When talking about historical heritage and energy efficiency, many actions may cross your mind: you may think of renovation works, improving insulation, repairing cracking, changing supplies, lighting... Any of these actions are valid, together with regular monitoring of consumption to analyse whether the measures taken are truly effective or not. In Europe, 35% of the building stock is more than 50 years old. Even if they are of a certain age, these buildings, of whatever type, are not considered historic. But of that 35%, a small percentage are buildings with decades or even centuries of history, and which deserve particular attention. In this article, some of the main features of energy efficiency projects in historical buildings are reviewed. The success story of the Art Nouveau Site of the Hospital de la Santa Creu i Sant Pau in Barcelona, a UNESCO World Heritage Site, is shared.