Beat the drum

The project was first mooted in 1983. Unsurprisingly, the concept changed markedly over its 16-year gestation. "It originally had a large visitor centre," explained Byrne, "but reality set in." The NEF realised that visitor numbers would be limited, and decided it better to use the money elsewhere.

Architect Weston Williamson won the contract in a competition – twice. It won one limited competition in 1994, for a different site, but that plot of land failed to materialise.

The NEF had to find a new site, and launched another competition in 1997. This time, though, the budget had been cut in half. Again, Weston Williamson rose to the challenge, working with consulting engineer Ove Arup & Partners.

The completed building sits on a greenfield site one mile east of Milton Keynes. In a new town like Milton Keynes, brownfield sites simply aren't available – which raises a barrier to the government's desire to build mainly on previously used sites.

Work had to start on-site before the end of the 1998-99 financial year because the Commission for the New Towns was changing status and there was a risk of losing the land. This forced the design team to short-circuit many of the usual processes in the run up to April 1999.

The team had only two months to finalise a design. How did they cope? They jettisoned the usual iterative process of weighing up a range of concepts, and ran with a single concept – basing decisions on experience rather than detailed testing.

On top of this, the National Energy Foundation's modest income meant that economy was of utmost importance. Arup associate director Chris Twinn was never under the illusion that huge sums of money would be thrown at the building.

A tight budget also meant that Arup was not given a full appointment. Instead, the consultant was contracted to produce a design, and make site visits afterwards – it had no administrative role on site.

The NEF did not have a hands-on involvement in the design work. Byrne said they "provided a broad specification and left it up to the design team to interpret their parameters". He felt this was natural, given the limited funds available and the need to use a fixed cost contract.

Twinn explained how low energy design often has a reputation for being "normal cost plus". He knew from prior experience, though, that efficient buildings need not cost more – "let the fabric do the work and you can take out mechanical systems".

Cost penalties, he said, come when you both enhance the building fabric and still need to install plant. Installation costs the same even if the equipment used is smaller.

Predictably, the NEF stressed energy efficiency in its specification for the building. Rather than using an energy target, though, Arup set an objective benchmark in terms of CO₂ emissions: initially 33 kg of CO₂/m²/y.

This figure was revised, to 45 kg/m²/y, when it emerged that the lighting control system had to be sacrificed to cut costs. The design team also aimed to achieve a 'very good' rating using BREEAM 93.

It was not actually BREEAM assessed because the money was not available to fund this. According to Twinn, "there is no real payback on a BREEAM assessment for an owner-occupied building – the real benefit is for a landlord to attract tenants."

The two-storey building measures 1026 m², allowing growing room for the NEF and its first floor tenant, National Energy Services. A 36 m-long rectangle running east-west across the site houses the office accommodation, while a single floor drum to the south provides meeting or exhibition space. Architect Chris Williamson said he wanted "to express the exhibition space separately to give more character".

Entry to the building is from the west through a large foyer, passing stairs, a lift, toilets and a small kitchen on the way to the office space. The offices are mainly open-plan, with some areas that were cordoned off as cellular offices as part of the fit-out.

There are now two cellular offices on each of the ground and first floors, and the first floor also has a new meeting room. The office spaces have a 12 m-deep floorplate, with generous windows on the north and south sides.

The principles behind the servicing strategy are simple, and – up to a point – intuitive: a well insulated, airtight shell, with maximum daylighting and natural ventilation.

Designers sought to keep loads to a minimum, installing aluminium louvres on south-facing windows to cut solar gains. They provided a heavy, exposed building fabric to absorb daytime heat, and ventilation shutters that can double as night cooling vents.

Ove Arup had tested similar concepts for its design of Anglia Polytechnic University. At Anglia, precast paving slabs were used to provide thermal mass. However, concerns about the slabs' structural integrity meant that bespoke batons had to be used for support on either side – pushing costs past the threshold allowed by NEF.

Since working for Anglia, Arup had searched for a cheaper material suitable for a high-thermal mass ceiling for the top floor. It struck on Cape's cement board products, which are available in different thicknesses and densities, and are supplied pre-finished.

Arup used two layers of Cape Pyroc fibre-cement board with a 1450 kg/m³ density, at a thickness of 30 mm (60 mm in total) for the first floor ceiling on this project. Twinn accepts that there could be a small risk of mishap if builders mistake the slabs for ordinary plasterboard in the future. However, he said the fixings were plastered out, "so you won't get Tom, Dick or Harry taking the ceiling to pieces".

"Until innovative products like this reach the mainstream," he added, "there will always be a fear of the unknown." It turns out that the ceiling innovation also brought with it some new difficulties in installation.

Contractors have a well rehearsed process for fitting plasterboard. They have no such process for the much heavier slabs used here. This meant there was a steep learning curve for contractors, with inevitable cost and time penalties.

Twinn said a heavy ceiling is worth the extra costs, though. It means that people on the top floor will not suffer uncomfortable summer temperatures – as happens in other buildings with lightweight roofs.

