are light and healthy, offer excellent heat and sound insulation, are free of damp caused by condensation, and have low embodied energy. Also, in these types of buildings, thermal bridges can be eliminated, and atmospheric carbon is locked in the building itself, giving it a negative carbon footprint from the point of view of the construction materials.
In the UK, many private homes and some social housing estates have been made from straw but it is relatively uncommon for local authorities to commission straw bale buildings. Until now.
The first straw bale housing estate was commissioned by North Kesteven District Council in the UK’s Lincolnshire in 2009, and built for £103,000 (A$187,000) per house above foundation, excluding balcony and porch. They were designed to last 200 years.
The construction was recently verified to be using 238 KWh/m2 per annum less than than the energy used in a traditional, solid-fuel heated property.
The cost of building a typical three-bedroom straw bale house, compared with a traditional construction can be found here.
This pair of social homes was built in Hastoe, Lincolnshire in 2013:
In the town of Hastings on the south coast of England work has begun on the straw bale construction of a new visitor centre for the area’s country park, following a public tender held by the local council. One of the driver’s behind the project, Hasting’s Borough Council environment and natural resources manager Murray Davidson told last month’s European conference of the Straw Bale Building Association that the tender process wasn’t plain sailing thanks to a raft of rules and regulations that had to be complied with.
Davidson is also vice chair of the Association of Local Government Ecologists and a member of the British Standards Institute panel responsible for the UK’s first British Standard for Biodiversity and Planning.
The pictures below show stages of construction, which is not yet complete: the site, plinth and frame with the first bales in place.
In the UK, all public authorities must comply with Public Contract Regulations 2015, and EU legislation that requires public bodies to seek tenders for works above £181,000 for goods and services and £4.5 million for capital works. These are published through the Official Journal of the European Union.
Individual local authorities also have their own procurement rules, which usually include value for money. This can be interpreted in a number of different ways because value may include the value of carbon, natural resources, and social value.
The Hastings Council published its intention to value sustainable construction, green energy and carbon neutral infrastructure and signaled that it wanted to commission a visitor centre made of straw bales.
Traditional building companies don’t know much about straw bale construction while straw bale construction companies are usually small, and so don’t respond to public tenders and don’t know their way around public sector procurement hubs.
Meanwhile, local authorities often lack knowledge about the type of contractors who can fulfil these tenders, and they often don’t have flexible procurement strategies.
Local government is also risk averse – risks can include potential public perception that taxpayers money is being wasted – and often sceptical about new technologies. Finally, the amount of time that an unusual procurement can take can pose problems for local government.
After holding two unsuccessful tenders, Hastings developed a new, collaborative way of tendering, and engaged with a consortium of artisan builders, according to Davidson, which had to be supported through the entire procurement process so that each side could be reassured that their time wasn’t being wasted.
It took five years from the conception of the idea for a straw bale building to the beginning of construction in May.
“But the benefits were enormous,” said Davidson. “Sustainable builders and small artisan straw bale builders are nice people and having a consortium of builders gives you even more nice people and a variety of skills, making it more collaborative and flexible.
“A consortium can also bring additional grant aid and funds to the project not usually available to local authorities,” Davidson said.
He said it was important to be clear in a tender about the questions being asked and the scoring process being used to rate the tender responses.
The winning consortium, or Constrawtium as they call themselves, consists of Huff and Puff Construction, SIA Design Build, Green & Castle, and Red Kite Design.
The building will be home to information about the park, its geology, habitats, species and heritage, and a café. It is funded by Hastings Borough Council together with an award of £475,000 from the European Commission’s Interreg North West Europe fund, taking some of the pain out of the full cost of the building of £770,000, due in part to the unfamiliarity of the tendering process.
According to the Council, the straw bales – which cost approximately £2,000 – have greater density and mass than normal bales because of the baling technique that has been used.
Prefabricated straw panels
Some companies are now making prefabricated straw panels, which speed up construction. They are plastered with clay and sealed against the weather. Because the bales are contained in their own wooden frame, the frame doesn’t have to be built in advance.
These panels form a modular building system and an all-in-one concept with structural elements and insulation, making construction an extremely cost-effective and low-skilled building process. A team of three people can assemble approximately 100m2 of wall per day. The work can be done by any skilled carpenter.
Loadbearing lintels, inclined tops and braced corners can be included without any change of materials. Passive house standard airtightness is possible by wrapping the building on the outside with a vapour-permeable airtight membrane.
The images below show a bungalow being constructed using the panels and the finished building.
Load-bearing or not load-bearing?
Some brave souls attempt to build straw bale homes without frames, so that the bales themselves support the building. This can work for two- or three-story buildings.
The images below show two houses on the same One Planet Development plot in Wales. The first is under construction awaiting lime rendering and for the roof to be finished, and the second is completed. Both were signed off by structural engineers and met UK building regulations.
David Thorpe is the author of the books The ‘One Planet’ Life and the new ‘One Planet’ Cities. From October he is teaching an online Post-Graduate Certificate in “One Planet” Governance.