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Indoor Overheating – From the Distant Past to a Cooler Future

Dr Argyris Oraiopoulos writes about the need to re-think how we define, assess and address indoor overheating.

Overheating problems from past centuries and the birth of the overheating criteria

16th October 1834: the ancient Palace of Westminster is destroyed by fire. The temporary replacement formed an experimental chamber where several different, ground-breaking ventilation strategies were tested, for delivery of the optimal thermal comfort conditions for debating. Design and testing was assigned to David Boswell Reid, a Scottish physician and Professor of Practical Chemistry at the University of Edinburgh. Reid’s ventilation system was designed following direct observations of thermometers, located in the chamber, and consultation with MPs during sittings. In summer 1839, Reid’s chief attendant of the ventilation system, Benjamin Riches, was ordered to keep the interior temperature at a level that would not exceed 67°F (19.4°C) – above this, the debating chamber was considered at risk of overheating, as the complaints from MPs would increase significantly1.

Fast forward about 100 years: the development of steel construction allowed a trend towards high-rise office buildings with large areas of glazing. The increased solar gains during summer months, however, increased employee discomfort and led to calls for summertime thermal criteria for buildings.

The studies that followed over the next decades, in assessing and attempting to predict this discomfort in these office spaces, interestingly included spot internal temperature measurements in unoccupied rooms… Parts of the indoor overheating criteria that resulted are still in use for offices but also for homes and hospital wards.

New Insights given by easier and more accurate measurement

Recent technological developments have given us small, low cost and relatively accurate sensors that can be deployed to capture long-term thermal conditions in homes through large-scale, nationwide studies.  At the same time, qualitative data from smaller scale surveys, combined with quantitative data on indoor temperatures, have started to shed light on what constitutes indoor overheating, and how it is perceived in homes across the UK2. These studies have revealed that the variety of temperatures sought by people in their homes in UK homes, are not captured by the past and current definitions of indoor overheating.

Recently, the criteria for overheating in living rooms moved from a static temperature threshold to an adaptive method, but failed to recognise the potential for adaptation in bedrooms during sleep.  Recent studies have confirmed that there are adaptive actions that can be taken to mitigate discomfort during sleep, similar to daytime choices (for example, reducing bed covers, opening windows, using fans), that do not cause significant disruption to sleep patterns. These actions are only beginning to be documented, and have not yet been reflected in overheating criteria or design methodologies.

Is it time to re-think indoor overheating?

Why should we rethink the definition/description of indoor overheating? A number of questions arise for policy makers, researchers and practitioners.

  • The developments detailed above suggest that there is no one-size-fits-all comfort state. There are different expectations and opportunities in homes, offices (and schools) and hospital wards that relate to comfort, productivity and health respectively. Why should there be a one-definition-fits-all when it comes to indoor overheating in all these indoor environments?
  • Summertime heatwaves will undeniably generate high levels of short-term heat stress to those indoors, but chronic, long-term overheating is already an issue in many settings. Are these two faces of the same coin? Are the same stakeholders interested in the impacts of both of these problems?
  • SInce the criteria are mainly applicable to naturally ventilated indoor spaces, why is humidity not taken into account, since it plays a significant role in thermoregulation in the human body?
  • If the definition of indoor overheating is not to be trusted and therefore the risk of overheating often misunderstood and inaccurately identified, then how can one develop and maintain successful policies for the mitigation of the overheating risk?

Are we storing up trouble for the future?

The latest research shows that in UK homes there is an overarching scepticism regarding the risk of summertime overheating. In many instances, it is seen as a one-off, short-term discomfort event that is easily and quickly dissolved3 since the UK has not been getting regular heatwaves on an annual basis.

But the latest climate projections confirm the long-standing predictions of more frequent and severe heatwaves during summertime4. Alongside this, the significant need for more housing, and the requirements of the building regulations are driving construction of small, low ceiling height, air tight flats with limited potential for effective natural ventilation.

Does this mean we are storing up trouble for the future?

