Introduction

Europe is the fastest-warming continent on Earth. As climate change accelerates, extreme heat events are becoming more frequent, intense and prolonged across Europe, challenging long-standing building design paradigms focused primarily on minimising heat loss in winter. Today’s buildings are often ill-equipped to prevent overheating, leading to serious health risks. According to the World Health Organisation (WHO), 200 000 people have died in Europe as a consequence of heat in the last four years and heat-related mortality could grow by 50% by 2050. Consequently, air conditioning usage is rising rapidly, including in regions where it was historically unnecessary. This raises multiple concerns about the urban heat island effect, energy-related greenhouse gas emissions, grid stability, Europe’s energy independence and – not least – summer energy poverty for households.

Drawing on scientific literature and selected European case studies, this article highlights the potential of passive cooling strategies to mitigate overheating and reduce or even eliminate the need for air conditioning in some parts of the continent. Measures such as external solar shading and ventilative cooling, including night-time ventilation, can maintain acceptable indoor temperatures under current and future climate conditions while lowering cooling demand significantly. When integrated early in building design or renovation projects, these approaches offer a resilient and low-carbon pathway to summer comfort.

Benefits and risks of large-scale air conditioning usage

Space cooling is increasingly recognised as an important adaptation measure to protect human health during extreme heat events. For vulnerable populations in particular, access to cooling can prevent heat-related illness and mortality. The recent heatwaves in Europe have made that clear. In hospitals, nursing homes, and other buildings that house vulnerable people or heat-producing activities, active cooling is already nearly unavoidable. Air conditioning technologies are becoming more energy efficient, and the ongoing decarbonisation of electricity systems means that the climate impact of cooling is gradually decreasing in many regions. However, while air conditioning can provide an essential short-term response to overheating, large-scale reliance on mechanical cooling also presents several long-term challenges.

Locally, air conditioning contributes to the urban heat island effect. Air conditioners cool indoor spaces by extracting heat and releasing it outdoors, effectively transferring heat from buildings into the surrounding urban environment. This can significantly increase outdoor temperatures during heatwaves. For example, if all Parisian buildings were air-conditioned to maintain indoor temperatures below 23°C during a heatwave comparable to that of 2003, outdoor temperatures could rise by as much as 2.4°C. 

Beyond its impact on urban temperatures, widespread air conditioning deployment would represent an important surge in energy demand. According to the IEA, cooling already accounts for 10% of global electricity consumption, and global cooling demand could more than triple by 2050 under current trends. Cooling-related greenhouse gas emissions could nearly double over the same period, reaching 7.2 billion tonnes of CO₂-equivalent annually by 2050 (equivalent to 12% of current global emissions). In the EU, energy use for residential cooling has already tripled between 2010 and 2019, though the baseline remains low compared to other parts of the world.

Growing dependence on active cooling also increases pressure on electricity networks, particularly during heatwaves when cooling demand peaks. The 2025 European heatwave illustrated how extreme temperatures can place significant stress on power systems, leading to a doubling of electricity prices and more frequent power outages. Grid failure during extreme heat events can pose serious health risks if vulnerable populations live in buildings that are dependent on air conditioning to maintain habitable indoor conditions.

While active cooling might be unavoidable in some European buildings as climate change accelerates, these challenges highlight the importance of limiting its usage, by adopting a passive-first approach to building design, renovation and operation.

A passive-first approach

The sustainable cooling hierarchy, introduced in UNEP’s Global Cooling Watch 2025, provides a framework for meeting growing cooling needs while minimising energy use and emissions. It advocates for a passive-first approach, recognising that the cleanest and cheapest cooling is the cooling demand avoided in the first place. The hierarchy consists of four steps in prioritised order:

(1) passive cooling, including climate-responsive urban and building design, shading, natural ventilation, insulation and nature-based solutions

(2) low-energy cooling, such as fans, evaporative cooling and hybrid systems that reduce reliance on air conditioning

(3) best energy efficiency, ensuring that any active cooling equipment is designed, installed and operated for maximum performance

(4) rapid phase-down of high-GWP refrigerants, particularly HFCs.

