A naturally quiet solution

Stockport Academy is a £24m state-of-the-art secondary school specialising in science-based subjects.  The four-storey building replaces Avondale High School and accommodates 1150 pupils. It provides a broad range of community resources covering ICT, sports facilities, public meeting spaces and a library.

The academy’s location beneath the flight path to Manchester airport dictated a sealed envelope as a means of attenuating aircraft noise. Natural ventilation using automated façade openings was favoured for communal spaces where acoustic isolation was not deemed critical, however, the main hall and sports hall posed a challenge. The limited size and number of windows in both halls 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 openable windows alone.

Aedas Architects worked closely with environmental engineer Buro Happold and Monodraught to develop a detailed natural ventilation proposal using BS5925:1991 (Code of practice for ventilation principles) and DfES Building Bulletin 101 (Ventilation of school buildings) to evaluate the needs of the academy. As a result Monodraught Windcatchers were installed as one of a number of active and passive energy saving technologies to provide natural ventilation in the 350m² main hall and 590m² sports hall.

As well as being cost-effective and reliable these simple yet innovative devices are designed to catch the wind from any direction and provide secure night-time cooling within the natural ventilation strategy. Equally important, the Windcatchers were able to meet stringent acoustic demands required by the building’s location close to the airport. In fact, independent tests by the BRE show that on average a standard Windcatcher has the effect of reducing noise transmission by 15dB compared to an open window, and an additional 17dB of sound attenuation was achieved by specifying Windcatchers with internal acoustic lining and fully-integrated silencer pods.

Using Windcatchers to naturally ventilate the main hall and sports hall also offered other advantages, the main one being that the units can be designed and sized to meet the exact needs of both spaces, without relying on opening windows and vents. This latter point also proved vital, as Windcatchers provide secure night-time cooling during summer without needing to leave windows open.

Whilst BS5925:1991 and DfES Building Bulletin 101 suggest a minimum ventilation rate of eight litres of fresh air per second per person (8l/s/p), as part of the ventilation strategy, Monodraught proposed a figure of 12l/s/p in accordance with CIBSE recommendations. Furthermore, a detailed Monodraught analysis indicated that over 60kW of heat gain would have to be ventilated from the main hall and over 20kW from the sports hall. The figures are based on occupancy rates of 740 and 150 people respectively; and include structural, lighting, solar and equipment heat gains.

To accommodate the anticipated heat build-up, Monodraught recommended using six Windcatchers to provide 10.5 air changes per hour (10.5 ac/hr) in the main hall, and two Windcatchers providing 1.5 ac/hr in the sports hall. All eight 1200mm² Windcatchers are designed to provide the desired ventilation rate at a wind speed of approximately 2 – 3 metres per second.  

The roof-mounted GRP Windcatchers, which comprise a series of external louvres linked to quadrants and internal turning vanes, bring captured external air down into the building via a damper system that controls the rate of flow.  Warm internal air is expelled simultaneously by the same route creating an overall effect similar to displacement ventilation.

Installation was carried out by Monodraught following completion of preparatory work by the roofing subcontractor. The Windcatchers, whose outer casings are finished in Merlin grey to complement the colour of the building’s metal doors and windows, incorporate anti-bird mesh and 50mm thick internal acoustic linings, 600mm deep acoustic silencer pods, motorised damper assemblies and white egg crate diffuser grilles.

They are controlled automatically using the Academy’s building management system and are linked to it via wall-fixed temperature sensors. During summer the dampers begin to open at 20°C and continue to open incrementally until they are fully open at 24°C. As mentioned, the system provides night-time cooling, which is pre-programmed. By opening the dampers fully at night the Windcatchers purge the halls of warm, stale air to make them cool and fresh for the start of the next day.
 
The dampers close automatically during this process if the internal temperature falls below 16°C. In the winter, the dampers are either closed or set to 5% open, in order to provide trickle ventilation. Manual overrides allow staff to fine-tune comfort levels and/or respond to unseasonal weather conditions. The override allows the dampers to remain open for up to one hour before they revert back to their pre-set positions.

The use of environmentally friendly technologies, such as Windcatchers was driven not only by Part L but also by the local planning authority, which stipulates a renewable energy content of greater than 20% on all new build projects. Besides delivering substantial energy and cost savings the Windcatchers contain few moving parts (none externally), which minimises maintenance costs – a priority for both the client and architect who were committed to reducing the operating costs of the building wherever possible.

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