Builders take you through the key principles of undertaking a radical retrofit of a home. The film uses two real-life projects in England to illustrate the key principles for whole-house retrofits, drawing on principles from the Passivhaus low energy building standard. The film looks at why it is important to pay attention to insulation, airtightness, continuity of insulation, and ventilation when undertaking a radical retrofit. The film also explores what to consider when choosing internal wall insulation strategies for radical retrofit projects.