From April 2018 properties with an EPC rating of F and G will be required to make the necessary changes to their rental properties to bring them to a minimum of an E rating. Currently there are around 465,000 rental properties in the UK with F and G ratings. However, a new report by the Building Research Establishment suggests about 100,000 properties may have been under rated due to a miscalculation on the value of uninsulated cavity walls. The report shows that these walls are more energy efficient that has been calculated and insulated cavity walls are less energy efficient. The report suggests some 100,000 properties may have been under rated and any on a G rating would move to an F requiring less work whilst any on an F rating would move to an E rating requiring no additional work.