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| Windcatchers are integrated into a cutting-edge academy school |
Designed by Aedas Architects, Stockport Academy is a £24m state-of-the-art secondary school accommodating 1150 pupils and specialising in science-based subjects, supported by a business and enterprise theme. Intended as a symbol for the transformation and regeneration of Cheadle Heath, the four-storey building replaces Avondale High School, and provides a broad range of community resources covering ICT, sports facilities, public meeting spaces and a library. In environmental terms the scheme features a range of passive and active technologies including a closed-loop ground source energy system, which has reduced carbon dioxide emissions by around 20 per cent. Substantial savings have also been made by naturally ventilating the 350 square metre main hall and 590 square metre sports hall using Monodraught Windcatchers. These simple yet innovative devices are not only cost-effective and reliable, but also facilitate secure night-time cooling – a key tenet of any natural ventilation strategy.
The brief was derived from the academy’s curriculum needs (detailed in DfES guidelines) and included the provision of nursing, sports science, physiotherapy and traditional science facilities. Project architect Imran Kassim says aspects such as massing, scale, cladding and landscaping are intended to set the building within its urban context, while at the same time creating an impression of distinctness – a place that balances the serious business of learning with the range of social experiences that make up a modern academy.
Located on a sloping site, the 10,300 square metre building is conceived as a solid block that has been carved away to reveal entrances, doorways and windows. Hollowed out of the centre is a three-storey atrium with a glazed roof. This forms the main communal space, allowing daylight into the heart of the plan, organising the internal circulation and separating the curriculum areas. Positioned around the atrium are the resource area, reprographics suite, informal study spaces and the dance and drama studios. The latter can be opened up, allowing performances to spill out into the atrium itself. Dead-end conditions have been designed out of the scheme, with the two stair towers on the south-west elevation acting as conduits from the outside play area into the building.
The main entrance for staff, visitors and sixth-form pupils is set into the western corner of the building and comprises a triple-height, external covered space overlooking the play area. The pupil entrance is at lower-ground level on the south-west facade, and takes the form of a two-storey recessed opening that brings natural light into the base of the atrium. Designed to maximise daylight, the traditional classroom spaces are located along the curved south-west facade, with access from balcony corridors facing into the atrium.
The main hall and sports hall are gathered together on the north-western side of the plan for reasons of access and efficiency. Entered at first-floor level and incorporating retractable seating, the main hall accommodates assemblies, performances and examinations. The sports hall is accessed at ground level and relates to an existing sports centre located opposite. Externally, the building is expressed as a single brick-clad form with a sculpted prow-like entrance, punched window openings and stair cores treated as individual elements extracted from the whole.
The academy’s location beneath the flight path to Manchester Airport dictated a sealed envelope as a means of attenuating aircraft noise. This in turn led to the adoption of mechanical ventilation and heat recovery – used in conjunction with a ground source heating and cooling system – for the teaching areas. Natural ventilation was favoured for the communal spaces, such as the atrium, main hall and sports hall, where acoustic isolation was not deemed as critical. In the atrium this was achieved by using a combination of high- and low-level automated facade openings. The main hall and sports hall posed a challenge as the limited size and number of windows – resulting from concerns over security, glare and internal noise transmission – meant that it would be virtually impossible to achieve the necessary levels of cross ventilation using opening lights alone.
The solution was to use Monodraught’s system of roof-mounted Windcatchers. Designed to catch the wind from any direction, the GRP units comprise a series of external louvres linked to quadrants and internal turning vanes. The captured air is brought down into the building via a damper system which controls the rate of flow. Simultaneously, warm internal air is expelled through the same route. The overall effect is similar to displacement ventilation. The use of environmentally-friendly technologies, such as Windcatchers and ground source heating and cooling, was driven not only by Part L but also by the local planning authority, which stipulates a renewable energy content of greater than 20 per cent on all new-build projects. In addition, both the client and architect were committed to reducing the maintenance and operating costs of the building where possible.
