The relevance of energy efficiency in buildings and its uptake

Globally, buildings account for 30% of final energy consumption and 28% of global energy-related emissions. Despite increasingly binding efficiency targets and new technologies, building energy demand grew by over 20% between 2000 and 2017. While space heating is currently the dominant end-use, research suggests that cooling needs are expected to increase 2–3 times in non-residential buildings and 3–20 times in residential buildings by 2050. This trend implies a decrease in global CO2 emissions for space heating by 34%–52%, alongside an increase of 58%–85% for space cooling. Identifying worst energy-performing buildings for prioritised renovation is essential, as over 80% of global economic energy efficiency potential remains untapped. Locating these structures is challenging, as 70% of floor area additions to 2050 will occur in places with limited building energy codes.

Building compactness and construction epochs are key estimators of building energy efficiency

Building compactness and construction epoch are primary proxies for energy efficiency. Compactness is measured by the shape factor, defined as the ratio of outdoor exposed building surface area to heated volume. Higher shape factor values indicate less compact structures, which generally consume more energy. The building construction epoch has long been used to associate construction practices with energy performance, with efforts dating back to the 1970s energy crises. Using building construction epoch as a predictor for energy demand using machine learning techniques shows promising potential for large-scale estimations.

Building energy efficiency and “climatisation vulnerability”

This study introduces the concept of ‘climatisation vulnerability’, defined as the strain on energy demand imposed by old building age combined with unsuitable compactness relative to climate. By exploring the relationships between building morphology, construction epoch, and climate zones, the indicator identifies climate-vulnerable buildings. This framework enables policy makers, governmental authorities, and researchers to explore options to reduce thermal discomfort risk, address energy poverty, and achieve significant energy and emission savings.

Results

The global analysis shows that 43% of buildings were constructed before 1980, when few energy efficiency measures existed. These buildings are primarily concentrated in temperate and cold-climate urban areas in high-income countries. Conversely, low-income countries have experienced more recent, lightweight, and less compact urban expansion. This exposes them to forecasted warming, which could increase energy demand and hinder climate adaptation. Current vulnerable areas are home to a population of 48 million persons, a figure that could increase by 10% to 30% by 2100 under likely socio-economic pathways.

Buildings are more compact in Eastern Asia and looser in Africa

Loose and very loose buildings are found predominantly in Sub-Saharan Africa, South Asia, Latin America, and the Middle East. More compact buildings, with a shape factor lower than 1, are found in South Korea, the Persian Gulf, Japan, and Alaska. Most building construction after 1980 took place in West Africa, South-Eastern Asia, and large parts of Australia. More recently, between 2000 and 2010, construction was dominated by China and East Africa, with the Horn of Africa seeing significant growth between 2010 and 2020.

Compactness has a distinct urban-rural gradient

The most compact buildings globally are found in urban centres, which host almost 75% of their footprint surface overall. More than 90% of built-up surface in global urban centres is compact. Increasingly looser buildings are located in suburban, peri-urban, and rural areas, where more than 85% of overall medium, loose, and very-loose surfaces are found. Urban sprawl towards rural areas favours relaxed building morphologies characterised by low-rise buildings and lower building compactness.

New buildings in urban centres became less compact throughout decades but follow different trajectories across world regions

A global analysis of over 9,000 urban centres reveals that newer buildings tend to be less compact than older ones, reflected by an increasing average shape factor. Four distinct clusters characterise these trajectories. Cluster 2, concentrated in the Global South, shows a recent sharp increase in shape factor and represents the most common building age-compactness relationship. Cluster 4, showing a linear increase in shape factor, is concentrated in Eurasia. Cluster 3, marked by decreasing shape factor, reveals more compact construction in Andean countries.

Similar building age – compactness relationships across cities indicate similar socioeconomic settings

City-level building compactness trajectories are linked to attributes including population size, population growth, and human development indicators. Cities with similar compactness trajectories over time tend to share similar socio-economic characteristics. Regional concentrations confirm that socio-economic factors influence urban development trends and building morphologies across global regions. Large urban centres are more representative of Cluster 2, while small and medium-sized centres are more typical of clusters 1, 3, and 4.

Country income marks disparity in building features linked with energy efficiency

Lower and lower-middle-income countries have experienced more recent urbanisation, with most buildings constructed after 1980. However, efficiency is not guaranteed, as only approximately 25% of lower-middle and low-income countries implement building energy codes, compared to around 50% for high-income nations. Buildings in low-income countries are less compact and more lightweight on average, making them more susceptible to daily temperature fluctuations without adequate climate control.

Inequalities are intensified in low-income areas

Climate change shifts impacts towards the equator, where cooling degree days are expected to increase by at least 75% and potentially up to five times by 2100. Global warming will relieve 40%–90% of the vulnerable population in cold regions like Europe and North America. However, the global vulnerable population could reach 48 million or increase up to 2–3 times by 2100, primarily in Sub-Saharan Africa and Asia. This disparity highlights existing social inequalities across the urban-rural gradient.

Discussion

The economic context of a country plays a significant role in shaping climatisation strategies. Low-income countries, where air conditioning is less affordable and energy use is less efficient, will experience dramatically higher heat stress. This will burden active climatisation and require greater adaptation efforts. In contrast, high-income countries may benefit from milder climates induced by global warming, though they remain susceptible to peak demand during heatwaves. Rural areas with loose buildings may benefit from opaque envelope insulation and active solar energy production.

Conclusion

Building compactness and construction epoch are effective proxy variables for assessing global energy efficiency. To mitigate high energy demand, building regulators should establish thermal mass standards and compactness limits. Adaptation measures such as reflective building materials, green space expansion, and permeable pavements should be deployed to prevent overheating. In rapidly urbanising low-income regions, setting minimum compactness standards and promoting passive ventilation designs are essential to curb the projected rise in cooling demand and ensure affordable thermal comfort.