The exposed ceiling on the ground floor is simpler. The bottom of the upstairs floor slab has been painted white, offering an elegant, light and inexpensive form for the ceiling.

Fresh air hinges on shutters

Air enters the building through a simple glazing system from Rational. The windows themselves are double-glazed and sealed, but they have shutters alongside them, opening on side-hung hinges to provide ventilation. The shutters are metal, offering excellent security when closed.

The architect went for shutters with softwood louvres outside to provide some security even when left open overnight. If Arup had been given a free hand, Twinn explained, it would have chosen more conventional permanent trickle ventilators as well as the shutters.

It is well known, he said, that occupants usually act to change the environment – in this case by opening the shutters – only when conditions get out of hand. The air would be fresher, he said, if there were permanent ventilators to provide background ventilation.

Nevertheless, on the day of our visit, air quality was quite satisfactory on both floors. Outgasing from the fit-out and furniture was imperceptible, and the air felt fresh. Further, the lower-than-usual ventilation rate undoubtedly improves airtightness and so raises energy efficiency in the heating season.

The building's heating strategy is a little quirky. Arup side-stepped the usual office building conventions of multiple boilers, pumpsets and controls. "We knew that we could never get that level of complexity within the budget," said Twinn. So the designers upgraded the usual standard of insulation – fitting 150 mm in the walls and 300 mm in the roof. Consequently, the overall heating demand is reckoned to be only 55 W/m². This means the building can use a blown-up version of a domestic heating system. A single condensing boiler, with a maximum 76 kW, feeds radiators with thermostatic valves.

The engineers tried to design-out the secondary hot water circuit. As a consequence, the 10 kPa pressure from the boiler's built-in pump had to be powerful enough to feed the whole heating system.

The heating circuit was designed to run water through five 'wild' radiators that are always open to allow the minimum flow through. This avoids the need for minimum flow valves, which have a costly pressure drop penalty.

The meeting/seminar room is heated using trench heaters sunk beneath the floor. This gives the benefit of clutter-free walls that are suitable for displays. On the day we visited, though, there was a problem with the trench heating, and the meeting room was decidedly chilly: no more than 15ºC.

The client had originally expressed interest in using a wood-fired boiler, and has space on the site for a coppice to provide fuel. Although the funds were not available for a wood burner at the design stage, the plantroom has been oversized to allow space for this to be added.

While the heating system is simple, procuring and installing the equipment was not plain sailing. The heating controls, for example, were supplied with instructions and labels in German – hardly an incentive for occupants to tweak the controls for their needs.

Worse than this, though, Arup had only limited control over the services equipment used in the building, and in many cases only knew what the contractor was using once the kit was installed.

It was also an act of faith that the contractor would follow Arup's guidance on putting the services together – faith that seems to have been abused in some areas. The contractor appears to have misunderstood the need to run hot water from the boiler through the five wild radiators continuously. So it neglected to fit a lockshield on the radiator in the stairwell. Instead, it fitted a thermostatic radiator valve.

Arup flagged this up on its snag list, but the contractor had not sorted out the problem on the day of our visit. Not resolving this problem means that, come spring – when most thermostat valves are closed – the boiler may cut-out because it runs too hot.

Turning to the cooling side of the equation, natural ventilation and the building fabric do all the work. Employees open the shutter next to them when they are warm during the day in mid-season. When temperatures rise further, the exposed concrete ceilings and walls absorb heat in the daytime, to be dissipated at night by allowing cool air though the shutters.

Since moving in at the end of June 1999, occupants have been quick to learn how to manage the shutters. Ian Byrne from NEF said some shutters on one side of the ground floor office are left open during working hours in winter, with shutters on both sides being used in summer. This provides ventilation without a draft or noise, he said.

Neil Cutland from National Energy Services agreed. He said he was very happy with conditions on the first floor: comfortable in summer and winter alike. Even during the hot week in August 1999, he said, his cellular office was "fine".

He puts the success down to a high level of local control. People near the perimeter can choose to keep the shutter nearest them open or closed – in much the same way as people adjust the thermostat on the nearest radiator.

On the first floor, the shutters are left open for night cooling, and Cutland explained that people seem happy to use them to ensure they are comfortable during the day. Here, of course, there is a minimal threat of theft because any would-be intruder has to climb up the outside of the building before gaining entry.

It's a different story on the ground floor. Byrne said the shutters here could not be opened at night for security reasons – the insurance company was not satisfied that the softwood louvres were enough to deter burglars. Of course, this means that there is no night cooling. Bryne claimed that even without night cooling the offices did not overheat during the hot month in summer 1999.

Likewise, measurements by National Energy Services' Vanja Kemenc suggest that employees on the first floor had acceptable conditions even when it was exceptionally hot outside.

