An innovative method known as Building Moisture Index (BMI) has been applied in hundreds of properties suffering of surface condensation and mould growth in order to quantify and rectify indoor atmospheric moisture imbalance. BMI system is a protocol, a method and a computer program that processes, assesses and reports environmental data in a fast, reliable and objective scientific-technical manner. An example is presented showing the causal factors diagnosed by the BMI code. The ventilation system was in this case improved, and moisture issues were automatically rectified, being this indicated by very high going down to low moisture imbalance scores, before and after installation, respectively. Following appropriate methods and protocols for environmental monitoring and assessment of moisture imbalance of indoor environments may support better retrofitting works to eliminate or avoid condensation damp and mould problems in residential buildings.