Background natural ventilation is provided at all times, but without any mechanical cooling. The risk of overheating is further reduced by automatic night ventilation in hot spells and cross-ventilation in daytime – no use of stack ventilation is relied on in hot day conditions.
The design team was formed when Richmond was preparing for Canon's architectural competition. Thinking that the site and brief might have potential for natural ventilation, Richmond contacted Nigel Griffiths whom he had seen give a presentation about the PowerGen building. Their design iterations resulted in a building with cloisters to the south and no general cooling. The approach seemed to work, but was viewed as a bit of a long-shot until the competition's interview stage, when the Japanese company's president took a liking to the simple and natural approach and the somewhat collegiate feel.
The completed building looks set to achieve a handful of accolades. Popular with the owners and occupants and recently formally opened by Prince Philip, it achieved an 'excellent' BREEAM rating and 'building of the year' nominations by the British Council of Offices, British Institute of Facilities Management, the Concrete Society and the Institute of Structural Engineers.
The three-storey building has three linked east-west wings – two of these have a linear atrium with a relatively narrow-plan office space either side; the third (to the west) forms a more conventional narrow-plan wing. Above the atriums, and forming a fourth level of the main atrium office wings, there is a service balcony with glazing and extract grilles to the roof air handling units.
Using the slope of the site, a single-storey restaurant floor extends from service basement level of the main building towards the site's original occupant: a well-proportioned hunting lodge which had been all but hidden by the previous office development.
Ventilation issues
Background ventilation uses two-speed fans delivering air via insulated ducts from floor diffusers. Return is via the atrium and grilles at the high level balcony level, or via extract grilles in the single-width west wing. Working at half-volume and reportedly getting close to the cube law one-eighth power consumption, the air handling units produce very low noise levels. The office air handling units are at roof level – a total of five, one for each side of the wings. The service balcony in the upper level of the atrium therefore is at the same level as the adjacent roof air handling units.
The ventilation is recirculating with fresh air being introduced to limit measured CO2 levels and to achieve a minimum winter supply temperature of 18°C. In winter start-up the ventilation will generally be fully recirculating.
Swirl diffusers – located in floor tiles and on flexible connections to underfloor distribution ducts – produce a column of air rather than a pool, so it can be introduced at temperatures down to 15°C, without causing discomfort. An air speed of below 0·25 m/s is achieved at a distance of 500 mm from the diffusers.
Maintaining comfort
The hot, still day was rejected as the design condition – a decision inherited from PowerGen – in favour of the cross-ventilated condition. It was not felt necessary to repeat the extensive real-weather dynamic calculations that established this for PowerGen.
The large (3 m) overhangs on all south-facing levels mean that design solar gains are marginally less in the south offices than in the north. Such overhangs might easily have led to a repetitive or oppressive facade, but the architect has varied the means of shading at each level to produce a light and interesting feel: the cloisters at ground level have brick piers and roofs; at first floor level translucent louvres form a brise-soleil, and at the second floor the oversailing roof ascends slightly. A shaped metal spacer rises up the building above each cloister column. East and west facades are smaller, have less glazing and automatic motorised louvre blinds.
Office equipment is typical 'one pc per desk' for corporate headquarters with administration, sales and marketing functions. Research showed a consistent requirement of 140 W/ workstation, reducing to 90 W at night. Copiers and other machines are grouped in areas with separate non-recirculated ventilation (and cooling in some cases) so as not to contribute to the potential cooling load of the main office areas. Heating is a perimeter system served by a single set of modular gas boilers.
Night ventilation is available to reduce internal temperatures in spells of hotter weather. The building energy management system (bems) controls Velfac opening windows for the toplight office windows and both lights of the high level service balcony windows.
The night ventilation control algorithm – reportedly coming top in BSRIA's recent head-to-head comparison – is intentionally simple. When in summer mode, the building is pre-cooled by opening night ventilation windows to achieve an internal temperature of 18°C. To quote Private Eye, "Er, that's it".
One potential thorn for the cross-ventilation approach – that the leeward office block gets atrium air which has been warmed from the upstream block – is answered by modelling. This showed that with the high level windows open and the wide atrium, the air entering the leeward block is cool enough to effectively reduce overheating.
