This may explain why the £22m building was completed on time and within budget. For most clients, this would be achievement enough, but Wessex Water has the added bonus that its building has been cited by the BRE as the most energy efficient ever.
Energy efficiency was high on the client's agenda, and figured in almost every aspect of the brief, from the performance of the finished building to the amount of energy used in the manufacture and transportation of its materials.
A modern air-conditioned city office block has an energy rating of approximately 300 kWh/ft sq. A well-designed non-air-conditioned office block in a business park might be about 200. The BRE considers 150 to be best practice. Wessex Water was aiming for 80, and it looks as though it has achieved 100.
The surprising thing is that when you walk around the interior – and there is a good deal of interior to wander around in – the low-energy brief is expressed, but in very subtle ways. There are no dinosaur-sized sections of Copex-like flue liner draped around the place, or gently rotating fans, or giant galvanised heat exchangers and other paraphernalia.
"Originally, we found ourselves dealing with the overall performance of our buildings more or less by accident," said Bennetts. "We just thought it would be so much nicer if people didn't have to work in neon-lit, air-conditioned boxes. In trying to make the places more agreeable, we found ourselves treating all the services and structure as apart of the whole design, rather than bolt-ons." In fact, the headquarters is an updated and refined version of the headquarters the practice built for Powergen in Coventry. The principles are the same: the concrete mass of the structure serves as a heat sink, and the ventilation system relies on opening high-level windows.
Admittedly, there is a back-up system if conditions get desperate, and the 10% of the building that contains the cafeteria, boardrooms and the magnificent electronic control room (the building's real raison d'être) does have mechanically controlled and conditioned air. Everything else is worked by nature.
The structure makes use of the recent discovery that only the surface 75 mm of a lump of concrete has any bearing on its capacity to store heat. At Wessex, the precast concrete coffered panels on the floors and ceiling are less than 75 mm thick – half the thickness as at Powergen. They help keep temperature down in summer by absorbing heat throughout the day.
The floor panels are simultaneously structural, constructional and decorative as well as being moulded to collect and channel the air, hide the cables and act as suspension points for the light fittings. There are about 2000 of these, so getting the prototype right was important.
The south-facing facade is fitted with a screen of mounted brises-soleil which reduce solar gain in the summer, while naturally lighting the offices and warming them in winter. Sunlight is also used to heat rainwater collected from the roof for use in the washrooms.
Bennetts Associates is nothing if not rigorous, and it closely monitors all its buildings after completion. While other architects are happy to lift Bennetts' design solutions straight out of the published projects, the practice is working hard to refine what it has learned for the benefit of its next client.
"Other architects often write to acknowledge that they've used our ideas," says Bennetts ruefully, "but some of this research is very expensive for us, and it's not as though they offer us any sort of royalty for all the work we're saving them." I suppose that everyone else's ecological misbehaviour ends up on the doorstep of the water companies, and so by setting an example, they can encourage the rest of us to clean up our collective act. It's hard to see how the whole project could have turned out better.