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Overheating in residential

Sustainability By Simon Wyatt, Partner, Sustainability – 10 August 2018

A red tinted photograph of a thermometer dial pointing at around 30 degrees Celsius


Simon Wyatt in front of office facade in a dark suit and blue shirt

Simon Wyatt

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There is a growing awareness of the risk of overheating in new residential developments. Several new developments have reached internal air temperatures in excess of 30oC. In extreme cases, temperatures of over 40oC are experienced, causing significant occupant discomfort and worse still, a public health risk for elderly and vulnerable people. In 2003, a European heat wave caused public health crises in several countries resulting in upwards of 70,000 deaths. With the impact of climate change starting to be felt around the world, these conditions are likely to be more common, as experienced in the summer of 2018. It is of the utmost importance that we are able to design residential developments which are not only able to cope with current weather conditions but also future projected climate change scenarios.

There are three main causes of overheating in modern residential developments:

  • Insufficient naturally ventilated openable areas in habitable rooms
  • Too much glazing – glazing areas should not exceed more than 50% of any façade
  • Community / district heating pipework distribution through poorly ventilated circulation spaces

Habitable rooms

There is a general misconception regarding the ventilation requirements for residential developments. The Building Regulations* require ventilation to be provided for two purposes:

  • Continuous background ventilation to meet the fresh air requirements of the occupants
  • Intermittent purge ventilation for rapidly removing pollutants (paint fumes, burnt toast etc)

* There is no requirement under the Building Regulations with regards to overheating, therefore it is vital industry standards such as CIBSE should be used for these purposes.

In modern residential developments, air tightness levels mean that uncontrolled ventilation rates have almost been eliminated entirely. Mechanical ventilation units with heat recovery are being used to meet the minimum fresh air requirements, and in some instances will meet the purge requirement if the supply rates are considerably enhanced. However, it is a misconception that the mechanical ventilation system will meet the overheating requirements. Openable windows or louvres in the façade are still typically required in these situations and where these are not provided, elevated temperatures in excess of 30oC can be experienced, unless comfort cooling is provided.

For purge ventilation, the Building Regulations require either enhanced mechanical ventilation rates which provide at least four air changes per hour (ACH) in the space, or an openable window with an area equal to 5% of the floor area of the room it serves (assuming an openable angle of more than 30o). In a number of instances, this will be sufficient to avoid overheating, but not in all. Detailed dynamic thermal modelling of all new developments should be carried out against the CIBSE TM59 Design methodology for the assessment of overheating risk in homes criteria to determine the equivalent openable areas required in each space. Special consideration should be given to night time conditions, as elevated temperatures can compromise sleep quality above an operative temperature of 26oC.

Typically, these openable areas are provided via openable windows. However, due to land constraints, we are seeing more and more residential developments being built on sites which are not wholly suitable for openable windows because of poor external air quality and/or high levels of ambient noise pollution. In these instances, a holistic approach is required to achieve a balance between internal air quality as well as acoustic and thermal comfort. Cundall has inhouse air quality, acoustic and building physics specialists who are not only able to give expert advice on their individual fields but can produce a combined assessment of the site conditions and recommendations. Upfront advice is most valuable at early stages of a project avoiding complex and expensive design solutions being required at later stages.

Each development is assessed against the local site conditions with bespoke solutions for each individual façades or spaces.

Building Regulations strategy Overheating strategy
Fresh air Purge ventilation
Building one
West façade – GF to 4F MVHR with NOx Filters Openable windows Comfort cooling
West façade – 5F to 12F MVHR Acoustic louvres Acoustic louvres
All other façades MVHR Openable windows Openable windows
Building two Trickle vents Openable windows Openable windows+ ceiling fans*

Example; ventilation requirements for new residential development

* Ceiling fans can help mitigate the effects of overheating. CIBSE suggest that they equate to a 2oC reduction in the operative temperature. To future proof a development, prudent design would allow for future installation of these.

It is vital that these requirements are fully understood by the design and construction teams at every stage of the project, or else the project runs the risk of overheating in use or having unacceptable levels of noise and internal air quality when windows are open to provide relief from overheating.

Pipework overheating

Community and district heating schemes are becoming the norm on most mixed-use and multiple unit residential schemes for a range of reasons. This has resulted in domestic hot water pipework being distributed through ceilings in communal circulation spaces, and where they have not been sufficiently ventilated, elevated temperatures have been observed.

Adequate ventilation should be provided in these spaces to remove the heat gains from the distribution pipework, ideally in the form of openable windows or louvres. Where this is not possible (i.e. completely internal circulation spaces) then mechanical system should be considered. In certain instances, the use of the smoke ventilation system could be considered for environmental control but will need to be agreed with Building Control.

Cundall also recommends that the pipework is insulated to beyond British Standard thickness.


New and existing residential developments are already struggling to provide comfortable environments for their occupants.

Climate scientists agree that heatwaves like the ones experienced in 2003 and 2018 are likely to become more extreme and more frequent in the UK by 2040 because of climate change. Higher temperatures put some people at increased risk of dying from cardiac, kidney and respiratory diseases.

We must act now to make sure all new developments are able to provide comfortable environments now and in the future, in the face of rising global temperatures.

To do this, there needs to be a greater level of collaboration between acoustic, air quality, mechanical designer and building physic engineers. Cundall’s in house specialists can provide bespoke, highly coordinated solutions, tailored to the site location and conditions.

For further information on how to avoid overheating please contact Simon.