Temperatures could soar to dangerously high levels in some homes insulated under the government's flagship Green Deal scheme, experts have warned.
Energy-saving measures designed to save on winter fuel bills and protect the environment could pose a risk to health during summer heatwaves, they add.
The government says it is aware of the problem and is taking steps to prevent overheating in Green Deal properties.
Homes in densely populated urban areas such as London are most at risk.
Heat can build up during the day and has nowhere to escape at night leading to poor air quality and a greater risk of heat stress for the occupants which, in extreme cases, can kill.