These heatwaves can have adverse health impacts, particularly on vulnerable people, and cause significant costs to the NHS.  They can also create an unprecedented demand for instant cooling in homes which, if satisfied by air conditioning units, will increase energy use, with detrimental consequences for carbon emissions and energy security.

The cost of these units has fallen considerably and is now comparable to other products that were once considered luxuries (for example, large screen TVs).  It is not difficult to imagine how a series of hot spells, followed by further price reductions, could drive sales of air conditioning units in a pattern similar to that of large-screen TVs (which have tripled in less than 10 years).

Is there a solution? Suggestions for policy makers, researchers and practitioners

As comfort expectations rise and tolerance of temperatures outside very tight boundaries is diminished, occupants (in homes, offices, schools, hospitals), if not educated and prompt appropriately, will seek to restore their comfort as quickly and as effortlessly as possible. And that is highly likely to involve energy intensive mechanical means.

In the UK, we do not regulate to ensure that the risk of overheating in naturally ventilated spaces is mitigated.  The European Energy Performance of Buildings Directive does however mention the need to reduce air-conditioning demand, since this is a much bigger issue especially in the South of Europe where temperatures often climb above 35ºC during summertime.

Strict regulations for the installation of air conditioning units do exist in some parts of the EU (Switzerland), where one has to prove that the indoor space needs air conditioning as the last resort in providing cooling. These however require strong political will and robust mechanisms in monitoring and enforcing legal actions.

An alternative to air conditioning units is the much less energy intensive solution of ceiling fans and external window shutters/louvres, a combination that has been used in the Mediterranean for many decades now.  Long-running research has shown the benefits of installing ceiling fans and their potential in providing cooling. However, the more passive methods of cooling require more planning ahead and a better understanding of building physics.

In the Mediterranean, this comes from long term exposure to heat and therefore experience. In the UK it must come by other channels such as education, engagement, communication, awareness, research and policy.

  • Education and Engagement

Residents of different indoor environments could be better informed of their passive choices in restoring thermal comfort and enabled to take these choices. Local government has a key role to play here, in engaging with local residents and supporting those most vulnerable to heat.

  • Communication and Awareness

Effective and timely advice is vital if people and businesses are to prepare for heatwaves and hot spells.  Simple heatwave alerts as part of a weather forecast will not prompt enough people to take actions in time. Prevention is a better strategy, and this relates to solar gains, at the building scale and also at the area-wide level.

  • Research

More research is needed to better understand where overheating is leading to discomfort, together with further mapping of vulnerable populations and buildings that are at risk of overheating. This will require the collection of qualitative data in direct combination with quantitative data, in the same fashion as Professor David Boswell Reid did, some 180 years ago.

  • Policy

The Government needs to be aware of the potential growth in the air-conditioning market and consider how this can be regulated. At the same time, it needs to develop and implement policies to improve the energy performance of building envelopes and ensure that landlords and building owners provide passive and affordable methods of cooling. Last but not least, it will need to work with the electricity providers to allow for an economic tariff during hot weather to protect those most vulnerable to increased indoor temperatures who cannot meet their comfort needs through natural cooling.


It is almost certain that the UK will be further challenged by future extreme weather events. It is also certain that technical, political, financial and behavioural solutions exist and can be deployed in time to prevent significant increases in electricity demand and a growing summer stress for the NHS, ensuring that the nation’s emissions’ targets are not jeopardised and the health of the population is not put at any further risk.



  1. Schoenefeldt, H. The Temporary Houses of Parliament and David Boswell Reid’s architecture of experimentation. Archit. Hist. 57, 173–213 (2014).
  2. Morgan, C., Foster, J. A., Poston, A. & Sharpe, T. R. Overheating in Scotland: contributing factors in occupied homes. Build. Res. Inf. 45, 143–156 (2017).
  3. Wright, D. L., Haines, V. J. & Lomas, K. J. Overheating in UK homes: Adaptive opportunities, actions and barriers. Windsor Conference Proceedings (2018).
  4. https://defradigital.blog.gov.uk/2017/03/17/using-climate-projections-in-your-decision-making/

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