A 1960s house in South London is staying 10C cooler than outside without air conditioning during extreme heat. It acts like an insulated flask maintaining a cooler temperature indoors by keeping heat outside

As extreme heat becomes a growing problem for homes, one 1960s house in south London is showing how older properties can be adapted to handle hotter summers without relying entirely on air conditioning.

According to the BBC, Marion Baeli’s four-bedroom home was around 10C cooler indoors when temperatures outside reached 35C. Baeli, an architect, has spent more than a decade upgrading the property to improve its ability to deal with both summer heat and winter cold.

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Her comparison for the house is simple: an insulated flask.

The idea is to prevent unwanted heat from entering the home in the first place, while also reducing the amount of heat lost during colder months.

What did she change in the house?

When Baeli and her family moved into the property, it had limited insulation and draughty double-glazed windows. The house became too hot during summer and struggled to retain warmth in winter.

The improvements were carried out in stages rather than all at once.

The walls, floors and roof were insulated, while areas where heat could easily move between the inside and outside were addressed. Some of the existing walls were later replaced with highly insulated timber-framed walls.

Triple-glazed windows were also installed to reduce heat transfer.

The house was then made much more airtight, with a ventilation system added to keep fresh air circulating without compromising the thermal performance of the building.

Together, these changes helped make the indoor temperature much more stable.

The house now uses far less energy

The changes were not only about staying cool.

Baeli estimates that the home's energy demand has fallen by 88%, while her annual energy savings are close to £750. During winter, she says, just one radiator is enough to heat the four-bedroom property because much less heat escapes.

The house follows the principles of the Passivhaus standard, an energy-efficient building approach developed in Germany that focuses on insulation, airtightness, high-performance windows and controlled ventilation.

However, keeping heat out still requires some simple habits.

Baeli closes the windows and curtains during the day and shades the glass to prevent sunlight from turning rooms into a greenhouse. Windows can then be opened at night when the outdoor temperature falls.

The retrofit was expensive

The transformation did not come cheaply.

Baeli spent around £80,000 on the retrofit, excluding additional architectural costs. The windows alone cost about £30,000, while making the house airtight cost around £5,000.

Because the work was too expensive to complete in one go, she spread the upgrades over about 10 years.

That highlights one of the biggest challenges with this approach. Retrofitting an existing home to a very high energy-efficiency standard can require significant upfront spending.

Building a home to Passivhaus principles from the beginning can be cheaper than trying to make an older property meet the same standard later.

What happens when nights stay hot?

There is also a limit to what insulation can do.

The house can stop heat from entering quickly, but it cannot magically cool itself. During one particularly hot night when the outdoor temperature remained at 27C, the indoor temperature also stayed at around 27C.

Baeli says that if such hot nights become more common, she may need to use air conditioning briefly to bring the temperature down. Once the home is cool, its insulation and airtight construction can help maintain that temperature.

For people who cannot afford a major retrofit, Baeli has a much simpler suggestion: shade the windows.

The BBC report notes that even partial improvements to insulation and energy efficiency can make a meaningful difference. With hotter summers becoming more common, the challenge is increasingly about adapting existing homes rather than simply installing more air conditioners.