After the heatwave: why do buildings take so long to cool down?

Heat waves are becoming more frequent and intense. Yet, even when the temperature drops to more bearable levels, the feeling of heat often persists for several days inside buildings. This discrepancy between the outside temperature and the perceived temperature of the occupants is explained by the way buildings absorb, store, and then release the heat accumulated during the heat wave.

Beyond the discomfort it generates, this phenomenon highlights the need to design buildings better able to withstand high temperatures. Summer comfort has now become a performance criterion on par with energy savings in winter.

Thermal inertia, an advantage… that can become a disadvantage

During a heatwave, facades, roofs, floors, and glazed surfaces absorb a significant amount of solar energy. Building materials gradually store this heat before releasing it slowly, sometimes over several days.

This thermal inertia offers numerous advantages when temperature differences between day and night allow the building to cool down naturally. However, when nights remain particularly warm, the materials no longer have time to dissipate this energy. Heat then accumulates from one day to the next, maintaining a high temperature in homes, offices, or public buildings.

Buildings heavily exposed to the sun, attic dwellings, buildings with extensive glazing, and urban areas subject to heat islands are particularly affected by this phenomenon.

Summer comfort is becoming an essential criterion in buildings

For a long time, the energy performance of buildings focused on reducing heating needs. Now, heat waves are leading professionals to place increasing importance on the behavior of buildings during the summer months.

Architects, engineering firms, project owners, and construction companies are now seeking to limit the risks of overheating from the initial design stages. Building orientation, window size, solar shading, natural ventilation, material selection, and landscaping are all parameters that directly influence occupant comfort during the summer.

This development is also part of the RE2020, which takes into account summer comfort in order to encourage the construction of buildings that are more resilient to episodes of intense heat.

Passive solutions to limit overheating

Before considering air conditioning, several solutions can effectively reduce indoor temperatures. Exterior sun protection, such as sunshades, blinds, or shutters, prevents a large portion of solar radiation from entering the building. Combined with high-performance glazing, they help limit heat gain.

Nighttime ventilation is also an effective tool when outside temperatures drop. It helps to remove some of the heat accumulated in the walls and prepares the building for the following day.

In both renovation and new construction, the choice of materials, the quality of insulation, airtightness, the presence of green spaces or reflective roofs also contribute to sustainably improving summer comfort while limiting the need for air conditioning.

This is an issue that is expected to intensify in the coming years.

With climate change, extreme heat events are expected to become more frequent and longer. For the building sector, the challenge is no longer just to construct energy-efficient buildings, but also buildings capable of remaining comfortable during periods of intense heat.

This development opens up new perspectives for manufacturers offering solutions for solar protection, ventilation, insulation, building management systems, and passive cooling. More broadly, it confirms that summer comfort is now an essential criterion for the usability of buildings and a key element in their adaptation to future climatic conditions.

Illustrative image of the article via Depositphotos.com.