How Passivhaus retrofits keep UK homes cool without air conditioning

On the hottest day of summer, Marion Baeli's 1960s south London council house sits ten degrees cooler than the world outside. The thermometer reads 35 Celsius. Her home has no air conditioning. Walking through her rooms feels like stepping into a different season—a small pocket of relief in a city struggling to breathe.

Baeli is an architect, and she designed this retrofit herself. She describes the house as a well-insulated flask: sealed tight, cap twisted firmly on, nothing wasted. The comparison is apt. Over half of all UK homes are already at risk of overheating in summer. Without serious intervention, that figure will climb to 92 percent by 2050. The math is simple and brutal. The country is getting hotter. Most of its housing stock is not prepared.

Baeli's solution follows the Passivhaus standard, an international building specification developed in Germany roughly 35 years ago. The principle is straightforward: design a home to maintain a constant internal temperature year-round by sealing it against external conditions. Homes built or retrofitted to this standard consume up to 90 percent less energy than conventional houses. When Baeli's family moved in a decade ago, the place was drafty and uncomfortable—cold in winter, hot in summer, with poor double-glazing and minimal insulation. The retrofit took ten years and unfolded in stages.

The first phase tackled the most disruptive work: insulating shared walls, the floor, and the roof. The team eliminated thermal bridges—those weak points where heat transfers rapidly, like gaps between rafters where insulation breaks. Two years later came phase two: cavity brick walls were replaced with thinner, super-insulated timber frames, and new triple-glazed windows were installed to slow heat transfer. With the house now airtight, a ventilation system was added to keep the air fresh without opening windows. The result is a home where temperature barely fluctuates. In winter, a single radiator heats all four bedrooms across three floors. Baeli's energy demand has dropped 88 percent. She saves nearly £750 a year on bills.

The cost was substantial: around £80,000 total, with windows alone accounting for £30,000. The airtightness work—one of the cheapest measures—cost £5,000. Baeli spread the expense across a decade because, as she acknowledges, most people cannot fund everything at once. The Passivhaus Trust, where she serves as a non-executive director, does not shy away from calling retrofitting expensive. Building to the standard from scratch is cheaper: new construction adds 2 to 5 percent to costs depending on the building type. But retrofitting an existing home is a different proposition entirely.

The system has limits. Baeli's house prevents heat from entering, but it needs cooler air to begin with. She closes windows and curtains during the day, hangs dust sheets over glass to block the sun's glasshouse effect. But on a June night when temperatures did not fall below 27 Celsius—a tropical night that broke the usual cooling cycle—her house remained at 27 Celsius, the highest temperature she has ever recorded inside. If such nights become routine, she may need to run air conditioning briefly to cool the space, then let Passivhaus measures hold that temperature steady.

The policy landscape is shifting. England's Future Homes Standard arrives in March, tightening building and energy performance requirements for new construction. Scotland is consulting on regulations designed to match Passivhaus standards, with guidelines expected later next year. Barratt Redrow, one of the country's largest housebuilders, is constructing 900 apartments in west London to the standard, complete with heat pumps, betting that the extra cost is worth staying ahead of rising regulations. Currently, about 1 percent of new UK homes are built to Passivhaus specifications. The Trust estimates this could reach 10 percent within a decade.

But here is the constraint: 80 percent of the housing stock that will exist in 2050 already stands today. New construction alone cannot solve the problem. Millions of homes must be retrofitted. A previous government support scheme for low-income households closed after reports of poor workmanship, and no replacement date has been announced. Jay Morton, incoming president of the Royal Institute of British Architects, notes that cost remains a barrier for many, but even partial measures matter. Implementing half of these improvements can significantly reduce energy bills. For those without the resources or will for a full retrofit, Baeli offers simple advice: shade your windows. It does not need to be elaborate. Whatever blocking of sunlight you can manage will make a measurable difference when summer arrives.