Can Sheffield university's Arts Tower become Sheffield's environmentally efficient building?

If imitation is the sincerest form of flattery, modernist architect Mies van der Rohe would have been tickled by Sheffield University’s Arts Tower. The 20-storey landmark, completed in 1965, is a homage to his Seagram Building in New York, completed seven years earlier.

The Arts Tower, which houses the architecture school among other departments, was designed by UK practice Gollins Melvin Ward & Partners. Its facade of exposed aluminium framing and repetitive glass panels was complemented by venetian blinds, the precise control of which was one of the Seagram Building’s signature touches. At ground level, Gollins Melvin Ward faithfully copied the fountains ranged symmetrically either side of the Seagram’s main entrance.

After three decades as Sheffield’s most prominent navigation point, the Arts Tower’s architectural credentials were affirmed when English Heritage granted it grade II* listed status. About the same time, it became clear its services and skin were on borrowed time.

“The building’s in a state,” is how Neil Cameron, the university’s estates director, sums up the situation. “Any building of this age requires work to keep it going, but faults with the services and drainage have reached such a pitch that we have come close to closing the Arts Tower. The cladding is life-expired. We are limping along from year to year, spending emergency money.”

Cameron is embracing the challenge of turning the Arts Tower into a modern, efficient academic facility while preserving its authenticity. The university is embarking on a £40m upgrade that will involve re-engineering the facade and internal services under the watchful eye of English Heritage. Recladding will account for £27m of the budget.

Mott MacDonald completed a detailed evaluation of the building last year. “With grade II* status there’s very little we can do to change the external appearance of the building. Whatever we do to the facade has to be true to the original,” says Eddie Murphy, Mott MacDonald technical director.

Even so, Murphy is predicting the refurbished tower will be Sheffield’s most environmentally efficient building, with a BREEAM rating of more than 80. A preferred solution has been approved by English Heritage and granted planning permission. The next step is to work it up with the help of a contractor. The aim is to have a firm appointed and ready to start work on the building early next year.

From a distance the Arts Tower still projects something of the sleek corporate America that inspired it. Up close, however, it becomes evident that the aluminium frames of its curtain walls are deeply pitted by corrosion. Glazing panels are cracked, and mismatched replacements upset the facade’s uniformity. Venetian blinds are bent and lopsided. It is as if Sheffield’s industrial decline has rubbed off on this paean to Park Avenue.

Its original cladding was a bespoke system. At ground level, double-height glazing panels 1.89m wide by 6.77m high are set behind the tower’s exposed concrete columns. Panels are gravity supported – their entire weight bears onto the ground-floor slab – with a clamp mounted on the first-floor soffit providing top-edge restraint. A generous 75mm air gap between the top of the glass and the soffit was designed to allow a path for exhaust air ventilation and for differential settlement.

From the first to the 19th floors, the cladding consists of naturally anodised aluminium framing spanning floor slab to floor slab and running in front of the reinforced concrete structural columns and perimeter upstand/ downstand beams. Single-glazed panels were bedded into I-section mullions with glazing tape and face-sealed with a mixture of putty and silicone. Each bay consists of a spandrel panel of 10mm-thick cast glass with a body tint, measuring 900mm wide by 950mm high. Above this is a fixed panel of 4mm annealed glass measuring 900mm by 700mm. The topmost panel is a 900mm x 1600mm vertical sliding sash glazed with 6mm annealed glass. Aluminium louvres screen the rooftop plant.

Murphy notes that when the Arts Tower was designed there were no performance codes to work to. “The architects only needed to worry about keeping the weather out,” he says. The building was never thermally efficient, but over the past 40 years the facade’s seals have hardened and shrunk. Inside there is a constant draught. “The only ventilation problem the building has is there’s too much of it,” Murphy quips. Architect HLM, which is working alongside Mott MacDonald on the project, calculates that the Arts Tower is at least five times leakier than Part L standards require.

With the airtightness of a sieve and single glazing, it is small wonder the building is freezing in winter and cooking in summer, as Murphy puts it. Unlike the fully air-conditioned Seagram Building, the Arts Tower relies on natural ventilation for summer cooling. But over the years many of the sash windows’ sprung counterbalances and opening catches have broken, making them inoperable, and the building occupants often won’t open the windows that do work.

