The UK, Europe and many other parts of the world are experiencing increasingly frequent and prolonged periods of extreme heat during summer months. Many homes and buildings were simply not designed for the temperatures now being recorded. Most lack external shading, and the position of doors and windows can restrict ventilation, meaning buildings heat up quickly and struggle to cool down overnight.
In built-up areas, the problem is compounded by the 'urban heat island' effect – cities and towns run measurably warmer than the surrounding countryside because dense development, dark surfaces and sparse vegetation all trap heat.
The risks are not evenly distributed. Elderly people, young children, those with pre-existing health conditions, and people living in top-floor flats, poorly ventilated homes or those with large areas of unshaded glazing face the greatest challenge in keeping cool.
CIBSE advocates a passive-first approach to managing overheating: keep heat out of the building, then remove any heat that does get in, before turning to mechanical cooling such as air conditioning. This approach is better for occupants, better for energy bills, and better for the environment.
What residents can do
1. Keep the heat out
Blocking heat gain is the single most effective measure.
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Use external shading where available: shutters, awnings, external blinds, or overhangs are all effective.
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Temporary solutions work too: external fabric screens or sheets fixed outside windows can make a real difference (even a fabric shower curtain over an external window frame can help) or reflective films on windows.
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Close blinds and curtains, particularly on south-, east- and west-facing windows, during the hottest periods of the day. Internal blinds are better than nothing, but external shading is far more effective because it stops heat before it enters the building.
2. Let the heat out
Ventilation removes the heat that has built up inside.
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Open windows when outdoor temperatures drop below indoor temperatures – typically in the late evening, overnight, and in the early morning.
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Close windows during the hottest part of the day, when outdoor air is warmer than the inside.
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Where it is safe and secure to do so, leaving windows open overnight is one of the most effective ways to purge accumulated heat from a building (including leaving windows open in different rooms which can support cross-ventilation) and pre-cool it for the upcoming day.
3. Reduce heat generated inside
Everyday appliances and activities add heat to the indoor environment.
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Turn off electrical equipment when not in use – televisions, computers and other electronics all generate heat, even on standby. Similarly, if you have a tumble dryer or dishwasher, try and run them overnight or early morning, rather than in the middle of the day.
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Avoid using ovens, countertop stoves and other heat-producing appliances during the hottest part of the day. Cook earlier in the morning or in the evening where possible, or try and use microwaves or slow cookers instead, which omit less heat.
- Take measures that will also reduce energy use e.g. insulating your water tank.
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Lighting can also make a difference – incandescent and halogen bulbs generate heat. Switch to LED bulbs if you haven’t already done so.
4. Fans and other low-energy cooling systems
Fans do not reduce room temperature, but they do make occupants feel cooler by increasing airflow over the skin.
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Ceiling fans are generally most effective (where ceiling heights allow – typically above 2.8 metres).
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Portable fans can also help, particularly where natural ventilation is limited. Using a spray bottle with water or put ice in front of a portable fan, which can increase evaporative cooling (provided it is safe to do so).
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Other passive cooling devices such as pedestal fans, misting fans, cooling vests, neck-coolers, cooling mattress toppers and thermoelectric chairs can also provide localised cooling and comfort. This may also reduce the burden on energy demand and bills compared to other mechanical cooling technologies, such as air conditioning units.
Longer-term improvements
As extreme heat becomes more common, it is worth investing in measures that will reduce overheating risk in future.
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External shading: Permanent shutters, external blinds, awnings or brise-soleil can dramatically cut heat gain through windows. Low-E window films can also restrict heat coming inside the building.
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Window design and layout: Consider window designs that allow safe ventilation, particularly overnight: secure night vents, restricted-opening windows, high-level windows or security grilles. Also consider optimal window-to-wall ratio if fitting new windows, as well as high-efficient glazing (double or triple). Where possible, introduce openings on opposite walls to enable natural ventilation.
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Ceiling fans: A highly energy-efficient way to improve comfort where ceiling heights allow.
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Greening your surroundings: Trees, plants and green spaces provide shade and reduce the urban heat island effect. Lighter-coloured or reflective surfaces on paving and external walls also help. Reducing large areas of dark paving close to a building can make a measurable difference.
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Air conditioning as a last resort: Where passive measures and low-energy cooling measures are not sufficient (particularly for vulnerable occupants), mechanical cooling may be necessary. Where possible CIBSE recommends efficient, low-carbon technologies such as reversible heat pumps.
Two risks to overheating homes and buildings that are worth keeping in mind
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Energy efficiency upgrades: While home improvements such as insulation, draught-proofing and new windows are important for reducing heating bills and carbon emissions, if not properly designed they can make overheating worse by trapping heat inside a building that was previously able to breathe. If you are planning or have recently completed retrofit work on your home, it is worth considering whether adequate ventilation and shading have been included alongside the insulation measures.
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Urban heat island effect: Cities and towns are measurably warmer than the surrounding countryside, because dense development, dark paving, sparse vegetation and waste heat from vehicles and buildings all trap and generate heat. During a heatwave, this effect intensifies, meaning urban residents face a significantly harder challenge keeping their homes cool. Greening the built environment – trees, plants, green roofs and lighter-coloured surfaces – can meaningfully reduce this effect over time and is something CIBSE actively advocates for in urban planning and building design policies.
When to be concerned about health
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Prolonged exposure to high temperatures poses real risks to health and wellbeing, particularly for older people, young children, and those with pre-existing health conditions.
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For advice on heat-related illness and what to do if you are concerned about yourself or someone else, please refer to relevant expert guidance, such as from the UK Health Security Agency, National Health Service or the Emergency Services.
CIBSE expertise on overheating risk
The buildings being designed and built today need to remain comfortable and safe as temperatures continue to rise. The practical advice in this factsheet is underpinned by an extensive body of CIBSE knowledge, research and technical guidance that supports this aim. For those designing, managing or assessing buildings, the key references are:
CIBSE also develops specialist climate datasets – updated in 2025 using the latest Met Office projections – that allow engineers to model how a building will perform not just in today's climate, but in the warmer summers projected for the 2030s, 2050s and 2080s. This helps ensure new buildings are resilient to future heatwaves, not just current ones.
As heatwaves become more frequent, understanding how to reduce overheating risk is increasingly important for both residents and industry professional. CIBSE continues to provide trusted guidance to help create safer, more resilient homes.