On a 35C day in south London Marion Baeli's retrofitted 1960s ex-council house built to the Passivhaus standard was about 10C cooler inside than outside, illustrating how the airtight, highly insulated approach can keep homes cool as UK summers warm. Passivhaus, developed in Germany about 35 years ago, aims to hold a constant indoor temperature by eliminating thermal bridges, adding thick insulation, and using triple-glazed windows plus a mechanical ventilation system; such homes are estimated to use up to 90% less energy than typical houses. More than half of UK homes are already at risk of overheating and at current climate rates that share would rise to 92% by 2050 without adaptation. Baeli's retrofit cut her household energy demand by an estimated 88% and she says it saves nearly £750 a year, but the total retrofit cost was around £80,000, with £30,000 spent on windows and about £5,000 to make the house airtight. The Passivhaus Trust notes retrofitting to this standard is expensive, and the standard has a practical limit: if night-time outdoor temperatures do not fall, the house needs cooler air to start with and can remain hot overnight, with Baeli reporting a night at 27C that left the house at the same temperature. Baeli spread the works over about ten years to manage costs, highlighting phasing as a common way homeowners make such retrofits affordable.
