Previous studies on retrofitting report temperature increases but show conflicting results regarding mould, damp, PM2.5 levels, and associated health outcomes. In addition, a major limitation of most prior studies is that they do not adequately control for confounders such as socio‑economic status and behaviour. This theme capitalises on major retrofitting projects led by our project partners, involving hundreds of retrofit homes, to provide empirical evidence and determine the impacts of retrofitting on indoor environments and health in real‑world conditions.