roof along with two spacious roof terraces giving views over north London. In addition to the perimeter extensions,


the floor plates have also expanded into the large central courtyard. The remainder of this opening is now covered by a transparent polymer roof of ETFE pillows to form the central atrium. In all, the addition of a new floor and extensions to the existing office floors has added over 60% more lettable space for the developers, Derwent London. The scheme now totals more than


The key to the success of this project was to involve the design team to develop the building to reduce energy consumption in conjunction with the building services solution


24,000 sq m of high-specification office space, along with an atrium, retail outlets and new roof terraces. ‘We’ve been able to generate a significant amount of new space,’ Hall explains. Energy reduction was at the heart


of this transformation. The re-used structure saved a significant amount of embodied carbon; the challenge for the building services design was to build on this achievement and come up with an energy efficient, environmental solution to complement the structural solution. The engineers were helped in their task


by the existing structure, which was based on a significant floor-to-ceiling height of 3.5m. ‘We inherited a building with fairly generous floor slab to ceiling slab heights, which opened up a lot of options for the type of cooling and heating systems we could install,’ says Hall.


PROJECT TEAM Client: Derwent London


Environmental and fire engineer: Norman Disney & Young


Architect: Allford Hall Monaghan Morris


Structural engineers: Adams Kara Taylor


Main contractor: BAM Project managers: Buro Four Cost consultants: Davis Langdon


Lighting consultants: Equation Lighting Design


26 CIBSE Journal May 2012


Cooling The designers looked at a number of options to provide cooling to the office floors – including fan coil units, chilled beams and chilled ceilings – before settling on an air displacement system. ‘In the end we decided to exploit the exposed soffits by opting for a displacement ventilation system concealed within a new 450 mm raised floor,’ says Hall. Displacement air is introduced to the


offices through grilles set in the floor. This arrangement restricts the minimum temperature at which air can be introduced to 19C which, in turn, limits the amount of cooling the system can achieve. ‘Because we’re introducing air close to people it cannot be too cold or they will feel a draught,’ Hall explains. The upside of supplying air at this temperature is that it allows the building to run in ‘free cooling’ mode for 80% of the time. ‘At 19C we don’t have to cool the outside air for most of the year, we don’t have to use the water-cooled chillers, so we


save loads of energy,’ says Hall. Another big advantage of this approach is that using mostly outside air helps keep the office environment feeling fresh. The downside is that the fans still have to run to push all this air through of the offices. Another potential drawback of the


system is that NDY was concerned that the displacement air would heat up in contact with the floor slab. The problem arises because air temperatures can reach 27C close to the ceiling soffit, heating the floor slab of the offices above. NDY had data from tests carried out in Germany showing that, depending on the temperature difference and the distance air travels in contact with the slab, the supply air temperature could increase by


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