Commercial energy consumption currently accounts for 8.6% of the total national energy consumption in India and it is predicted to surge in upcoming years. To tackle this issue, building envelope insulation is being promoted through codes and standards to reduce the cooling and heating demand and hence reduce the overall energy demand. However, with prolonged exposure to humid ambient conditions in warm-humid locations, building materials undergo decay in their hygrothermal properties, which induces mould growth and increases the energy that is needed to tackle the latent cooling load. Mould growth, in turn, harms the occupant and building health. Therefore, this study attempts to evaluate the mould-growth index (MGI) in the coastal city of Mangalore, Karnataka, India using the heat and mass transfer (HAMT) model. The MGI for one autoclaved aerated concrete (AAC) wall assembly in a representative commercial building has been studied by integrating EnergyPlus through the Python plugin. The simulated results suggest that the annual mean MGI for the AAC assembly is 3.5 and that mould growth will cover about 30–70% of the surface area. Furthermore, it was concluded that surface temperature, surface humidity, and solar radiation are key parameters for mould growth on the surface of a material.