As heatwaves sweep across the UK, keeping homes comfortable has become a major challenge for millions of residents. More than half of British homes are currently at risk of overheating during summer heatwaves, a number expected to rise sharply as temperatures break historical records. Yet in south London, one 1960s home managed to stay a remarkable 10°C cooler than the 35°C heat outside—without using any air conditioning.
The four-bedroom ex-council home belongs to architect Marion Baeli, who spent over a decade retrofitting the building to meet the Passivhaus standard. Developed in Germany around 35 years ago, Passivhaus design focuses on sealing a building against external weather to maintain a steady indoor temperature year-round. Baeli compares the system to a vacuum flask, noting that keeping the structure airtight prevents heat from invading in summer while retaining warmth during winter.
Achieving this performance required a multi-stage transformation. Baeli began by insulating shared party walls, the floor, and the roof, while eliminating "thermal bridges"—areas where heat escapes or enters quickly. She later replaced drafty brick cavity walls with super-insulated timber frames and installed triple-glazed windows to slow heat transfer. A mechanical ventilation system continually supplies fresh air inside the sealed structure.
These changes reduced the home's energy demand by 88%, saving Baeli nearly £750 annually on utility bills and requiring just a single radiator to warm all three floors in winter. However, converting an existing house to this standard is costly. The entire project cost approximately £80,000, with triple-glazed windows accounting for £30,000, while sealing the house airtight cost around £5,000. Baeli spread the costs over ten years to make the project affordable.
Daily habits also play a vital role in keeping heat out. During hot days, Baeli closes windows, draws curtains, and hangs external dust sheets over glass surfaces to block direct sunlight. On rare "tropical nights," when outdoor temperatures remain high, indoor cooling can be difficult, but active management helps minimize heat buildup.
The success of such retrofits comes as building standards shift. Incorporating Passivhaus principles into new construction is significantly cheaper than retrofitting older buildings, adding only 2% to 5% to initial construction costs. With new energy regulations such as England's Future Homes Standard taking effect soon, developers like Barratt Redrow are already experimenting with large-scale Passivhaus developments to future-proof UK housing against a warming climate.