In a recent building simulation project, the significant effort required to calculate and enter casual gains and air flow data into the ESP-r modelling software was found to impede progress and increase the risk of error. A software tool, implemented as a Microsoft Excel spreadsheet using VBA, has been developed that avoids these problems by generating the required ESP-r data files automatically. The use of the spreadsheet had a major impact on project progress. Creating the input data for a run became a matter of selecting some options and pressing a button to create the files, where it would have taken well over an hour in the past. In addition, errors were reduced and opportunities for new investigations increased.