More aircon is not the answer. How do we make buildings more heat resilient?

The challenge is particularly significant because so much of the building stock that will be occupied in 2050 already exists today. UK Green Building Council estimates that 80% of the buildings expected to be occupied in 2050 have already been built.

The construction industry’s clients need to understand how their current portfolios will behave under more extreme temperature scenarios, rather than on future schemes.

For air-conditioned buildings, one of the main risks is that existing systems may not have been designed for the temperatures now being experienced. In many cases, installed equipment may provide insufficient cooling to maintain comfort, particularly during longer or more intense heatwave events.

In more extreme conditions, systems can overheat and fail, removing the ability to provide cooling when it is needed most.

In an office, that failure could mean disruption, with staff sent home and business activity affected for the duration of the incident. For residential buildings, manufacturing facilities or hospitals, a loss of cooling and ventilation can become a much more serious operational risk.

For these buildings, extreme heat is not simply a comfort issue, but a resilience, safety and continuity issue.

Naturally ventilated buildings face a different but equally important set of risks as external temperatures rise and traditional cooling strategies become less effective. When the outside temperature is higher than the internal temperature, natural ventilation is no longer viable for cooling. On hotter days, windows and vents should be closed to prevent hot air from entering buildings. 

That has consequences beyond thermal comfort, particularly in densely occupied spaces where reduced ventilation can also affect internal air quality. Rising carbon dioxide levels can reduce productivity and cognitive function, compounding the wider effects of heat on people and business performance.

Building operators therefore need to consider temperature, ventilation and occupancy together, rather than treating overheating as an isolated issue.

Occupants also need to understand the impact of their actions and their role in managing internal conditions. This could include encouraging them to use blinds to reduce the impact of direct sunlight and asking people not to open windows, which could reduce the effectiveness of air conditioning.

The first step is to understand the specific overheating risk across a building, estate or wider portfolio based on evidence rather than assumptions.

Modelling at building, estate or portfolio level is already a standard approach for informing decarbonisation investment pathways, and a similar approach can be applied to extreme heat. By combining computational modelling with site surveys, it is possible to test how buildings might perform under future conditions.

Modelling allows operators to identify which assets are most at risk from business interruption and what investment is needed to mitigate that impact. For some buildings, the answer may be to maintain or replace existing air-conditioning systems before they reach a critical point.

For others, the priority may be facade upgrades, shading, passive measures or changes to occupation patterns during periods of extreme heat.

There are also operational interventions that can make better use of existing buildings without immediately moving to major capital works. Actively ventilating buildings at night, for example, can help cool the building fabric and maintain cooler spaces during the day.

Adjusting occupation times may also reduce exposure during the hottest periods, particularly where business activities can be planned with greater flexibility.

For new buildings, there is more scope to create a heat-resilient design from the outset. With advanced modelling tools, combined with CIBSE’s updated future climate data, building designers have the tools to understand heat impacts early in the design process. That early understanding is critical because the decisions made at concept stage will shape the building performance for decades.

The temptation may be to respond to higher temperatures by defaulting to an air-conditioned solution, but that should not be seen as a simple decision.

Increased energy costs and the UK government’s net-zero targets mean that building designers must carefully balance resilience, performance, carbon and whole-life cost. Where an air-conditioned solution is proposed, an investment decision will need to be made about system sizing, replacement programmes and the location of equipment to minimise overheating risks.

These challenges point to why passive design sits at the heart of the response, whether that’s with buildings being naturally ventilated or mechanically cooled. Shading, high-performing facades and increased planting around buildings can all improve resilience while limiting reliance on energy-intensive cooling.

These measures also help create buildings that are better able to withstand hotter summers without undermining wider decarbonisation goals.

Weather patterns over recent years have shown that designing for heat resilience needs to become part of how buildings are designed, built, operated and maintained in the UK.

The challenge now is understanding where the risk sits and what can be done about it. For existing buildings, that starts with assessing overheating risks through modelling and site surveys, identifying vulnerable assets and developing targeted investment plans.

For new developments, it means using future climate data and advanced modelling tools early in design to ensure buildings are resilient to the temperatures they are likely to experience throughout their lifespan.

Organisations that act now will be better positioned to protect occupants, maintain business continuity and avoid costly interventions later.

Ruth Shilston is global lead for engineering sciences at Mott MacDonald.