A project has helped 330 social housing tenants reduce their energy bills and cut carbon emissions by 16,000 tonnes.
An average semi-detached property has made savings of £260 a year, according to EDF Energy.
The energy supplier worked with Westdale Services Ltd and Rykneld Homes to install insulation and double glazing into the inefficient properties in the North East Derbyshire area.
The project is also said to have helped tackle fuel poverty.
A local resident who has benefited from the scheme said: “It is brilliant and what a saving on gas bills! The heat holds in for hours and you still feel warm all day.”