This study aims to analyse the causes of roof moisture damage during renovation and explore the use of whole building heat, air, and moisture (HAM) simulation to develop moisture safety strategies for roof renovations using prefabricated additional insulation elements. The primary cause of mould growth, particularly on oriented strand boards (OSB), was caused by excess moisture that had dried out from mineral wool on the attic floor. Roof insulation became wet due to rain during the construction period. However, the implementation of effective moisture safety measures, such as carrying out construction in dry conditions beneath a temporary roof, using ventilation to mitigate local water damage, and employing ventilation with heating or mechanical drying, can effectively prevent mould growth. The innovative aspect of this study lies in demonstrating that the whole-building HAM modelling conducted using IDA-ICE software allows for precise calculations of moisture drying, thus facilitating the development of effective moisture safety strategies. Nevertheless, further development of whole-building HAM tools (two moisture movement potentials) and additional information on boundary conditions during construction are still required. The findings underscore the significance of prioritizing moisture safety during industrialized renovations involving prefabricated timber frame insulation elements. The study emphasises the importance of thorough inspection, the establishment of rules for moisture safety, and the analysis of designed solutions during renovations utilising prefabricated insulation elements. The Finnish mould index serves as a reliable indicator for evaluating moisture safety strategies.