OVERHEATING | VENTILATION AND ACOUSTICS to sleep disturbance, diminished quality of life and health issues. It is unreasonable and unethical to design new buildings for people to live in very noisy conditions for this amount of time. A holistic industry standard that considers noise and overheating together is needed. This could include a time- or dose-dependent noise standard such as a degree-day approach with an absolute maximum, in a similar way to that used for limiting overheating in CIBSE TM52 and TM59. Noise guidance COOL SOUNDS Limiting overheating and noise control must be considered in tandem if residential properties are to be comfortable, healthy homes, says Swecos Katherine Holden, who suggests alternatives to mechanical cooling L imiting overheating is a major issue in many new residential developments, particularly in the south-east of England, or in areas with noisy external environments, such as cities, town centres and near busy roads, railways and airports. CIBSE TM59 offers guidance on limiting overheating in new homes, while BS 8233 deals with limiting noise ingress but achieving the standards of both, simultaneously, is often challenging. Many planning authorities have requirements for limiting overheating and internal noise, but they are often not applied together. Recent overheating analysis by Sweco shows that windows in a typical apartment block in London without mechanical cooling may need to be open for around one-third of the occupied hours in the summer to comply with CIBSE TM59. Even then, internal temperatures could exceed 30C for up to 59 hours per year in living rooms and still comply. With windows open, internal noise levels could be as much as 50dB, leading The forthcoming Acoustics Ventilation and Overheating (AVO) Residential Design Guide adopts a risk-assessment approach to faade design for reducing noise from transport sources. It offers guidance on approximate external noise limits related to different background ventilation strategies from Part F of the Building Regulations. It does not give specific guidance on overheating strategies, however. Conflicting design factors Many building design factors some conflicting need consideration. For example, the fabric energy efficiency (FEE) test in Building Regulations Part L1A tends to favour higher g-values to pass, as the notional building g-value is 0.63. This reduces the opportunity to limit overheating by reducing solar gains with high-performance glazing. The FEE calculation method could be revised to give more weight to the impact of overheating and mechanical cooling relative to heating energy for example, by lowering the g-value for the notional building in Part L1A. The need for more glazing to meet daylight criteria can also oppose limiting overheating. This can be particularly critical where the home is overshaded by adjacent buildings for part of the day. The daylight criteria could be reviewed to identify alternatives to average daylight factors, including climate-based daylight modelling. Drivers for natural ventilation Natural ventilation is preferred for most developments in the UK because it is likely to be cheaper, simpler and more energy efficient. New developments in noisy areas, however, could require mechanical cooling to avoid unacceptable noise levels and overheating, unless they are carefully designed or the design criteria are changed. As well as costing more and being more complex, installing mechanical cooling would create two vicious cycles: n Increasing carbon emissions would increase climate change (see left) n Heat rejection would increase the urban heat island effect. Building services overheating solutions for different noise risks To limit overheating with natural ventilation in warmer parts of the UK, open windows in, our experience, typically need at least 1m2 of effective free area per room. They would only provide minimal noise reduction, of around 7dB. Using attenuated windows with plenums or attenuated vents could be feasible for cooler parts of the UK. If windows must stay closed because of high external noise levels, mechanical ventilation or cooling would be required. Homes in the South East would not pass TM59 with mechanical ventilation alone, however mechanical cooling would also www.cibsejournal.com October 2019 43 CIBSE Oct19 pp43-44 Sweco.indd 43 20/09/2019 18:32