Once considered a Mediterranean concern, extreme summer heat has become a Europe-wide reality. Even in traditionally cold climates such as Finland, prolonged heat now keeps people awake at night, makes working in uncooled buildings painful and may cause hundreds of premature deaths..

Not just comfort: a matter of productivity

High summer indoor temperatures are not merely a question of comfort — they measurably reduce the performance of occupants. In summer clothes and when the activity level is low, such as reading or watching TV, 26°C is still a comfortable temperature. Studies have also shown that productivity falls by around 2% for every degree above 26°C, so in overheated rooms work efficiency can easily drop by more than 10% compared with optimal conditions, when temperature is over 32°C that typically exist during heatwaves. The cognitive effects are well documented

• reduced attention and concentration (particularly important in schools);
• mental fatigue and reduced working memory;
• more errors in tasks requiring sustained attention;
• impaired decision-making and a reduced ability to solve complex problems.

A serious health risk

The health impact is more serious still. Prolonged heatwaves cause excess mortality: in Finland, a heatwave lasting several weeks can cause up to 400 premature deaths, a figure that, set against the roughly 80 fire deaths recorded annually, cannot be dismissed as small. Most heat-related deaths occur because extreme heat aggravates pre-existing conditions such as cardiovascular and respiratory diseases, kidney failure, diabetes and neurological disorders.

An elderly person with heart failure, for example, may suffer a fatal heart attack during a heatwave. The death certificate will likely record “myocardial infarction” rather than “heat”, even though the extreme temperature was the triggering factor. The risk is concentrated among older adults and people, even young people, with chronic illnesses, groups who also spend most of their time indoors, whether at home or in a care facility. People in general spend around 90% of their time indoors, so the quality of those conditions is very significant.

A holistic approach: passive and active measures together

Addressing this requires a holistic approach that combines passive measures – the building itself and e.g. free cooling with night ventilation – with active measures – mechanical cooling. The two are complementary, not alternatives, and the right combination differs between new and existing buildings and climate zones.

New buildings: update design methods to a changing climate

The most effective moment to manage a building’s summer performance is at the design stage, which is why building regulations require summer temperature assessments – in Finland, new residential buildings must be designed so that 26°C is exceeded for no more than 150 degree-hours. Other countries have similar requirements. Yet a building can look fully compliant on the drawing board and still leave residents sweating, because design typically relies on 30-year average summer temperatures created for energy calculation rather than the temperatures of an actual hot summer, let alone the summers yet to come.

Design methods must therefore be updated to reflect climate-change weather conditions: systems should be dimensioned against hot-summer temperatures, not only the required average summer. Passive design choices – light-weight and high-performance building envelope, building orientation, a cool environment with sufficient trees and shadow, glazing solar-transmittance values, and internal and external solar shading – play a key role, allowing indoor temperatures to be controlled energy-efficiently and significantly reducing the risk of overheating. In new buildings, air conditioning should either be installed from the outset or, at the very least, the space and infrastructure for its future installation should be provided.

Existing buildings: simple, low-cost measures and adapted behaviour first

In existing buildings, the means of controlling indoor temperature are more limited, but simple and low-cost passive measures, combined with adapted behaviour, are a realistic first step. The priority is to limit solar heat gains: lower the shutters before leaving home in the morning, use external shading such as awnings, and ventilate naturally whenever the outdoor air is cooler than the indoor air. If shading devices are not mounted, it is advised to place them in retrofit. Internal gains should be reduced by switching off bathroom underfloor heating and avoiding ovens and other heat-generating appliances during the hottest hours of the day.

Mechanical ventilation should be run more efficiently, using cool night outdoor air to pre-cool the building mass. Eventually, if a centralized ventilation system exists, an air humidifier can be added to be used when the hot outdoor air is enough dry for evaporative cooling without increasing indoor air humidity. Retrofitting the envelope – improving wall & roof insulation, replacing inefficient windows and adding outside solar shading – is among the most cost-effective measures, cutting energy consumption for both heating and cooling.

When passive strategies are no longer enough

Passive strategies alone, however, are insufficient when high temperatures persist for several days or weeks. The building’s thermal mass gradually stores heat, night ventilation is no longer enough, and indoor temperatures climb to uncomfortable – and unhealthy – levels. Under these conditions, mechanical cooling becomes essential. When outdoor temperatures exceed 30°C it is generally impossible to maintain acceptable indoor conditions without it, and cooling becomes necessary once daily temperatures are high – for instance, with a minimum of 25°C and a maximum of 35°C. Health rules reflect this: e.g. in Finland indoor temperatures in homes should never exceed 32°C, and 30°C for the elderly in care homes.

The role of humidity

Finally, humidity must also be considered. Perceived heat depends as much on humidity as on temperature, and discomfort becomes particularly severe when occupants sweat despite performing no physical activity. Increasingly common warm nights bring both difficult sleeping and persistently high indoor relative humidity throughout the day. In coastal regions, humidity can matter almost as much as air temperature, and there the only effective solution is a mechanical system able both to lower the indoor temperature and to dehumidify the air.

Cooling as a basic necessity – and a right

For all these reasons, residential cooling should be treated as a basic necessity rather than a luxury. Solar shading and air conditioning should be extended to buildings that currently lack it - particularly schools and nursing homes, but also dwellings – and should be regarded as a right for vulnerable populations such as the elderly people and the people with underlying health conditions.

Key messages

“Air conditioning should be extended to buildings that currently lack cooling systems, particularly schools and nursing homes to limit health issues and productivity lost.”

“Air conditioning systems are essential in dwellings, particularly for vulnerable populations such as the elderly or individuals with underlying health conditions.”

“Passive techniques, such as high-performance building envelope improvements or nocturnal natural ventilation, can significantly improve indoor environment quality and reduce energy consumption if mechanical cooling is employed because necessary.”

“Air conditioning systems and smart solar shading should be considered as a right, and homeowners’ & buildings’ associations should facilitate the installation of the equipment by allowing the use of common areas in block buildings also for installing photovoltaic panels to drive AC systems with solar energy.”

“Protecting vulnerable occupants must be a priority for policymakers, building owners, and our industry. As summers become increasingly extreme, cooling should be recognized as a fundamental requirement for safe buildings, just as heating is essential during winter.”

“Adapting to climate change begins with taking decisive action today—accelerating the evolution of the rules, standards, and practices that govern how we design, build, and operate our built environments. This is critical to creating safer, more resilient spaces for the future. If not now, when?”