Previous studies on retrofitting have reported temperature increases but conflicting results on mould/damp and PM2.5 levels and health. 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. WS2 will capitalise on major retrofitting projects, led by our project partners for hundreds of retrofit homes to provide the empirical evidence and determine the impacts of retrofitting on IAQ and health in real-life situations.