This paper presents the development of a new energy rating system for existing houses, combining the information from utility bills with on-site measurements and computer simulation. The proposed system was tested on a sample of 45 houses in Montreal, with the following main objectives: (i) to evaluate the quality of results given by the system (e.g., energy performance of houses, thermal resistance of exterior envelope, air infiltration rate), and (ii) to evaluate the methods of measurement and simulation (e.g., time required on site to collect data, time for computer simulation, accuracy of measurements and simulation).