Spotlight on lighting

Twinn explained that lighting design was complicated on this project by a cost-cutting process Arup was not part of, and the constant need to look for low-cost alternatives. Despite the handicaps, though, Twinn is pleased with the lighting. Indeed, if you don't look too closely, the offices and meeting room are very pleasing to the eye and have a light feel.

The designers were united in trying to allow in as much natural light as possible. There are generous windows in the office areas, and the first floor office also has a single skylight running the length of the space.

The skylight stands proud of the roof, with vertical glazing that faces north to avoid solar gains. Beneath it is an attractive light shelf which serves a double purpose: cutting glare, and reflecting some sunlight deeper into the office.

The ground floor meeting and exhibition room also benefits from daylight courtesy of a skylight. This long narrow fixture, supplied by McKenzie-Martin, again wears custom-built, powder-coated light shelves.

The skylight, which merges into a small south-facing window, appears to create some glare problems: ad hoc cardboard shading was placed where the ceiling met the wall – apparently blocking light from behind a projector screen that was placed (perhaps unwisely) in front of the window.

One occupant expressed a little dissatisfaction with the daylighting on the ground floor. He said that the large office windows created glare problems in winter. Although the light shelves work well in summer – reflecting light deep into the office and controlling glare – winter sun comes in below them. This forces most people to close their blinds when it's sunny, and means the electric lights come on.

Good lighting control was also a priority at the design stage. Sadly, though, Arup's original plans to use a simple lighting control system had to be sacrificed. Chris Twinn explained that it had been "an easy target" for stripping out when the pennies were being spread too thin.

However, Twinn was pragmatic about the sacrifices: "That's the nature of the beast. If the client has a limited amount of money, and wants good value, there is a pecking order of priorities." The lighting switches are still zoned parallel to the windows so that, when there is sufficient natural light near the perimeter, lights there may be extinguished without turning off the lights deeper into the floorplate.

When we visited, the sky outside was grey and all the office lights on both floors were illuminated. Neil Cutland, director of first floor tenant National Energy Services, acknowledged that electric lights are left on nearly all the time. They are only really switched off when one of the directors steps in – typically on a very bright day – to turn off the lights.

Arup had aimed to provide 250 lux on the working plane in the offices – recommending that individual task lighting be provided if needed. In fact, light levels measured higher. The upstairs office, for example, varied from 300 to 380 lux at desk level. Part of the variation was a result of the curved ceiling and changing floor-to-ceiling height.

The electrical circuit follows the precedents of simplicity set elsewhere in the building. The design team had investigated using photovoltaics to generate some electricity. However, the client was unable to secure funding for solar cells, so the idea was dropped.

Nevertheless, the roof is oriented to carry a photovoltaic array, so maximum sunlight falls on it, and even has a grid of fixing points below the roof surface. This effectively future-proofs so that solar cells may be fitted if and when they become cost-effective.

So how efficient is it?

The design team put a lot of effort into keeping down energy consumption. The early indications are that this paid off – up to a point. It gets full marks for gas consumption, coming in well below DETR's benchmarks for good practice open plan offices with natural ventilation (see table 1).

However, electricity use over the first 25 weeks seems unimpressive compared to government benchmarks – missing the target for this type of building. The NEF suspects this is due partly to the external lighting being used unnecessarily in the evening and at weekends.

It was originally intended to pressure test the building when complete, but this was another casualty of the cost pressures. Twinn explained that when a low energy heating system is correctly specified, it does not have a capacity margin to spare countering unplanned leaks.

This means that airtightness is important and could pose a threat – not just to running costs, but to the heating system's ability to keep the building warm. It is a myth, Twinn said, that airtightness is unimportant for naturally ventilated buildings. All buildings should have appropriate ventilation, but should not be over-ventilated in winter.

Surprises in store

Post-completion fit-out raised a few eyebrows with the services design team. An unfortunate choice of locations for new partitions could conspire against some of the low energy features.

One partition, for example, comes between the ground floor office space and its lighting control panel. This means that to turn off the lights, office workers have not only to leave their desks and working areas, but to go through a door to the hall outside.

Another new partition, between the drum-shaped meeting room and the entrance foyer, cuts off the windows intended to ventilate the drummed area.

Chris Twinn benignly accepts that such changes are inevitable: "The building as designed is only the first stage. It must be flexible enough to meet occupants' needs.

"The real problem," he said, "is that engineers aren't very good at communicating how to run buildings to occupants." The client didn't appreciate that putting up partitions in these places would affect how the building works.

Fit-out surprises apart, the designers seem to have come up with a building that meets occupant needs at the right price. Despite having poor ground conditions and a large site, which bumped up preliminaries and external works, construction costs worked out at a very reasonable £960/m².

Quantity surveyor Brian Halford, from Davis Langdon & Everest, said this compared very favourably to the £1150/m² average for this building type. Avoiding bells and whistles meant that services cost "about half" the norm for office buildings, said Halford.

This building is ideal for beating home the message about energy efficient construction: step-change energy savings are possible, and do not always force up building costs. Experience and ingenuity proved more than a match for pressures on design time and money.