Historical mixed-media objects consist of a wide range of materials that can be organic and inorganic in nature. They are often fragile due to the nature of the materials used, and their advanced state of degradation. Well-considered preservation conditions are needed to slow down their degradation. Since each material has its specific requirements, it is challenging to give preventive conservation recommendations. To support the decision making, a risk-based approach is developed. The proposed methodology (1) offers an objective basis to define compromises in preservation conditions, and (2) allows the comparison of different scenarios in order to select the most appropriate one. The methodology is first explained theoretically. Subsequently, its two application aspects are illustrated on the basis of a case study regarding the preventive conservation of a medieval Enclosed Garden.