On the basis of a deeply concerning report by the Comptroller and Auditor General, we took evidence from the Department for Energy Security and Net Zero (the Department) and Ofgem to find out why an estimated 98% of external wall insulation and around a third (29%) of internal wall insulation fitted under the Energy Company Obligation 4 (ECO4) and the Great British Insulation Scheme (GBIS) up to mid-January 2025 have major issues that need fixing.2 We also took evidence from TrustMark and the United Kingdom Accreditation Service (UKAS).