Improving energy efficiency is the first and most important step toward achieving sustainability in buildings. Buildings designed with sustainable solutions are usually energy-efficient because sustainable design is aimed at producing green buildings. In the case of historic buildings, however, this is quite challenging. Integrated building energy simulation programs have been used in various ways by building professionals to respond to these challenges and to improve building design with respect to sustainability. This poster presents a new approach toward optimizing thermal indoor energy using a digital mock-up simulation of a historical building to explore its potential applicability for other sustainable building projects. The Gunzo room located in an old building situated in a historical site of the Cluny Abbey in France was the object of the experiment. Common to many old structures, the building contains rooms with thick walls and large windows that result in temperature fluctuations leading to occupant discomfort. In the present study, we assessed the temperature characteristics of two versions of the room (before and after renovation) based on three types of indicators: Monthly Temperature Profile, Passive Gains Breakdown, and Passive Adaptivity Index.