_{Table 1. The Sustainable Cooling Hierarchy. Source: UNEP Global Cooling Watch 2025.}

Different variations around the same idea were introduced by other organisations and researchers; examples include Konsam et al.’s cooling pyramid, the Ladder of Cooling proposed by ES-SO and the cooling hierarchy adopted in the 2016 London Plan. The same philosophy will form the basis of a new upcoming European Technical Specification (CEN/TS) focusing on the design of ventilative cooling systems.

All these frameworks make it clear that good city planning and building design are key to preventing unnecessary use of active cooling. At building level, factors such as the thermal performance of the building envelope, its thermal mass, roofing materials and window placement play an important role in reducing overheating risk and cooling demand. In existing buildings, however, these elements can be costly to improve on, if possible. For this reason, this paper focuses on the ‘low-hanging fruit’ of overheating mitigation: passive measures that can realistically be integrated into light renovation and building operation strategies to improve thermal comfort and reduce the cooling needs of the existing building mass.

External shading as the first line of defence

A large body of evidence demonstrates the effectiveness of passive cooling strategies in mitigating overheating while reducing energy demand. 

External shading consistently ranks among the most effective measures for reducing overheating. By blocking out solar radiation before it enters the space, external shading outperforms internal blinds and often delivers greater benefits than more costly alternatives such as increased thermal mass, external wall insulation or changes in glazing design (full review here). Studies primarily from the UK report reductions in heat-related mortality of 30–73% and decreases in overheating hours of 39-50% when external shutters are installed. In hot climates, horizontal shading is one of the most efficient strategies, yielding a 3.6°C decrease in indoor temperature on average across studies, against 1°C for vertical shading. On average, external shading can achieve a 10-15% reduction in cooling load.

Up to 50% cooling energy savings with ventilative cooling

One of the most effective passive cooling strategies is ventilative cooling, corresponding to the use of outdoor air to remove excess heat from buildings. Night ventilation is a particularly powerful strategy because it uses cooler night-time air to flush heat from both indoor spaces and the building structure, especially when combined with exposed thermal mass. Ventilative cooling works best when openings are available on different sides of the building and at different heights, enabling higher airflow through cross-ventilation (relying on wind-driven pressure differences) and the use of the stack effect (buoyancy-driven airflow). According to the studies compiled in the UNEP Global Cooling Watch 2025 report, wind-driven ventilation and night ventilation can reduce indoor temperatures by 6.4°C and 5.1°C on average, respectively. IEA EBC Annex 62 case studies report indoor temperature reductions of 4–8°C through night-time ventilation.

Ventilative cooling can also substantially reduce cooling demand. Across hot-climate studies reviewed by Hu et al., cross-ventilation reduced cooling loads by more than 25%; the UNEP report mentions 11-20% average energy savings. However, well-designed natural ventilation strategies have shown the potential to reduce cooling demand by 50%; the Annex 62 reports cooling energy savings of 30–50% in office buildings, while an EU Commission case study found a 52% reduction in cooling demand with cross- and stack-ventilation.

The greatest benefits are achieved when shading and natural ventilation are combined, particularly when using night-time ventilation. For example, in 2080 climate scenarios for London, this combination could reduce night-time overheating by up to 65%, compared with 25% for shading alone. Reducing temperatures at night presents important benefits for sleep quality with consequences for health (both mental and physical) and productivity.

_{Figure 1. Passive cooling strategies include external shading and ventilative cooling. Source: The VELUX Group.}

Automation can improve the reliability of passive cooling

Automation can help passive cooling measures deliver their full potential by removing the need for occupants to constantly manage windows and shutters. This is particularly relevant for solar shading: manually operated shading is often less effective because the periods with highest solar gains frequently occur when people are away from home. Unless occupants close their shading devices before leaving home on warm days, overheating can remain close to levels seen in homes without shading. In contrast, automated shading controls can reduce thermal discomfort by more than 50% by responding proactively to indoor and outdoor conditions. Window automation can theoretically achieve large indoor temperature reductions too. Real-world evidence from a renovated Danish home shows that automated control of roof windows and shading can lower peak indoor temperatures by around 3.5°C. However, acceptability of automation is a concern, especially in residential scenarios. It is therefore important that automated controls are transparent, as simple as possible and – most importantly – that users have the possibility to adjust them and override them. By empowering occupants to better control their windows and shading devices, automation holds the potential to make passive cooling measures more reliable.

