Nearly two million homes in England and Wales cannot be upgraded to an Energy Performance Certificate rating of C or above, according to analysis by Rightmove.
Across England and Wales, 59% of homes have a D, E, F or G rating and there is the potential for this to be reduced to 11% of homes if improvements were made.
The property website says that 1.7 million homes with the lowest ratings do not have the potential to meet a C rating even if their owners carried out upgrades.
The government is aiming for as many homes as possible to reach a C rating by 2035 in England and Wales, with an earlier target of 2030 for private rented homes.
A further 11 million homes in England and Wales do not yet have an EPC rating, probably because they have not been sold or let since the certificates were introduced.
Rightmove looked at the areas with the largest share of poorly-rated homes and found that Gwynedd in Wales has the highest proportion of homes with an EPC rating of D or below at 77.4%, which could be reduced to 21.7% with the help of green upgrades.
Castle Point in Essex was next with 77.2% of homes rated D or below, which could be reduced to 13% after improvements.
Tower Hamlets in London has the lowest proportion of homes with a current EPC rating of D or below, at 27.4%.
The most common improvement that is recommended in EPCs is to install solar panels, although these will cost thousands of pounds.
The second most common suggestion is to switch to low-energy lighting, which is one of the cheapest improvements households can make with an average cost of £38.
Among the other most cost-effective measures Rightmove identified are insulating the hot water cylinder at an average cost of £23, draughtproofing single-glazed windows at £100, increasing loft insulation at £223 and upgrading heating controls at £400.
Rightmove director of property data Tim Bannister says: “It is encouraging to see that there are some energy efficiency improvements that can cost less than £100, so it is definitely worth checking your EPC if your home has one to see if there are small changes you could make to try and improve your rating.
“The bigger challenge is for those homes with much lower ratings that will cost a substantial amount of money to improve.
“There are a number of homeowners who don’t feel an urgent need to make changes now unless it makes a big difference to the cost of their household bills or if it is going to make their home more attractive to a potential buyer if they’re planning to sell.
“It is early days with some lenders now starting to introduce green mortgages as incentives, but homeowners need to be better informed that how green your home is will become increasingly important as we aim to move towards a net zero society, and they need more help to understand why making improvements are so important for the long term.”
