Approved Document O – Overheating and Noise

As the UK moves into the colder months, we are left reflecting on a summer of new and unnerving record temperatures. It certainly seemed timely that the date set for an Association of Noise Consultant’s meeting on the newly published Approved Document O on Overheating coincided with the hottest day recorded in UK history, with the meetings moving online due to the issue of…overheating.

Many would have found these hottest days of summer to be an overtly uncomfortable time, and the increasing concern surrounding the wellbeing of occupants during periods of overheating is the driving factor behind this document. The aim of the new Building Regulations document is summarised as limiting unwanted solar gains in summer and providing an adequate means of removing excess heat from the indoor environment.

Another element of wellbeing that is covered in the document is the issue of noise levels, specifically during night-time hours, when noise can play an important role in the quality and ability of sleep. Overheating and noise have always been issues that lie on the opposite sides of a tipping scale. The dichotomy between them centres on the simple fact that façade openings are a good thing for fresh air and cooling, and a bad thing for noise intrusion. The wider a window is open, the more noise comes in. Air quality also plays a part and has many similarities with noise in this respect.

The document states that the following parameters are to be achieved with regards to noise:

• 40dB LAeq,T, averaged over 8 hours (between 11pm and 7am)
• 55dB LAFmax, more than 10 times a night (between 11pm and 7am)

The first parameter is an overall average of sound pressure over the course of the night. The latter parameter refers to instantaneous peaks of noise, which have been shown to have a significant effect on sleep disturbance.

What do these decibel levels mean for acoustic design? In short – they are low and could be difficult to achieve in many instances.

The other issue with these targets is that, unlike overheating parameters, they are not measured according to a percentage occurrence over a year. Even if a proposed strategy was to show that windows could be open for only one night a year and satisfy overheating requirements, if the acoustic predictions show that the noise limits are exceeded for that one night, then the approach is not compliant with Approved Document O and must be discarded.

So how do these targets for internal noise levels relate to potential methods for controlling overheating? The paper describes two methods to control overheating – a simplified method, and a dynamic thermal modelling method.

For the simplified method, different approaches apply depending on whether the location is a ‘high risk’ or ‘moderate risk’ location, where the higher risk category has a greater chance of overheating, and therefore requires a greater degree of cooling. The relevant parameter that states the amount a window should be opened in the simplified method is expressed as a free area of 13% of the floor area for high risk locations, and 4% for moderate risk locations.

A joint publication from the Association of Noise Consultants and Institute of Acoustics has conducted some number crunching on these open window areas based on typical bedroom sizes and has found that having windows open by these amounts means that, in order to meet internal noise targets, external levels must be no higher than:

High risk locations:

• 44dB LAeq,T, averaged over 8 hours (between 11pm and 7am)
• 59dB LAFmax, more than 10 times a night (between 11pm and 7am)

Moderate risk locations:

• 49dB LAeq,T, averaged over 8 hours (between 11pm and 7am)
• 64dB LAFmax, more than 10 times a night (between 11pm and 7am)

An approximate noise map of major roads in England (see below) gives further context for London – where all levels greater than 50 dB LAeq,T are indicated. Not only are limiting levels of 44 dB going to be more extensive than these, but the noise map is only a rough guide and large areas outside the roads mapped on this service will still exceed this level – with instantaneous max events not indicated at all and very likely to play a significant role in congested city areas.

If the simplified method were to be a first pass approach to complying with Approved Document O on a development, but a noise assessment found the above levels would be exceeded, then it cannot be adopted. Dynamic thermal modelling must be used instead.

The proposed dynamic thermal modelling process closely follows the approach outlined in CIBSE’s TM59. The document and subsequent FAQ guidance clarifies that windows can be modelled as partially open at night – this can be an important mitigating tool for noise since, as previously mentioned, one way of attenuating noise is to reduce a window opening to be as small as possible.

Of course, this can and will be dictated by how large the opening must be according to the thermal modelling to achieve appropriate overheating mitigation. However, this works in both directions – the opening of the window can and will equally be determined by limits on how much noise the opening will allow in. It is important for both parties to understand early on that these limits are applicable to the discipline across the table – as they will dictate the limits applicable to their own.

Whilst other forms of mitigation for acoustics do exist, and can be adopted to further reduce noise, it is widely acknowledged that items like noise attenuating louvres, or attempts at shielding noise via balconies, can often have a limited effect in the face of the significant reductions that are often needed.

It may be obvious at this point why the onerous limits on noise are a cause for concern. Should a stalemate between the thermal and acoustic requirements be reached, then there will be no other option but to adopt a solution that relies at least partially on mechanical ventilation or cooling. This is not a desirable outcome for those seeking a more passive solution, but as a result of these new compulsory regulations, there will be many instances where no other option is feasible. It is very likely that projects will start with a more passive goal of ventilation in mind, only to have the unexpected and unwelcome news that a much higher level of mechanical ventilation is needed than was first accounted for. This does not sit well with the drive towards zero carbon design.

It is therefore important to consider these issues at the earliest possible design stage. Early and clear communication between the acoustics and mechanical/building physics engineering teams should be established – and clients must be well informed on the issue in these early design stages as well. The more knowledge / awareness of these new requirements, the more constructive the communication between team members and subsequent approach and outcome are likely to be.