Active cooling as a last resort rather than the default response

In parts of Northern and Central Europe, well-designed combinations of passive strategies can maintain acceptable indoor temperatures for much of the summer and may eliminate the need for active cooling in many buildings – at least in the short- to medium-term. The UNEP Global Cooling Watch states that ‘strategic combinations of multiple passive interventions can achieve total temperature reductions of 6–9°C, often removing the need for mechanical cooling in many tropical and temperate buildings.’

Both shading and ventilative cooling reach their limitations in extreme climate conditions. The performance of ventilative cooling depends strongly on outdoor conditions and is most effective when night-time temperatures fall sufficiently below indoor temperatures; its effectiveness declines during prolonged heatwaves and ‘tropical nights’, when cooler outdoor air is no longer available to remove accumulated heat. Similarly, exterior shading is unlikely to provide sufficient protection during prolonged, extreme heatwaves. In warmer locations, in poorly performing buildings and among vulnerable populations, active cooling will remain necessary as Europe gets warmer, particularly during extreme heat events. In these buildings, cooling demand should first be minimised through passive measures. Efficient active solutions, for example high-performance heat pumps, district cooling networks or hybrid systems combining fans with air conditioning, can then address the remaining cooling needs.

Increasing human and building resilience during severe heatwaves

Such a prioritisation reduces peak electricity demand and limits pressure on energy infrastructure during heatwaves. Buildings that are dependent on active cooling (typical examples include highly glazed buildings without shading or operable windows) are likely to become dangerously inhospitable during a power outage. It is critical that buildings retain a degree of passive survivability: the ability to maintain safe and habitable indoor conditions when power supply is constrained or cooling systems fail.

For healthy subjects, research also suggests that regular exposure to moderately warm environments can strengthen physiological adaptation to heat and improve thermal resilience. Rather than shielding occupants from all temperature variation, buildings can support this adaptive capacity by allowing a certain degree of thermal fluctuation. In this sense, future resilience to climate change requires both resilient buildings and resilient humans, and passive cooling strategies play a key role in both.

A holistic approach to building renovation is needed

Energy renovation is essential not only to reduce heating energy consumption and associated CO2 emissions, but also to ensure that homes remain healthy and comfortable in summer. Households living in energy-inefficient buildings are often exposed to a double burden: cold, damp and unhealthy indoor conditions in winter, and increasingly severe overheating during summer heatwaves. Renovation strategies should therefore address both challenges by integrating passive overheating mitigation measures alongside energy-efficiency improvements. The Living Attic demonstration project in France illustrates this potential: a full-house renovation brought the energy performance rating from F to A. Moreover, thanks to the use of dynamic shading and automated ventilative cooling utilising the stack effect, indoor temperatures remained 8°C below outdoor temperatures in heatwave conditions.

Conclusion

Future-proofing Europe’s existing building stock requires a decisive shift towards a passive-first approach to cooling. The evidence reviewed in this article shows that measures such as external shading and ventilative cooling can substantially reduce overheating and cooling demand while improving resilience during heatwaves. These strategies should become a standard component of renovation projects, ensuring that the necessary energy renovation of the building stock does not worsen summer liveability. UNEP also suggests that solar protection and optimisation of natural ventilation potential should become mandatory in building codes. Where additional cooling is needed, hybrid approaches can combine passive and highly efficient active systems, reducing energy demand and cooling system size. Such an approach can help safeguard occupants’ health during heatwaves while reducing dependence on active cooling and its impacts on the grid, climate and urban environments.