How hot is too hot? Thermal comfort in the UK’s changing climate.

The UK climate is getting hotter. Predictions suggest that the south east might be 5°C hotter on average by the end of the century (Hulme, 2002). The government is also legally committed to reducing our CO2 emissions. These projected changes will require us to drastically rethink the way we live, and carefully design and modify our buildings with these considerations in mind.

Before we look at the risks of overheating, it’s important to remember how deadly cold winters can be to those who can’t afford to heat their homes. Last winter (2014/15), fuel poverty lead to the deaths of 15,000 people in the UK. This winter the effect is likely to be less dramatic, as temperatures have been so mild that met office records have been broken, but excess winter deaths will be with us in the UK for some years to come – unless homes fuel poverty is tackled.

UK Average Temperature Anomaly over the last 100 years. (Data source: Met Office ) UK Average Temperature Anomaly over the last 100 years. (Data source: Met Office )

The main determiner of domestic energy use (and CO2 emissions) is heating. The amount a home is heated depends on many interrelated factors such as the physical characteristics of the house, the weather outside but also on the preferences of the occupants. The occupant preferences are complicated to model but the key factor at play is people’s thermal comfortPut simply, this is whether people feel comfortable in a given set of conditions. Thermal comfort itself depends on a whole host of factors like an individual’s metabolism, how much people wear or what activities they’re up to – gyms need to be cooler than offices, for example.

As a rough rule of thumb, offices should at least 20°C, but what about an upper limit? How hot is too hot? The answer, as with many things in building science, isn’t simple. Zero Carbon Hub have published a review that looks into the various measures of overheating. The industry is split as how exactly to define overheating – some say there should be maximum acceptable temperature for buildings (around 28°) others say it depends on the outside temperature because people adapt and are more accepting of warmer temperatures in the summer. So where does this leave us?

The most recent example of the deadly impact of high temperatures comes from 2003. Across Europe, the extreme temperatures claimed the lives of over 70,000 people. In the England the toll was lower, at around 2000 excess deaths. This demonstrates that the heatwave was not felt equally everywhere – France suffered particularly badly where the high temperatures killed around 15,000 people.

Average temperature anomaly July 20 – August 20 2003 (source: Reto Stockli and Robert Simmon, based upon data provided by the MODIS Land Science Team) Average temperature anomaly July 20 – August 20 2003 (source: Reto Stockli and Robert Simmon, based upon data provided by the MODIS Land Science Team)

Of course, like excess winter deaths, the deadly effects of high temperatures are not evenly distributed through society – the worst effected are usually older, sick or less well off. The UK government has responded, based on the projection that the 2003 extreme temperatures will be ‘normal’ by 2040. The NHS has published a Heatwave Plan for England which outlines what needs to be done in the event of a heatwave.

Responsibility also falls on landlords, local authorities and building owners to ensure that buildings are fit for habitation and work. The UK building stock is diverse and measures that work for one situation might not be effective in another. Careful attention is required to ensure any retrofit an owner installs, whether they be the installation of solar shading or improved ventilation, do not increase the energy consumption of the building. Air conditioning systems are often less efficient than natural ventilation.

The house I grew up in was a poorly insulated 17th century Welsh farmhouse, so the risk associated with overheating there are far less than those of being too cold in winter. But in general the UK housing stock is not currently equipped to deal with temperatures that are projected to occur in the decades to come. As ever, it tends to be that those in society least able to make changes to their living conditions are those at greatest risk of harm.

A balance needs to be struck by policy makers to ensure the overheating risks are addressed without compromising the essential work in improving housing efficiency and reducing our CO2 emissions. Given the lessons of 2003, this problem is far greater than issues of comfort, but potentially one of life and death.


M Hulme, Tyndall Centre for Climate Change Research, & UK Climate Impacts Programme. (2002). Climate change scenarios for the United Kingdom :  UKCIP02 Norwich: Tyndall Centre.

Academic references I have collected are available here


Published by

Harry Kennard

Energy, climate change and health researcher

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