Ordinary residential buildings dating from late nineteenth century and early twentieth century are an essential part of the European urban built environment. Today, it is important to address the comfort of residents through renovations improving the buildings’ energy performance. Nevertheless, strategies aiming for a drastic reduction of energy consumption – such as outside insulation currently applied to new buildings – are often incompatible with their original architectural character. Considering the absence of systematic protection measures, that could sometimes be too restrictive, ordinary buildings are often at risk of being completely disfigured, namely through interventions concerning the windows. This contribution examines the original features and renovation strategies for windows of ordinary residential buildings in French-speaking Switzerland. The aim is to compare the energy performance of different interventions, as well as assessing their architectural impact. We first identify four types of windows. The joinery techniques used are examined by looking at building manuals of the time and by conducting metric surveys in situ. Secondly, three types of intervention strategies are compared: adapting windows with single glazing to insulated glazing; replacing existing windows for some types of new windows; adding a second window with insulated glazing in front or behind the original window. The interventions are examined in collaboration with joiners to observe the construction processes. For each type of intervention, the architectural and thermal insulation impact are compared using the same criteria, drawings at the same scale and by calculating heat transfer coefficients. The aim is to show that an apparently simple procedure such as window renovation can induce important changes to buildings. Not only are the aspects to consider often contradictory, but the multiple intervention strategies are often little known. It is therefore essential to contribute to research concerning historic windows and to make public alternative solutions to their replacement.