1099 Built Heritage 2013 Monitoring Conservation Management Indoor measurements of microclimate parameters in the Mithra- eum in the Baths of Caracalla (Rome, Italy) Alessia Caratelli 1 ; Anna Maria Siani 1 ; Giuseppe R. Casale 1 ; Alessandro Para- vicini 2 ; Chiara Bertolin 3 ; Dario Camuffo 3 1 Sapienza Università di Roma, Physics Department, Rome, Italy; 2 Tecno-el srl, Rome, Italy; 3 National Research Council, Institute of Atmospheric Scien- ces and Climate, Padova, Italy 1. Introduction Although in the last decade a growing number of studies on environmental conditions of museums and buildings housing artifacts have shown that a stable microclimate is beneficial for their conservation [Camuffo, 1998, 1-432; Camuffo,2001, S127-S140; Pavlogeorgatos, 2003, 1457-1462], this is not always put into practice. The continuous monitoring of the microclimate to assess the artifacts own historic microclimate and to develop a preventive control program for the best conditions of preservation, is becoming a com- mon procedure [Camuffo,1998, 1-432; Corgnati et al., 2009, 1253-1260]. The building, as envelope, can be considered a filter for the outdoor environmental conditions and sometimes this can be sufficient to ensure adequate microcli- mate and air quality [Strada, 2002, 89-92]. When the indoor environment is strongly affected by external forcing, this requires to be studied carefully. The Mithraeum was built in the early third century A.D. in an underground corridor of the North-West exedra of the Baths with an independent access from the Baths complex. The Mithraeum consists of five communicating ro- oms (Fig.1): two rooms (į and η) adjacent to the entrance (e) with two wall openings of about 1.0 x 1.5 meters (F2 and F3); room (İ) with no ceiling adja- cent to the Central Hall (h) thus precipitation and sunlight reach the Mithraeum continuously. The Central Hall still preserves a mosaic floor and on the wall on the North-West side (P1B) a fresco of Mithras. The site has been closed to the public for the last 10 years. Different decay processes occurred in the Central Hall. Green and dark patinas on the wall P1A were probably due to biological colonization. The high porosity and the mobilization of salts, together with the humidity in the inner layers of the wall, could have facilitated the colonization and growth of such organisms [Macedo et al., 2009, 3476-3490]. The fresco has suffered from exfoliation and flaking that could be caused by salt crystallization [Zehender, 2007, 353-367]. In this paper we performed the analysis of the indoor/outdoor exchange of air mixing ratio and of temperature, of the impact of natural lighting, and of fungal growth, to prevent the decay processes and to suggest solutions to improve the microclimate conditions. 2. Methodologies The following parameters were monitored from December 14, 2010 to May 3, 2011 before the beginning of the restoration:  Indoor air temperature (T2in and T3in);  Outdoor air temperature (T1out);  Indoor Relative Humidity (RH2in, RH3in);