EVEN WITHOUT RENEWABLES IT IS STILL SUSTAINABLE
Bennetts Associates is relying on designed-in sustainability, rather than renewables, for its refurbishment of Ashburton Court in Winchester.
Britain's cities are strewn with soulless 1960s office blocks perched atop podiums of car parking and divorced from the surrounding streetscape. Ashburton Court, one such ensemble which forms part of Hampshire County Council's (HCC) central campus in Winchester's historic city centre, is undergoing a radical makeover by Bennetts Associates, setting a precedent for how this building type can be updated. Early feasibility studies examined a newbuild option on another site and concluded that refurbishment of the existing buildings - a monolithic horizontal block and a second L-shaped block connected by upper-level bridges with car parking in the interior courtyard of the site - would be less expensive and preferable due to proximity to other HCC departments.
The public face of the building was critical because the brief, says Bennetts' Peter Fisher, 'was essentially for a town hall without a debating chamber'. A key challenge was balancing the desire for transparency with the need for security. To free up the site, Bennetts proposed moving part of the parking outside the city centre and infilling the released space with office accommodation.
The top oor of the buildings will be cut back, pedestrian bridges removed, and new one-storey buildings built at the centre of the block along a new internal street to house a reception area, a restaurant and café, an auditorium and meeting rooms. The buildings are being stripped back to their concrete frame; services and cladding are being renewed. Analysis of the building's concrete frame, slabs and foundations using Envest, the BRE's Whole Lifecycle Environmental Impact Analysis software, showed that they comprise about half of the building's embodied energy - another compelling argument for refurbishment.
Both client and architect saw sustainability as integral to the project from the outset. Bennetts director Julian Lipscombe explains that planning permission was relatively straightforward because there was a unanimous feeling that anything done to the building would be an improvement. Interestingly, renewables were a major discussion point with the planners, who were keen to understand how a building could be sustainable without renewables. Winchester has not adopted a mandatory 10 per cent, but rather a suggestion of renewables. After extensive analysis, Bennetts was able to demonstrate that the real issue was overall energy consumption, not the percentage of on-site generation.
Ashburton Court, a 90m-long horizontal linear building, is oriented east-west - the worst possible solar orientation for controlling heat gains - and acoustic studies by Arup Acoustics determined that noise levels in the streets on three sides of the building were such that having operable windows for natural ventilation was impractical. Restrictive ceiling heights limited the feasibility of horizontal runs of new services.
The architect, with the input of services engineer Ernest Griffiths, maintained that natural ventilation, which would take advantage of the thermal mass of the concrete slab, could be achieved without relying on windows on the street facades. Clean air is drawn from internal courtyards across the office floor-plates and up ventilation ducts on the building exterior. While the ventilation strategy looks like stack-effect, it is in fact a form of cross-ventilation driven by wind troughs at roof level. Fisher explains that wind-driven cross-ventilation is more reliable in office buildings because it relies on wind movement rather than, as stack does, on temperature differences, which during summer days can be very small. Wind blowing over the building generates negative pressure inside open-topped 'wind-troughs' irrespective of wind direction. The fine-tuning was determined by iterative thermal and daylighting computer modelling and physical wind-tunnel modelling, which confirmed the pressure differentials between each duct and corresponding courtyard facades for wind directions on 16 compass points. These were fed back into the thermal model, using weather data, to predict hourly annual temperature profiles.
New vertical elements on the street elevations were consistent with the intention to break up the buildings' massive horizontal facades, and took the form of 1.5m-deep brick-clad projections which also provide solar shading to the adjacent windows, so that the building will be partially 'self-shading'.
Another challenge in the design of the new facades was to maintain a glazing ratio below the maximum desirable 40 per cent. Functions that require mechanical ventilation were located in the liberated podium level. In winter, excess heat is recovered and circulated to the offices through an air plenum.
Bennetts' analysis showed that wind turbines would have cost an extra £400,000 in order to save approximately 2kgCO 2/m2/year, while the use of natural rather than mechanical ventilation reduces CO 2 consumption from around 55 to 30kgCO 2/m 2/year. When HCC planners saw the magnitude of the CO 2 reductions, they were convinced despite the fact that the project now includes no glamorous renewables.
Fisher acknowledges that it was a 'mammoth challenge' to unravel the complexities of this building and to define a 'clear sensible strategy' for its renewal. The solution is elegant and simple and not particularly high-tech, though it would have been impossible without the computer simulation and wind studies available today. Fisher is gratified that the architecture - rather than mechanical systems - is 'doing the work': the new vertical ducts provide natural ventilation, give a rhythm to the facade and contribute to solar shading. Wind, through innovative crossventilation, is used in its raw state, rather than being converted to another type of energy. 'It's more primal', Fisher observes.
Ashburton Court is being stripped out and the first phase will complete in winter 2007. The impact of the transformation will then be assessed to see if this is a way forward. It is heartening to learn that evaluation of comfort and energy consumption in this landmark project, too rare in these days of preoccupation with all things green, will be undertaken by the Carbon Trust.
See overleaf for more diagrams.