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Climatic Facades
All the offices have bottom-hinged double-glazed windows which are motorized to allow natural ventilation. This is controlled by the building management system or by the occupants using wall mounted switches. On the outside of the building these opening windows are protected from the weather by a layer of glass with a full width opening of around 125mm top and bottom to admit ventilation air. In the void between is a venetian blind, motorized for remote control of rise and fall and of blade tilt.
With this window configuration the client has two principal building management options. First, windows controlled shut with air-conditioning – air inlet and extract at ceiling level plus cooling by chilled ceilings. Secondly, there is a low-energy option of natural ventilation with air-conditioning off.
Building management will vary with height, informed by weather stations at four levels. The triangular-plan form with its rounded corners and gently convex facades is more aerofoil-like than a sharp-cornered rectangle, so has lower surface wind pressures than a rectilinear building.
The final cladding that resulted from competitive tendering reduced its cost but it does not have the same performance attributed of the system that was first designed. As the schematic sections show, in the original design exhaust air from the room was vented up between the fixed double and single glazing and out through the mullions, so keeping this void close to the internal temperature. Above was an opening, louvre-protected top light. This aimed to give good mixing between ventilation and room air, and allow some night cooling by natural ventilation. Despite the lack of an exposed ceiling and cellular layout, modelling suggested a useful night-cooling contribution. While the windows could be opened for almost all the time, modelling suggested natural ventilation would be an energy benefit for up to 60 per cent of the year. The designed mid-pane U-value was 0.9W/m2K (requiring work to provide matching frame performance). Estimated building energy consumption was 140-150Wh/m2/year for heating and cooling.
The as-built design is expected to work somewhat differently to the original. Hot-weather performance as-built is expected to be worse, in that heat will tend to build up (by greenhouse effect) between the single-glazed screen and the double-glazed opening light. Thus there will be some preheating of ventilation air and radiant warming of the room. On the other hand, while a large bottom-hinged opening light is not ideal for fine ventilation control, it has the single glazing to shield it from the weather while the original opening top light was unscreened. For cooler periods and for rain and wind, the as-built design is expected to work better and be used more frequently.
Overall the as-built building is expected still to be used for natural ventilation some 60 per cent of the time and to have similar seasonal energy performance. Mid-pane U-value is a bit higher at 1.1W/m2/K.
For both cladding systems, the double glazing is also side-opening to allow cleaning of the glass and blind in the void. Office design temperatures are 20°C minimum in winter and 27°C maximum in summer. The minimum ventilation rate is around three air changes per hour to give good fresh-air distribution and to cope with localized hot spots and smoking. The building's air inlets are at floor 5 and at the top of the building. The lighting was designed as high-efficiency, with high-frequency ballasts and dimming control rather than switching on-off in response to daylight variation, which the engineer feels is too distracting.
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