In Europe, older buildings are responsible for 40% of the total primary energy consumption. This gives great potential for reducing energy and CO2 emissions. However, during the last decade, the retrofit processes applied to existing buildings have resulted in unintended consequences such as overheating, mould growth and the increase of indoor pollutants. In practice, retrofit is dominated by energy and economical approaches that concentrate on specific standards of U-Values and increasing airtightness. Thermal comfort, although a requirement, is not taken into account during the retrofit design process. This paper discusses how those unintended consequences can be minimised if thermal comfort is properly considered and assessed prior to retrofit interventions.