Aukett Fitzroy Robinson’s Cheshire Oaks scheme proves the value of a rigorous post-occupancy process, writes Hattie Hartman
On the face of it, an out-of-town superstore with a car park for almost 1,000 cars hardly sounds like one of the UK’s greenest buildings. The 300m² green wall which clads one elevation of the car park looks like the ultimate in greenwash. But at Marks & Spencer’s Cheshire Oaks shop on the outskirts of Chester, the green is more than skin deep.
Designed by Aukett Fitzroy Robinson, the hybrid glulam and steel structure of this 42,000m² shed, the second largest new build in the M& S estate, is clad in 400mm thick hempcrete panels. North-facing clerestory windows and extensive glazing screened by timber louvres make for a soaring light-filled retail space, unusual for this building type. A fabric-first approach means that the average U-values exceeded the building regulations by 45 per cent, and detailing was carefully considered to minimise thermal bridging and maximise airtightness, with a result of 2.93m³/m² per hour at 50 Pa. But the design alone does not guarantee performance.
Recently released building performance data from the Technology Strategy Board (TSB) corroborates Cheshire Oaks’ green credentials. In its first year of operation, the building’s energy consumption outstripped design predictions by 29 per cent. Not only is that contrary to common practice - which says wait until the building’s second year to get decent data, so that it can bed down across all four seasons first - but it confounds most current findings where actual energy consumption far exceeds the predicted loads simulated during design. The results were so surprising that the TSB asked the building performance team at Faithful+Gould to double check their figures.
The secrets to this success are easy to unpick. The starting point was an ambitious brief from Marks & Spencer for a landmark sustainable store, which would embody in built form its eco-ethical program Plan A. This store was to be a visible manifestation of the Plan A initiative. ‘We wanted a store which would demonstrate that sustainability makes commercial sense,’ says Munish Datta, head of Plan A property.
Sustainability was addressed in the round, covered by 21 of Plan A’s 180 targets, ranging from embodied energy to community engagement. Stretching targets set jointly with the design team encouraged innovation. The hempcrete panels were a first, with two quick wins: easier detailing for airtightness and reduced waste. An integrated team including the client, the architect, the services engineers, the contractor and key suppliers developed and bought into the targets.
It is worth pointing out that Cheshire Oaks is not an energy-hungry supermarket, but primarily a retail store. Only 15 per cent of the floor area is devoted to the food hall, with the remainder retail and café (7 per cent). This also explains why lighting is such a high proportion of the electricity consumption. M& S opted against LEDs inside the store due to poor colour rendering, although the technology has now improved so the retailer is now specifying LEDs in all new and refurbished stores.
Faithful+Gould’s Sean Lockie attributes Cheshire Oak’s remarkable performance to the fact that: ‘They did everything right and it all worked well. And the POE process was exceptional. We had everyone that needed to be there at six meetings over 12 months. Any issue could be addressed immediately with an integrated solution.’
Lockie also stresses the importance of independent assessment, noting that: ‘There is nowhere to hide when an independent assessor comes in, points a thermal camera at your facade and finds a lot of red.’
From a client’s point of view, Datta says that POE was important to protect the building’s legacy. POE was planned for from the outset of the project, with more than 50m installed throughout the store. Datta notes that: ‘If you’re not set up for metering in the design phase, you won’t be able to measure.’
Marks & Spencer topped up the TSB funding, adding another 10 per cent for ‘soft’ results from staff and customers, undertaken by University College London. This included comparing staff and customer perceptions about Cheshire Oaks with a peer store. The intangible impacts of the light-filled retail space with its prominent timber structure and the exterior landscaping came through forcefully in all the focus groups. Datta attributes the building’s success partly to the behavioural change amongst staff who were more engaged to switch off lights and close down refrigeration curtains.
The fact that Cheshire Oaks is outperforming its design predictions has made headlines across the industry. It shouldn’t be so remarkable. We’re in a sorry state with so many buildings, especially so-called sustainable buildings, failing to perform. The results at Cheshire Oaks show what is possible with a committed project team. As Suzette Vela Burkett of Aukett Fitzroy Robinson says: ‘Post-occupancy is the true test of a building’s success.’ So why don’t we do it more often?