Building Performance Simulation (BPS) has become an important tool in the upgrading of the existing commercial building stock. It allows a user to predict energy savings from upgrades, and thereby optimise the energy, financial or other benefits of an energy efficiency upgrade. The current paper presents the findings from a qualitative investigation into the use of BPS in the Australian commercial building retrofitting industry. The interviews had a particular focus on challenges for lower quality (known as secondary grade or mid-tier) office buildings. Twelve semi-structured interviews were undertaken with stakeholders in the building energy retrofitting industry, including consultants, government administrators and building managers. Thematic analysis of the interview data revealed numerous challenges for BPS users, primarily related to the lack of reliable, accessible data regarding building operations and energy use in Australia. The poor data availability encouraged the use of simulation to inform decisions but required the modeller to use assumptions and heuristics to develop the model. This, in turn, necessitated a reliance on the expert knowledge of the simulation user, and made the interrogation of a building model by a third party problematic. (C) 2018 Elsevier B.V. All rights reserved.