One of the university’s architecture students, Yufan Zhang, carried out a study of the building’s performance, focusing on the facade and blinds. She photographed the four facades daily over the course of a year, noting where windows were open and where blinds were positioned. She also asked the 230 permanent occupants each month about the way they were using the building and their comfort levels.

She found that only 11%-12% of the 1760 windows were used. In summer people chose to combat the sun’s heat by closing the blinds, Murphy says. “Yufan found that when the windows were open, gusts of wind sometimes blew paper off desks. The building’s users also told her that if the windows were open and the blinds partially drawn they tended to slap against the glass and perimeter columns. It was noisy, panes had been cracked, and the blinds got damaged. Sometimes blinds got sucked out of the windows.”

Blinds down, windows shut, it emerged, was the Arts Tower’s default user setting, even when the sun was not blazing. “That contributed to the dire situation of the lights being switched on even though there are floor to ceiling windows and there’s abundant daylight,” Murphy adds. He set out to remodel the way the building worked and, in the process, try to change user behaviour, to reduce the energy squandered on winter heating and year-round lighting.

Five refurbishment options were proposed. Each was modelled, giving a carbon footprint composed of anticipated winter heating and summer cooling requirements (see box). The options that offered by far the greatest reduction in the Arts Tower’s carbon footprint – down from 17kg of CO2/m2 floor area to 5kg and 3.5kg respectively – also involved the most conspicuous changes to the structure. English Heritage would not entertain them unless refurbishment of the facade was technically impossible or grossly expensive. In October, trial dismantling of the facade at the corners, where eddying winds have subjected the skin to the worst of the local weather, suggested there would be no significant problems.

Of the solutions allowed by English Heritage, the next best involves installing new double- glazed panels at low level. High-density insulation will be injected behind spandrel panels and mid-height fixed panels replaced with bucket-type windows to introduce low-level ventilation. Sash window glazing will be replaced with solar-control laminated glass to block infra-red light and blinds will be mounted beneath new light shelves, installed 300mm beneath floor slab soffits. This will get blinds out of the sash window airstream. The light shelves will bounce daylight onto ceilings, obviating the need for artificial illumination even when blinds are drawn.

“With this solution we are improving airtightness by a factor of more than five. The whole cladding system’s going to come off and be put back on with new neoprene seals. That, we hope, will get airtightness down below 10m3/hour/m2 of facade, which is the threshold for Part L,” Murphy says. Seals will be a standard, off-the-shelf type, which is important for long-term maintenance and reparability.

Murphy says adding the low-level bucket window will allow for natural buoyancy ventilation, with cool air entering near the floor, collecting heat and venting near the ceiling via the sash window. Wind tunnel tests will be carried out on mock-ups of the new window bays to ensure that moving the blinds down, out of the airstream, really does eliminate slapping.

BREEAM points will be picked up for reusing the cladding system. “Because we are dealing with a listed building, we’re interfering with the existing framing as little as possible, just enhancing what’s already there,” explains Clive Atkinson, Mott MacDonald’s facade engineering project leader. New components will be clearly defined, and “anything put in has to be undoable – though it’s hard to imagine any circumstances in which you’d want to go back to the original system”.

It is thought to be the first time such a sympathetic approach has been attempted on a building of this age. Additional BREEAM points will be won for other savings. Achieving these will be helped by the fact that, despite the tower’s inefficiency, its occupants like it on the whole and are willing to forgive character flaws they would not in a less charismatic structure.

In July the temperature in the Arts Tower frequently reaches 29ºC – 5ºC above the generally accepted upper comfort limit. “We planned to put in chilled beams to reduce the internal temperature and proposed an extremely efficient cooling system operating at 14ºC,” Murphy says. “We were planning to produce water at 14ºC through using the wet bulb principle to cool external air, with chillers kicking in when external air temperatures went above 15ºC.”

The university has chosen, instead, to rely entirely on natural ventilation. “The core of the building, containing lifts and stairs, is quite cool, so we’ll help by opening doors, allowing coolth to percolate into outer areas,” Murphy explains. “We also looked at improving internal ventilation by removing some of the internal dividing walls.” That will enable air from the east and north sides of the building, which are shaded, to circulate to the south and west sides, which suffer most from solar gain.

The effect will be to reduce the building’s carbon footprint from 8kg/m2 to 5kg/m2.

“We reckoned, when we anticipated putting a cooling system in, that we would get an energy improvement on the existing building performance of 25%,” Murphy says. “Relying only on natural ventilation will get us a 30-35% saving easily.”