With the UK breaking temperature records while homes become increasingly air tight, James Healey from the Institute of Acoustics looks at hitting the tricky balance between overheating and external noise
This summer saw the hottest temperatures the UK has ever experienced; 40.3˚C was recorded in Coningsby, Lincolnshire on 19 July. People living in houses with cross-ventilation – and in quiet, non-polluted areas – have probably learned that closing curtains and windows when the sun shone helped reduce solar gain and minimise internal temperatures, while at night, opening the windows brought the benefit of colder air.
However, if you live in a flat that doesn’t enable cross-ventilation, or if you live in an area with night-time noise or poor air quality, this wasn’t an option. In fact, if you live in a modern house or flat, then it’s likely that opening a window will not have provided much benefit. This is because we have ignored the impending effects of climate change and designed houses to reduce energy consumption by making them more airtight and better insulated for the winter, without considering the consequences in the summer.
Guidance & regulations
In 2019, the Government released studies into the risk of overheating in our homes. In the reports it confirms that there are currently around 2,000 heat-related deaths each year in England and Wales, and this has been expected to rise to over 7,000 deaths per year by 2050. The findings of the studies have culminated in the release of a new Building Regulation – Approved Document O – specifically addressing overheating in homes and covering applications for all new dwellings, care homes and student residential buildings submitted after 15 June 2022.
The Institute of Acoustics and Association of Noise Consultants (the main industry bodies) formed a working group covering Acoustics, Ventilation and Overheating which released a national guidance document in 2020 on how to consider the balance between overheating and noise in new homes. This guidance document has informed Approved Document O, with the government recognising that overheating strategies must be usable.
The Building Regulation stipulates that where the prescribed noise or air quality conditions are exceeded, an alternative method other than opening windows is required. However, this approach is needed in order that any overheating strategy developed is usable – there is no point developing a strategy to avoid one risk that results in causing another. Given the need to consider environmental factors within the overheating strategy, this is likely to mean that locations within city or town centres, or next to roads, rail lines with 10 or more trains at night or airports with night flights, will require an alternative to opening a window. When implemented correctly, this regulation, along with others, should improve the living conditions in our residential buildings and lead to reduced heat-related deaths.
However, Approved Document O contains a significant amount of ambiguity and is open to interpretation. In recognising the need to have a clear, precise and usable method of compliance, the Acoustics, Ventilation and Overheating working group again convened to write guidance to clarify and fill the gaps.
Impact on architecture
The regulation correctly requires designers to exhaust all passive means of removing excess heat before using air conditioning. When you couple this requirement with a need to consider closed windows in a poor air quality or a high noise environment, designs need to focus on reducing heat gains as much as possible to reduce the use of mechanical systems. This inevitably leads to architectural features which are not common in the UK.
Reducing the g-value of glass (a measure of how much solar heat is allowed through) makes a difference, but this alone will not be enough in many UK locations. Reducing glazing also make a difference, but where an open window can be used to remove excess heat, the window openings need to be sufficiently sized to allow the necessary air flow. Increasing the proportion of floor area of a room compared to glazed area, or recessing windows, also reduces heat gain a little, but there are obvious limitations to how far this can be taken.
The regulation permits the use of adjacent buildings to provide shading, but it does not allow modellers to account for external foliage or internal blinds, as either could be removed and result in non-compliance.
After these measures have been exhausted, the next effective step is to consider external shading. This is the most controversial element, as (outside of London) our residential buildings do not have any deliberate shading elements and introducing them changes the appearance significantly. Balconies on apartments are a good way to shade the windows below and where they have solid upstands and provide noise reduction, but more commonly we are likely to need to consider elements such as brise soleil, awnings, external venetian blinds or shutters to provide reduced solar heat gains.
Another unfavourable feature if the objective is to avoid mechanical systems, is to use an acoustic louvre when needing to avoid an open window. However, the size of the louvres needed without a mechanical system (and sometimes the appearance) mean that this option is often not chosen. This points us towards more mechanical systems to ensure a usable overheating strategy. While on the face of it this sounds bad, when you consider that if a design is done correctly, and that these systems are operated infrequently (in the current climate), this is perhaps not as bad as it might seem. Nevertheless, the goal should be to naturally cool a building wherever possible – and therefore we will need to improve our outdoor environments in the places we build.
James Healey is director of overheating & acoustics at the Institute of Acoustics