The airtightness of a building envelope impacts upon the magnitude of uncontrolled air leakage and associated ventilation energy losses. A building's airtightness can be assessed using a steady state fan pressurisation technique. This paper describes a study on the largest building in the UK ever to have had its airtightness tested. Power law regression analysis revealed a good correlation between flow rate into the building and observed pressure differentials. Building internal - external pressure differentials were measured during the testing and compared with predicted values from a CFD model. The CFD analysis showed that using resistance areas derived from Effective Leakage Area calculations gave reasonable agreement between the predicted and measured differential pressures. However, further work on boundary conditions is required to improve the agreement.