The aim of this paper is to describe our Building Simulation (BS) approach to study indoor microclimate parameters in heritage buildings. Literature already exists over this subject: in the area of museums - Thomson, Camuffo; this issue has been particularly emphasized, for example, by Bernardi; others, as De Guichen, proposed specific methodologies for museums. Nevertheless, in our opinion, today there isn’t a vision which includes the history and the objectives of the architecture. We consider indoor microclimate in heritage buildings as a result of several strategies to guarantee indoor comfort. We named it “Historic Indoor Microclimate”, and we adopt BS to simulate actual, past and future indoor microclimate. In this paper we show three case studies of Italian historic buildings: Villa La Petraia, Florence; Villa Barbaro, Maser; Palace of Venaria Reale, Turin, which can clarify the importance of the application of the BS on cultural heritage. Moreover, the paper proposes an index: “Heritage Microclimate Risk”, useful to study the indoor microclimate aggressiveness.