Contrary to received wisdom, straw bales are being increasingly used to insulate wooden buildings due to the many benefits they offer. Clean, in plentiful supply and high-insulation, this local and cost-effective material is the king of energy efficiency. Forget about the story of the Three Little Pigs!
- Crédits : © anandoart -Thinkstock
Egis, in association with the firm DCL Architectes, is currently working on a straw building project for a new school in the Netreville district of Evreux in Normandy, France. This construction method is straightforward and locally-sourced and has a low impact on the environment. The combination of 36-cm thick walls and roof and triple glazing delivers a passive building – in other words, a building which could conceivably manage without heating. The walls will be built from prefabricated straw bale panels to save building time. Given the challenge of using bio-sourced and local materials in a facility open to the public (ERP in French), the most appropriate option to emerge was a construction with a wooden framework, insulated with straw bales. A preliminary carbon footprint assessment was conducted on the scale of the building, winning over the contracting authority to the suitability of this construction method.
Guy LEFRAND, maire d'Evreux et président de l'agglomération Evreux Portes de Normandie
Insurable and accepted by all verification bodies since the 2012 publication of the Straw Construction Professional Rules (“CP revised”), the technique of wooden straw bale panels had undergone a LEPIR II fire retardancy test in 2009 at the Scientific and Technical Centre for Buildings (CSTB). This test concluded that straw bale panels subjected to a severe fire burned very slowly and therefore provided the fire stability to allow safe evacuation and the intervention of the emergency services.
Furthermore, this dry building technique enables contractors to make extensive use of prefabrication, thereby shortening construction times. We also scheduled work so as to erect the wooden structure independently from the insulation panels, thereby dissociating the two work schedules.
Most importantly, straw has excellent insulating properties which achieve with ease the results required by the German Passivhaus energy efficiency label awarded to new buildings whose heating requirements fall below 15kWh/m²/year. It is also a long-lasting and cheap material which fosters a healthy interior. Straw does not require much grey energy as it needs relatively little transformation, reduced transport and can be composted at the end of its life. Finally, like wood, it is a “carbon sink” as it traps carbon dioxide during its cultivation.
Our role as an engineering firm is to advise architects in their design and building choices so that they can meet the future objectives of the French Responsible Building Regulations. It is our duty to recommend wood-straw construction to them as early as bid stage whenever it is appropriate to the type of structure to be built.
These future regulations stipulate a reduction in the carbon footprint of construction materials and require buildings to be solidly heat insulated. Straw enables this dual requirement to be satisfied through excellent insulating properties and a negative carbon emission value. In the case of Evreux, the aim of building a passive building has led to an extremely well-insulated building which requires virtually no heating. In a school, the internal heat gains resulting from its high occupant density are adequate to heat the premises during the day. The decision to build a wooden structure with straw bale insulation not only achieves the passive level, but also delivers an exceptional construction and operation carbon footprint (less than 1kg CO2e/m²NFA/year).
Today the only true technical impediment is the maximum authorised construction height (three storeys). But in reality, the chief obstacle to the choice of straw is people’s ignorance about this material which still suffers from prevailing negative bias. In the collective mind, straw has the image of a farm-sourced, even antiquated material which is highly inflammable, despite tests proving its fire retardancy which outstrips that of plastic insulation materials!
Today there are between 100 and 400 houses made of straw in France, while an increasing number of public facilities use this eco-friendly construction method. For many years the technique had been experimental by nature, but its deployment is today gaining in professionalism. Prefabrication makes for fast and safe building projects, the industry guidelines of 2012 have made these structures insurable, while their energy performances exceed the requirements of the future building regulations in France. At Egis, we strongly believe that a bright future lies ahead for straw construction. Although public awareness is still low, straw is unanimously acknowledged as being the zenith in eco-construction. We promote it to our clients and partners both for housing and tertiary building projects as it offers one of the best trade-offs between traditional selection criteria and the impact on the environment, health and living conditions.
The Sustainable housing Olympics, or Solar Decathlon China 2018, recently took place in Dezhou, China. Twenty-two teams from all over the world (10 countries represented) competed to build the most efficient, eco-responsible and bio-climatic house possible.
France, represented by Team Solar Bretagne, finished in third place, neck-and-neck with the German team (1st place Italy, 2nd place China). Egis advised the team from 2015 to 2017 to help it design an innovative housing concept using bio-sourced materials such as wood or rice straw. This human and educational adventure has opened up new perspectives for sustainable building both in China and in France, where a scheme will soon be launched with the Rennes metropolitan authority to develop innovative dwellings around Brittany.
Straw construction has since 2006 been overseen and driven by the French Network for Straw Construction. This association brings together the different stakeholders in straw bale construction and boasts around 550 individual members, 250 professionals and 50 associations and training colleges. Its aim is to popularise the use of straw as a construction material, spread best practices in its application and promote its performances.
Building Envelope Engineer – Carbone Specialist
Sustainable Development and Energy Specialist