The renovation of heritage buildings presents a dual challenge: improving energy performance while preserving architectural integrity. Traditional wooden windows, often key aesthetic and historical features are also common sources of heat loss due to poor airtightness. This study investigates the airtightness of several window typologies through in-situ measurements and evaluates the impact of different renovation strategies using a multicriteria decision-making approach. The airtightness measurements are normalized and analysed across various renovation scenarios, ranging from minor interventions to full replacements. A case study of a late 19th century institutional building in Brussels illustrates the practical implications of these strategies, assessed through environmental, economic, comfort, and architectural criteria. Results show that targeted interventions, such as the addition of sealing joints, can achieve substantial improvements in airtightness and thermal comfort at relatively low cost and minimal impact on heritage value. The study proposes a replicable, component-scale methodology that supports more informed, balanced decision-making in the sustainable renovation of heritage buildings.