We present a series of ‘lessons learned’ in hygrothermal modelling from real-world investigations of moisture in buildings. These are illustrated through two case studies which used various tools including on-site monitoring and hygrothermal simulation. Our focus is to demonstrate the benefit of hygrothermal modelling, which can deliver greater understanding of the movement of moisture through the building fabric than traditional approaches. One important outcome is that condensation through occupation, often considered the main culprit of moisture-related damage, is not always the cause. Construction defects and a significant increase in the moisture sensitivity of modern designs (and materials) are often the factors that determine the failure and decay of contemporary building systems.