Bristol fashion

How do you take a grade II listed, 1970s office block with unimaginably deep floor plates and transform it into affordable, low energy residential apartments fit for modern living? That’s the challenge that developer Urban Splash threw down to the design team for its latest project, on the outskirts of Bristol.

The building in question is the former UK headquarters of Imperial Tobacco. Completed in 1975, it was designed by the American architects Skidmore, Owings and Merrill – the architects behind the Sears Tower and the Lever Building in New York.

What made the office building stand out was the five storey steel structure supporting its concrete floors, which was exposed to view as part of its design. Below this were a further two levels constructed from reinforced concrete beams, slabs and columns, all cast in-situ.

Urban Splash’s goal is to convert the former office building into seven storeys of apartments as well as a number of commercial and live/work units at the podium level.

No specific accommodation requirement was set; instead the developer specified that ‘the building was adapted in a way that best used the structure’.

However, it did require that the design team explore all possible options for energy efficiency and the use of renewable sources with the aim of achieving a minimum EcoHomes rating of ‘Very good’.

“The aim is to be as carbon neutral as possible,” says Anna Flemming, associate director of Atelier 10, services consultants on the scheme. “So far, using a number of measures, we have got it down to around 44 kg of CO2 per typical two-bed apartment, compared to around 360 kg for a more conventional design. The last 44 kg is down to the gas boilers”. There are also proposals to offset 100% of the landlord’s energy requirements using renewables.

The first stage of the transformation has already begun, with planning consent for the scheme recently granted. Enabling works are due to start early next year with the project due to go out to tender in June 2007.

The original factory

The original project included the production factory clad in Cor-ten steel with roof top trusses spanning a column free space some 180 m x 90 m, an energy centre and the office building all linked by underground tunnels. Construction work started in 1970 and was completed in 1975.

The factory and energy centre were demolished in 1997/8. The office building has been vacant since Imperial Tobacco scaled down its UK operations 14 years ago and was Grade II listed in April 2000.

Thin film photovoltaics

At Lake Shore, up to 500 m² of thin film pvs will be integrated into the laminated glass balustrade on the south façade of the new build apartments on the north side of the development. These will be enough to offset the annual energy demands for the landlord’s lighting and the car park lighting.

Integrating the pvs into the laminated glass of the balustrades as a substrate reduces the extra-over cost associated with framing and support that would otherwise be incurred. The only noticeable difference is the orange tint to the glass. The typical cost of the glass balustrade is around £350-380/m², and with the integrated pv it is only marginally more at around £400-450/m². German firm Oskomera is currently working on the details of the design.

Renewable energy

The design team is considering both wind energy and photovoltaics for the sceme.

The viability of using wind energy is questionable – however, Urban Splash is willing to pursue it because it could provide up to 20% of the landlord’s energy requirement. Under consideration is the installation of between three and six ‘Quiet Revolution’ type wind turbines. These were chosen partly

for their ‘funky look’ and also because they’re not limited by wind direction. The 16 m high devices also fit into the desired power band, producing a peak of 5-6 kW each and up to 12 000 kWh per year.

Atrium

Cutting the atrium through the centre of the building reduces the depths of the apartments to manageable proportions and provides circulation routes.

The atrium is enclosed by an ETFE roof. This allows a daylight transmission of over 80% while allowing some control of solar gain and enables the back of the apartments to benefit from the daylight in the atrium space.

This roof is raised one storey above the top floor and has ventilators built in to the upstands for ventilation and smoke control. In summer these will open to encourage air to pass through the space and in winter the vents will be closed so that the atrium acts as a thermal buffer space.

Ventilation

All apartments have individual continuous mechanical extract ventilation. This allows the normal trickle vents to be omitted from the windows, reducing the heating demand associated with air infiltration by more than 50%. Apartments will be pressure tested to achieve a maximum infiltration target rate of 7 m³/h/m² @50 Pa.

Ground source heat pumps

Apartments are heated by an underfloor system supplied from an energy centre which includes a closed loop ground source heat pump installation and gas-fired boilers. Around 80% of the annual heating load will be met by the ground source system, which comprises 50-60 boreholes, sunk to around 250 m to compensate for poor ground conditions.

To balance the heat extraction from the boreholes during winter months, the heat pumps will be reversed to provide cooling to the apartments in the summer. This will be delivered through the underfloor system and act to lop off the peak temperatures. However for the more exposed apartments on the upper floors fan coil units will be installed, fed from the same system.

To help improve the system’s performance in cooling mode the temperature of the waste heat will be reduced by 1-2°C by first running it through the lake before it is returned via the boreholes to the ground.

Daylighting

The depth of the floor plates is around 85 m, which was typical in US-style corporate offices of the 1970s. This makes it impossible to achieve recommended daylight factors in all parts of the apartments, particularly in the kitchens.

Large, full-height viewing windows in each apartment partly compensate for this, along with a large atrium punched through the structure’s centre. This is covered with an ETFE roof, allowing light to penetrate into the atrium and in turn into the apartments.

Typical one bed apartments are arranged in pairs as interlocking ‘L’s across three structural bays. The apartments are identical, except one has a bedroom orientated out towards the landscape and the other has a bedroom onto the atrium (in this case, the bedroom is raised maintaining privacy).

To minimise heat loss in winter and solar gains in summer, all the glass used on the facades will be high performance double glazed units with low E glass and inert gas fill, with a U-value of 1.5 W/m²K. The glass has a high visible light transmission (over 75%) in order to maximise daylighting, but is set back from the structure, providing some shading from high angle sun.

Electricity

The biggest hit on reducing the carbon emissions for the scheme is the bulk purchasing of green electricity. This will be distributed to tenants and owners and metered by a management company as part of the leasehold agreement. The long-term benefit of this is that technologies such as biofuel CHP and hydrogen fuel cells can be easily incorporated into the scheme, further reducing carbon emissions.