A single chiller serves fan coils in the meeting rooms, and in 'management centres' where office equipment such as copiers are concentrated – a good move for thermal comfort and potential chemical hazard reasons. The chilled water also serves the lecture theatre supply air handling unit but to date there has been little demand for cooling.
The chiller uses R407c refrigerant and a high level of sound attenuation – the project set its own noise control criteria which were tighter than local authority or BREEAM standards because the site is so quiet.
Apart from the areas mentioned above and two server rooms with direct expansion cooling units, the rest of the building is not cooled. Space has been provided in the main air handling units for cooling coils, but these have not been installed.
Lighting and other services
The general office lighting scored a welcome first at the time of visit, in that most of the office lights were visibly dimmed by their local lux detectors. This was true at the perimeter, in central office areas and at the atrium sides of the offices, and it indicates an unusual coincidence of three events:
- light level detectors are installed and enabled
- light level setting were reasonable (350 or 400 lux)
- sufficient daylight in the offices to provide this level of lighting
The office lighting consists of twin-tube fittings in lateral booms at 3 m centres within the concrete coffer ceiling system. Ten percent backwash and white acoustic wings provide a pleasing ceiling brightness.
Presence and lux detectors control the light fittings. Research was carried out to ensure that the detectors provide effective control – this concerns the number of fittings controlled by each sensor and the range of the sensors: too little overlap and control is patchy and over-sensitive, too much and you lose the effectiveness of local controls. Halide atrium lighting is controlled from the bems.
A photovoltaic roof cladding system provides a 33 kW peak of electrical power and therefore ranks as one of the UK's largest installations. Supplied by a Canon subsidiary as a demonstration, it appears to be effortlessly integrated both electrically and architecturally.
Office cabling uses an Ackermann spider system with a neat underfloor hub and umbilicals providing 'potential flood wiring' – each desk has a circuit breaker. A hot-desking area on the ground floor has special desktop power and network connection pods.
The building in use
It is little surprise that the building is quiet with small air movement or noise. There is a feeling of space in the open-sided atrium, which has a relatively generous width, especially in relation to the three-storey height, and the office plan depth of only 11 m. Thoughts that this is all a bit luxurious in space planning terms are countered by the fact that the atrium provides at least two crucial functions with great efficiency.
First, office circulation is in 1·5 m balconies within the atrium volume, ie outside the 11 m office plan. Being relatively light structures and with the office edge defined by columns, these balconies provide a low cost circulation area which does not encroach on the atrium or the office. The atrium is in fact 12 m wide to the structural columns, 9 m between the balcony edges. By acting as the return path for ventilation air the atrium also provides a further modest saving in floor area.
Second, at ground floor the cafe-style seating provides surprisingly effective informal meeting space provision. Fears that using this seating for work would cause noise or privacy problems for nearby office workers, or for those at the meetings, have apparently been unfounded.
The arrangement of shaped ceiling coffers with rounded lighting booms provides a clever aesthetic solution to a difficult problem: how to achieve the structural strength of the coffers while incorporating a lighting system and acoustic damping, but without creating obtrusive ceiling beams or dark concrete coffers. The concrete coffers were formed on-site in reusable moulds, which while individually expensive, prove cost-effective in bulk.
What of the future?
Is this the corporate naturally ventilated building which works, or a company president's indulgence? Although not yet tested by a scorching summer, it promises to be a technical success. It also looks great, is easily managed, and was built at moderate cost in corporate headquarters terms. But the building has advantages that may not be widely available: a sloping, unobstructed site facing the prevailing wind; enough land to allow a building of only three-storeys with an 11 m floor plan and a wide atrium; a major corporate owner-occupier interested in a natural approach, and where individual offices are taboo.
Nevertheless, whingeing about a site's advantages would be petty if this building works out half as well as it promises, and it could provide another valuable step away from estate agents' standard requirement for air conditioning.
Source
Building Sustainable Design
Postscript
John Field is director of Target Energy Services, www.targ.demon.co.uk
Credits
Client Canon UK Quantity surveyor Bellamy & Wareham Architect David Richmond & Partners M&E consulting engineer Ernest Griffiths & Son Structural and civil engineer Curtins Consulting Engineers Main contractor Wates Construction Mechanical and electrical contractor Haden Young Commissioning contractor Balcomm Services