Nearly two thirds of UK homes fail to meet long-term energy efficiency targets, according to data analysed by the BBC.
More than 12 million homes fall below the C grade on Energy Performance Certificates (EPCs) graded from A-G.
It means householders spend more on energy bills and pump tonnes more CO2 into the atmosphere than necessary.
The government has said it needs to go "much further and faster" to improve the energy performance of homes.
