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It might have the highest BREEAM rating of any office building in the UK, but the Co-operative Group鈥檚 new head office in Manchester wears its eco credentials lightly. With its dramatic lozenge shape and imposing play of steel, concrete and glass, it is every inch the prestige corporate HQ. The facade is adorned with twisting white columns and a bronze-anodised glazing frame, while the interior boasts a stunning atrium rising the full height of its 15 floors. This is statement architecture, designed to project a sleek and go-ahead image for its client.

It is all a very far cry from the kind of driftwood-and-windmill look that sometimes accompanies the very highest levels of energy efficiency. So how has this new 50,000m2 Manchester landmark managed to environmentally outperform every other office block in the land?

Mike Hitchmough, head of offices at architect 3DReid, explains: 鈥淲hat we realised early on was that if you go for BREEAM 鈥榦utstanding鈥�, there鈥檚 no half-way house. Energy planning has to be fundamental to the design from the beginning. There鈥檚 no way you can do it with add-ons.鈥�

For the Co-op, that meant ensuring from the start that the building would have a great deal of thermal mass to retain heat and iron out inefficiencies caused by diurnal temperature fluctuations. These days, of course, there is nothing too unusual about that, and large amounts of concrete are routinely specified in commercial buildings for just this purpose. What is striking about the Co-op building, though, is the way that concrete has been deployed to achieve the required mass, and the techniques Hitchmough has used to turbo-charge the thermal mass effect.

The floor slabs are a hybrid of precast and in-situ concrete on steel beams. These have been created by lowering some 2,000 large precast concrete coffer units, weighing up to 6.5 tonnes each, onto steel beams and then adding a topping of in-situ concrete. Three in-situ slip-formed concrete cores add to the thermal mass, but since these areas of the building are less occupied, it is the coffers that play the biggest role in the heating and cooling strategy. The exposed concrete ceilings provide a huge area of thermal mass to the occupied office space, absorbing heat in summer, and retaining it in winter.

The ceiling makes a further contribution to the building鈥檚 BREEAM rating - through the unusual decision to paint it. 鈥淭he finish that was achieved on the coffers was beautifully smooth and some might think it sacrilege to paint concrete of that quality. But we used two coats of white Kiem Optil mineral paint to increase the amount of light that reflects into the floors from the exterior and from the atrium. That enabled us to save more energy by reducing the lux level of the lighting.鈥�

Energy planning has to be fundamental to the design from the beginning. There鈥檚 no way you can do it with add-ons

If the coffers鈥� thermal mass is central to the design, equally important is the way in which air moves around the building, and perhaps the Co-op鈥檚 most unusual energy-saving devices are the enormous subterranean earth tubes used to temper air brought in to ventilate the building. There are three main tubes, 3m in diameter, running from precast intake 鈥渃himneys鈥�. These are situated at ground level some distance from the building (see box, overleaf). Formed from in-situ concrete, the tunnels suck air deep below the basement levels before releasing it up vertical shafts for distribution throughout the building.

鈥淪ince the temperature down there is a fairly constant 12潞C, this heats the air in winter and cools it in summer,鈥� says Hitchmough, adding that the natural stack effect of the atrium helps spread this tempering effect. 鈥淭he whole building is naturally ventilated - just not through windows. To make it even more efficient we have placed heat exchangers at the top of the building to extract any spare warmth and send it back down the system.鈥�

As you might expect from a building with the rare accolade of a BREEAM 鈥渙utstanding鈥� rating, the Co-op HQ has many other environmental features to admire. Such heating as is required is provided by a cogeneration CHP biofuel boiler that runs on oil from rapeseed grown on the Co-op鈥檚 farms, as well as waste cooking oil. A double-skin glazing system adds extra insulation, and the lighting is LED. But Hitchmough says that passive measures such as the earth tubes and high thermal mass form the bedrock of the design. 鈥淥ur approach was that active measures would only kick in as a last resort - so, for example, the chilled beams that are attached to the coffered ceiling units only start up when the concrete is saturated with warmth. And in winter, because the air is pre-warmed by the earth tubes, heating should be kept to a minimum.鈥�

How long the Co-op鈥檚 HQ retains its crown as the BREEAM king of office buildings remains to be seen: one imagines that its usurpers are already beyond the drawing-board stage. But whatever look they go for, it seems increasingly likely that the design will include a substantial concrete element, and you would not bet against earth tubes becoming a more regular feature of 21st-century commercial development.