European Union countries would be required to renovate their least energy efficient buildings by the end of the decade to cut emissions and save fuel, under rules proposed by the European Commission on Wednesday. The contribution buildings make to climate change is under scrutiny, as EU policymakers map out how each sector will contribute to the bloc"s target to cut net greenhouse gas emissions 55% by 2030 from 1990 levels. Buildings account for roughly 40% of EU energy use, and most are heated by fossil fuels. The EU executive proposed on Wednesday that by 2030, all buildings in the EU with the worst energy rating - a "G" energy performance certificate - must be renovated to a higher grade. G-grade homes must be renovated by 2030 and then F-grade homes by 2033.Non-residential buildings must be renovated faster - G-grade ones by 2027 and F-grade by 2030. The proposal says a G grade should include the worst-performing 15% of a country"s buildings - meaning millions would need renovating using methods such as insulation or efficient heating systems. Louise Sunderland, senior adviser at the non-profit Regulatory Assistance Project, said setting minimum grades was essential to kickstart renovations, but it did not incentivise deeper renovations to lift buildings into the best performing A, B or C grades and deliver significant energy savings. "This is a massively missed opportunity," she said. The long-term aim is for all EU buildings to have zero emissions by 2050, when the EU has committed to eliminate its net emissions. The Commission said member states should not offer public support to fossil fuel boilers from 2027, and urged countries to spend EU funds on job-creating construction and renovations that prioritise vulnerable households. read more Advocates see renovation as a chance to win public support for green policies, for example if subsidised renovation programmes improve their quality of life with warmer homes and a fall in heating bills that have climbed in recent months because of record gas prices. But nervousness that cheaper bills may not always be the result make buildings regulation politically sensitive. Some countries have opposed a separate EU proposal to impose CO2 costs on heating fuels, fearing its social fallout. The buildings proposal must be approved by a majority of EU countries and the European Parliament. Reporting by Kate Abnett; editing by Barbara Lewis and Our Standards: The Thomson Reuters Trust Principles.