The Department and Ofgem believe that an estimated 98% of external wall insulation and around a third (29%) of internal wall insulation fitted under ECO4 and GBIS up to mid-January 2025 have major issues that need fixing. Between 32,000 and 35,000 homes are likely to be affected. This is based on audits of a statistically representative sample of 758 homes that had external and internal wall insulation installed under ECO4 and GBIS before mid-January 2025. The site visits, which were commissioned by Ofgem, took place between June and August 2025.14