Abstract Nowadays, increasing buildings’ energy efficiency represents an opportunity to improve their thermal performance and adapting them to the global warming effects. However, finding the most suitable intervention for heritage buildings represents a challenge since the best energy retrofit solution has also to achieve the preservation of the building’s original characteristics. This work seeks to evaluate the thermal performance of a dwelling located in the Historic Centre of Asuncion. Subsequently, several retrofitting solutions for the building envelope were assessed to determine the most efficient solution for the building and climate under analysis. As results, energy-efficient solutions guaranteeing optimal conditions of habitability are proposed.