Deadly Overheating Isn’t Coming: It’s Already Here

Overheating in UK homes is no longer a projected risk. It is a current condition. Eighty per cent of homes in London already overheat, and the 2022 heatwave, during which UK temperatures exceeded 40 degrees for the first time, produced more than 3,200 excess summer deaths. The majority of heat exposure occurs indoors, in homes, care settings and hospitals, not outside. 

HEARTH, the UK’s national research hub on net zero, health and extreme heat, was established to address this gap. It is a five-year, £7.4 million programme funded by UK Research and Innovation and the National Institute for Health Research, led by Oxford Brookes University. The programme brings together six universities, including University College London, the London School of Hygiene and Tropical Medicine, and the Universities of Leeds, Edinburgh and Exeter, alongside four non-academic public sector partners: the Greater London Authority, Oxfordshire County Council, Forest Research and Sussex NHS Foundation Trust. 

Four settings, one systemic problem 

HEARTH focuses on four settings where high exposure meets high vulnerability. In homes, design constraints including single-sided ventilation and restricted window opening compound the problem. In care homes, similar physical restrictions are worsened by the fact that older residents often do not recognise heat as a health risk. During the 2022 heatwave, hospital service delivery was affected in settings where indoor temperatures rose unchecked. In prisons, residents have almost no environmental control, and research links sustained heat to increased interpersonal violence. 

The programme’s approach is deliberately transdisciplinary. It connects climate projections, indoor and outdoor environments, human behaviour and health outcomes across all four settings, rather than treating any of them in isolation. 

Sustained heat, not just acute spikes 

A central finding across HEARTH’s early literature reviews is that sustained heat, prolonged periods of elevated indoor temperatures over days or weeks, poses greater cumulative risk than short acute heat spikes. Buildings that cannot lose heat overnight accumulate temperatures progressively. Care homes, for example, have been observed to overheat before, during and after heatwaves, with no mechanism to dissipate accumulated heat. 

This dynamic places particular physiological stress on older adults, whose thermoregulation is impaired. Older people show reduced sweating capacity and impaired thirst perception, increasing the risk of dehydration, heat stroke and kidney injury. HEARTH’s research also highlights that older residents in care settings frequently fail to identify heat as a risk to their health, often remaining in warm clothing and maintaining existing heating routines during hot weather. 

Infants and preschool children, those with chronic illness, and residents in social housing with limited cooling options face equivalent or compounded risk. For these groups, housing conditions, not only physiology, determine exposure. 

The passive cooling priority 

HEARTH’s early evidence is consistent on one point: passive cooling strategies, principally ventilation and external shading, should take precedence over mechanical or energy-intensive solutions. Internal blinds are a widely used but inadequate response. They reduce light but do not intercept solar radiation that has already entered the building fabric. 

External shading is significantly more effective, but UK housing design creates a structural obstacle. Windows in the UK typically open outwards, making the retrofit addition of shutters impractical. For new-build housing, this is a design choice that can be addressed through specification changes, including inward-opening or tilt-and-turn windows, deeper window reveals, fixed louvres, overhangs or balconies. The Part O building regulations provide a regulatory mechanism through which some of these constraints may begin to be resolved. 

The risk HEARTH identifies is that the current national retrofit programme addresses insulation without considering ventilation, or installs external wall insulation without integrating external shading. These approaches risk creating homes that retain heat more effectively without any complementary means of losing it. 

Policy ambition and research design 

HEARTH’s policy objectives span building regulations, the National Adaptation Programme, NHS estate planning and local authority decision-making. The programme is developing heat health indicators and decision support tools intended to support evidence-based policy, including health economics assessments for interventions such as external shading in hospitals. 

From a Healthy Homes Hub perspective, this research reflects a wider principle that indoor environmental quality cannot be considered through a single lens. Thermal comfort during hot weather is as much a health determinant as thermal comfort during cold weather, and the design, retrofit and operational decisions that shape one will affect the other. 

The programme’s core message for the sector is clear. Overheating must be integrated into retrofit and new-build programmes now, not treated as a future consideration or a separate workstream from net zero. The two are inseparable, and the policy frameworks to align them already exist. 

Practical steps for housing providers 

• Audit your current retrofit programme to identify where overheating management is absent, and establish whether ventilation and shading are being considered alongside insulation measures. 

• Treat every planned retrofit as an opportunity to integrate climate resilience, including ventilation improvements and external shading, rather than addressing net zero and heat risk as separate workstreams. 

• Review window specifications in new-build schemes to assess whether current designs accommodate external shading now or in future. 

• Move beyond temperature monitoring alone when assessing overheating risk; consider duration of elevated temperatures, nighttime conditions and the vulnerability profile of residents in each property type. 

• Prioritise passive cooling measures, including ventilation strategies and external shading, before considering mechanical or energy-intensive cooling options. 

• Identify residents most at risk from sustained heat, including older people, those with chronic illness, residents in top-floor flats and those with limited ability to adapt their environment, and develop targeted support plans. 

• Sign up for HEARTH’s quarterly newsletter at hearth.ac.uk