Down on the farm

The team behind the largest building to be constructed from straw in Europe has set itself a mission: to get away from the barns and farmyard image and make straw an acceptable construction material.

With the need to build more energy-efficient buildings, the industry might increasingly look to straw as it has impeccable environmental credentials and is an excellent insulator. As an exemplar, it has been used in the groundbreaking £7.8m EcoDepot in York. Set for completion next month, the building will be used by York council to house its commercial services department. The depot, which besides timber-framed, straw-clad panels comprises a range of energy-efficient devices, is said to reduce energy use by

The straw-panelled EcoDepot is said to reduce energy use by three-quarters compared with standard buildings

three-quarters compared with similar-sized buildings of a standard design. The project team has estimated that the cost/m² of energy at the 1350 m² EcoDepot will be cut from £40 a year now to just £5.

The material’s insulating qualities – three times better than the requirements of the Building Regulations – have been essential to realise such energy performance. Brian Dunn, senior contracts manager at main contractor Carillion, says: “There has been discussion about using straw in construction for a long time but products haven’t been readily available. We found AgriFibre, which could produce panels in any shape and in the required quantity.”

For the EcoDepot’s external walls, architect White Design Associates specified 500 mm thick straw bale panels inserted into a timber glulam structure from Lamisol. The panels were prefabricated on a local farm by compressing straw and rendering the resulting boards, which incorporate openings for doors and windows, with lime. To let the straw breathe, the specifier did not use any paint or covering on the straw panels apart from the lime render and plaster.

White Design director Craig White hopes such panels can bring farmed products such as straw and hemp into common use in the construction industry. He says: “By taking materials that are normally perceived as being deep, green and hairy and combining them with modern methods of construction, a new product has been delivered.” Rapid assembly, essential in the one-year programme, was made possible by the product being delivered on site ready for fitting.

Dunn says straw is a fire-resistant, robust material whose natural properties have been enhanced at EcoDepot by three coats of lime render on either side. As for durability, he admits it is too soon to be completely sure of long-term performance. However, the results of research carried out at Bristol University for the past two years have been very satisfactory, says Dunn.

Openings allow natural light into the EcoDepot, reducing reliance on artificial lighting

Straw cladding panels come at a price: £290/m² compared with £120/m² for brickwork. But Dunn says: “In the short term it’s more expensive but the maintenance will be minimal.” Payback period is expected to be 25 to 30 years and the building will save about 155 tonnes of carbon dioxide per year.

The EcoDepot has been designed with an array of energy-efficient features that are set to generate a saving of about £46,000 on the council’s annual energy bills compared with its old depot, which was run on fossil fuels. The architect emphasised openings throughout to let in natural light and reduce reliance on artificial lighting. The team also opted for natural ventilation, which influenced the layout of the shallow plan building – it is 15 m wide to allow natural cross-ventilation. A window sensor system has been specified and the temperature and CO₂ levels are controlled by computer, which opens windows according to the needs of the building. The concrete ceilings absorb heat in the day and the slabs are cooled at night.

Another sustainable initiative is the way the EcoDepot saves water. In the old building, drinking water was being used to wash council vehicles. The price of such waste: £25,000 a year. The project team specified a rainwater harvesting system that is expected to halve consumption.

To raise awareness among end users, the project team installed prominent displays and read-outs to tell users how much energy is being consumed at any one time.

White is convinced the EcoDepot can change the way people perceive sustainability. He says: “Research shows that if people are made aware of their energy use, they will reduce their consumption by up to 30%.”