This article presents the SIM-FAST software : it is a tool which allows to evaluate the energy consumption and the financial cost of French buildings in the Residential sector. Thanks to the limited number of parameters which must be entered to describe the building, it provides rapid appraisals in less than one hour. It provides global results in terms of both energy consumption and financial cost as well as time-based change of temperatures and flux. It operates under Windows 95 with 12MB RAM.