Retrofitting buildings will be an integral part of meeting government targets around reducing energy usage in coming years. If carried out using the correct methods and materials, with good design and installation, retrofit can be successful in improving the energy efficiency of buildings. It is important to recognise, however, that buildings can also suffer from defects if retrofit of insulation is not successful. Buildings that are of traditional construction are particularly vulnerable to defects associates with retrofit as they have particular requirements around moisture and condensation as well as ventilation. This paper considers retrofit of traditionally constructed buildings with a focus on some of the defects that may be found as a result of unsuccessful retrofit. As the volume of such work increases, so too does the likelihood that those assessing buildings will find defects related to or exacerbated by retrofit. This is not to say that traditional buildings cannot or should not be improved, as research has shown they can and should, but rather seeks to highlight a potential future cause of building defects.