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ACOUSTIC AND VENTILATION ASSESSMENT IS NECESSARY

TECHNICAL & PRACTICE

Achieving successful ventilation in schools while complying with guidelines for acoustics can lead to conflict in the design brief. But there are ways to resolve these issues, including greater involvement of clients and end-users.

Good acoustics and proper ventilation are critical to the successful design of schools. The BB87 document, issued in 1997, provided guidance on both these issues. In 2003, BB93 (Acoustic Design of Schools) was incorporated into the Building Regulation process via Part E, followed in 2006 by BB101 (Ventilation of School Buildings) via Part F. Although these regulations are intended to support the design of good school buildings, recent work for the Scottish Executive by the authors with Sara Wigglesworth concluded that the design of schools in Scotland often involves compromise when it comes to these regulations. The same is true in England, Wales and Northern Ireland, despite BB93 and BB101 effectively being Building Regulations - not the case in Scotland.

Clients typically request compliance with the latest standards for air quality, daylighting, acoustics and overheating, but schools often suffer from extensive derogation of these design aspirations. Our research for the Scottish Executive shows that briefs for new and refurbished schools are conflicting, and that resolution usually occurs too late in the design process for an informed client decision to be made. Clients must be educated to understand engineering trade-offs at an early stage, and end-users must be involved in discussions so that any issues are made clear in the project brief.

DAYLIGHTING BB90 states that daylight should always be the prime source of lighting during daylight hours. However, this requires a daylight factor of 5 per cent or more. Lighting can be one of the biggest energy users within a school building. Clearly, maximising daylight has significant sustainability benefits, but avoiding glare and solar heat gain can affect the design. For schools of more than one storey (i. e. most secondary schools and many primary schools), lower floors can't realistically be expected to achieve the 5 per cent daylight factor. Use of clerestories or skylights can significantly improve daylight factors, and provide opportunities for better ventilation, but can mean excessive rain noise.

VENTILATION Learning environments require good air quality. Although indoor air quality (IAQ) depends on a complex set of factors, it is understood that if CO 2 levels are kept low, good IAQ is likely to be achieved. Levels of 1,000ppm-1,500ppm CO 2 generally indicate good IAQ in classrooms, and the levels reduce as outdoor air passes through a room. Ventilation is also a primary means of preventing overheating in summer. The problem is the use of natural ventilation through opening windows. This is inherently problematic. The production of air pollutants is largely independent of external weather conditions and so maintaining the same degree of air quality requires the same ventilation rates in summer or winter (continued overleaf).

BB101 requires the capability to ventilate at 8l/sec/ person. The minimum ventilation flow rate required for a typical classroom, 3l/sec/person, equates to 2.1 air changes on a continuous basis, with a capability to achieve 5.7 air changes an hour. This heat loss can be a major element of the energy consumption of the school - a large quantity of air which, in the winter, is introduced into the space simply via open windows. Such a design can't be considered good or sustainable.

Research has shown that levels in excess of 4,000ppm of CO 2 have been found in UK schools, indicating unacceptable IAQ.

This normally occurs in winter periods and is largely due to windows not being opened during teaching sessions. Reasons cited for the failure to open windows were their location, which made opening difficult, and draughts from open windows in the winter.

Some school specifications now require CO 2 sensors in classrooms to inform teachers of poor air quality. This will result in greater awareness of winter IAQ levels, but will opening windows be seen as an acceptable reaction?

NOISE Open windows provide limited attenuation to external noise. In many schools, open windows could cause internal noise levels that exceed those considered desirable by BB93 and BB101. However, many schemes are naturally ventilated and considered successful, while not achieving strict compliance. Natural ventilation and strict compliance can't be assumed to be compatible.

THERMAL COMFORT BB101 sets a standard for overheating that includes a maximum of 120 occupied hours at over 28infinityC. This reflects guidance in BB87, but it is not a particularly high design aspiration. At a recent conference, the National Union of Teachers (NUT), concluded that teaching should not take place at temperatures exceeding 26infinityC. Passive design methods can be effective, but it remains questionable whether the NUT objective can be achieved.

THE ANSWER?

Clients undertake the commercial preparation of a tender process with a degree of rigour, retaining financial and legal advisors. It is less common to see engineers retained to develop the brief.

Environmental design compromises are developed in isolation from end-users. Often the outcomes are a consequence of the architectural design, which is the focus of interaction between the client and bidding teams. Clients often assume that 'engineering matters' will be resolved by specification statements.

It is often impossible to comply with all the regulatory requirements. Yet architects must be more proactive in tackling these important issues early in the design process so that the tradeoffs can be assessed and an informed decision can be reached.

Peter Bailey is an acoustic engineer and director of Hawksmoor Engineerg.

Gordon Hudson is a mechanical engineer and an associate director of Mott MacDonald.

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