London’s mayor is spending £2.5 million to fix ‘greenhouse’ eco-homes that overheat in summer. Sadiq Khan’s own flagship development, built to Passivhaus standards, now needs retrofitting because residents reported indoor temperatures above 30°C. Yet Khan still refuses to allow air conditioning, as reported by The Telegraph. This is not an isolated failure. It is a warning for every UK homeowner chasing an EPC rating without thinking about summer comfort.
Why eco-homes overheat
Passive design works brilliantly in winter. Triple glazing, airtight seals, and thick insulation trap heat inside. But in summer, those same features trap heat out. Without external shading, cross-ventilation, or mechanical cooling, indoor temperatures can exceed outdoor ones by 5–8°C. The London development used fixed shading that proved inadequate for south-facing glazing. The result: residents swelter while policy makers argue about air-con. Ofgem’s data shows that UK homes already experience overheating in 20% of heatwaves, a figure that will rise with climate change.
What this costs homeowners
Retrofitting passive cooling is not cheap. External blinds cost £200–£500 per window. Mechanical ventilation with heat recovery (MVHR) systems run £2,000–£4,000 for a typical 3-bed semi. Full active cooling, like a heat pump with cooling mode, adds £2,500–£6,000. Khan’s £2.5m bill for 30 homes works out at over £80,000 per home, far beyond what most households can afford. The Energy Saving Trust estimates that 60% of UK homes could benefit from simple shading and ventilation upgrades, but only 15% have taken action. Grants like ECO4 and the Home Upgrade Grant can cover some insulation and ventilation work, but not air-con. Homeowners should check eligibility via gov.uk before summer hits.
EPC ratings miss the heat problem
The current EPC system measures energy efficiency, not thermal comfort. A home can achieve an A rating while baking its occupants. The government’s proposed EPC reform includes an overheating risk indicator, but it is not yet mandatory. For now, homeowners must do their own assessment. Look for south- and west-facing windows, lack of external blinds, and minimal cross-ventilation. The Passivhaus Trust recommends a ‘summer comfort’ check using free online tools like the PHPP. The catch is that most UK homes were built before anyone worried about overheating. Retrofit solutions like loft insulation can worsen the problem if they reduce airflow. The lesson: do not seal your home without planning how to cool it.
What to do now
Check your home’s solar gain before June. Install external blinds or shutters on south-facing windows. Ensure trickle vents are open in summer. If you are planning a retrofit, include a ventilation strategy. For existing homes, a whole-house fan or ceiling fan costs £100–£300 and can reduce perceived temperature by 3–4°C. Khan’s mistake was assuming passive design alone would work. Yours does not have to be. Act now: assess your home’s overheating risk this weekend. If you are eligible for ECO4, apply before 31 March 2025. The next heatwave is coming.
Frequently Asked Questions
No. Current UK grants like ECO4 and the Home Upgrade Grant do not cover air conditioning. They focus on insulation, heating, and ventilation. However, some local authorities offer discretionary funds for cooling measures. Check with your council or visit gov.uk for the latest.
It depends. Air conditioning units consume electricity, which can increase your home's energy use and lower the EPC score if not paired with renewable generation. However, heat pumps with cooling mode can maintain efficiency. Always consult an accredited assessor before installing.
