Moisture in the building envelopes increase the energy consumption of buildings and induce mould growth, which may be amplified within the area of thermal bridges due to their different hygrothermal properties and complex structures. In this study, we aimed to (1) reveal the moisture distribution in the typical thermal bridges (i.e., wall-to-floor thermal bridge, WFTB) and its surrounding area, and (2) investigate the mould growth in the building envelope that includes both WFTB and the main part of the wall. The transient numerical simulations that lasted for five years were performed to model the moisture distribution. Simulated results indicate that the moisture distribution presents significant seasonal and spatial differences due to the WFTB. The areas where moisture accumulates have a higher risk of mould growth. The thermal insulation layer laid on the exterior surface of WFTB can reduce the overall humidity while uneven moisture distribution, which may promote mould growth and water vapour condensation.