Windcatcher specification
The decision to use Monodraught Windcatchers to naturally ventilate the 2095 cubic metre main hall and 6435 cubic metre sports hall was taken for a number of reasons. First, the units can be designed and sized to meet the exact needs of the individual spaces, without relying on external elements such as opening windows and vents. Second, the system can provide night-time cooling during the summer, without the security risks of having to leave windows open. Third, the Windcatchers themselves contain few moving parts (none externally) and therefore incur minimal maintenance costs. Last but not least, the ventilators were able to meet stringent acoustic demands resulting from the building’s close proximity to Manchester Airport. Independent tests carried out by the BRE show that on average a standard Windcatcher has the effect of reducing noise transmission by 15dB compared to an open window. The team’s decision to specify units with internal acoustic linings and fully-integrated silencer pods resulted in an additional 17dB of sound attenuation.
From the outset the architect worked closely with environmental engineer Buro Happold and Monodraught to develop a detailed natural ventilation proposal. The team used BS5925:1991 (Code of practice for ventilation principles) and DfES Building Bulletin 101 (Ventilation of school buildings) as a starting point for evaluating the needs of the project. Both documents suggest a minimum ventilation rate of eight litres of fresh air per second per person (8l/s/p). Monodraught, however, proposed a fresh air figure of 12l/s/p. Detailed analysis undertaken by the manufacturer indicated that over 60kW of heat gain would have to be ventilated from the main hall and over 20kW of heat gain from the sports hall. The calculations were based on occupancy figures of 740 and 150 people respectively, as well as structural, lighting, solar and equipment heat gains.
In order to accommodate the anticipated heat build-up, Monodraught recommended 14 air changes per hour in the main hall using six Windcatchers, and 1.5 ac/hr in the sports hall using two Windcatchers. All the ventilators measure 1200mm square and are designed to provide the desired ventilation rate at a wind speed of approximately 2-3 metres per second. The Met Office gives a mean wind speed across the UK of around 4-4.5m/s – so sizing below this average gives a high degree of assurance that the required airflow will be achieved. During occasional periods of little or no wind, air movement is still created by stack effect; ie the warmer less dense internal air is dispersed at roof level and replaced by cooler and denser outside air.
A Merlin grey finish was chosen for the outer casing of the Windcatchers to complement the colour of the metal doors and windows used throughout the building. The manufacturer liaised with the structural engineer and roofing contractor to ensure that structural loading and detailing issues surrounding the installation of the ventilators was fully co-ordinated into the programme.
Installation and commissioning
The roofing subcontractor formed the structural apertures and then fitted 200mm high metal aprons/ apexes to each void ready to receive the Windcatchers. Installation was carried out by Monodraught following completion of the preparatory work. The ventilators, which incorporate anti-bird mesh and 50mm thick internal acoustic linings, are bolted to the upstands. The acoustic silencer pods, motorised damper assemblies and white eggcrate diffuser grilles (located beneath the units) are suspended from the steel roof structure using a system of drop-rods and support frames. Located directly above the dampers, the silencer pods comprise a series of v-shaped foam splitters housed in 1750mm square by 600mm deep galvanised steel enclosures.
The Windcatchers are controlled automatically via the academy’s building management system. The units are linked to the BMS via wall-fixed temperature sensors (two each in the main hall and sports hall) located at a height of approximately 1.5 metres from the floor. The average temperature reading from the sensors determines the amount by which the dampers are opened or closed. During summer operation the dampers commence opening at 20°C and then continue to open incrementally until 24°C, when they are fully open.
The system is pre-programmed to provide night-time cooling. This involves opening the dampers fully at night, thereby purging the halls of warm, stale air and making them cool and fresh for the start of the next day. The dampers automatically close during this process if the internal temperature falls below 16°C. In the winter, the dampers are either closed or set to five per cent open to provide trickle ventilation. Manual overrides located on the sensors allow staff to fine-tune comfort levels and/or respond to unseasonal weather conditions. The override permits the dampers to remain open for up to one hour before they revert back to their pre-set positions.
Project team
Architect: Aedas Architects; structural engineer, services engineer: Buro Happold; qs: Parker Browne; project manager: Townson Associates; main contractor: Bowmer & Kirkland; services contractor: NG Bailey; client: United Learning Trust.
Selected subcontractors and suppliers
Passive stack vents: Monodraught; steel frame: FH Dales; aluminium roofing: Keyclad;Technal windows, doors, curtain walling: Dortech; brickwork: Steve Barratt Brickwork; groundworks: Adana.
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