To achieve a low carbon future, existing housing stock needs to be retrofited for energy effiency. We have detailed a project for the retrofit of a 3-bed detached house to PassivHaus standards (EnerPhit – the Passive House Certificate for retrofits). Every retrofit is going to have its own strategy and complexities and this project is based on adding a ‘jacket’ of external insulated render around the existing masonry walls. The detailing of a project like this is crucial because air tightness and thermal continuity are key to achieving a low energy building.