Mosques differ from other types of buildings by having intermittent operation schedule. Due to five prayer times per day throughout the year, mosques are fully or partially, yet periodically, occupied. Historic mosques, possessing cultural heritage value, need to be evaluated in terms of thermal comfort. The adaptive thermal comfort method presented by ASHRAE 55 is recommended to be used to analyze thermal comfort conditions of unconditioned buildings. The aim of research is to analyze thermal comfort conditions of historic mosques throughout the year, and to conduct an adaptive comfort analysis. The selected Salepçioğlu Mosque was built in 1905 in Kemeraltı, İzmir, Turkey. The objective is to improve indoor thermal comfort levels by applicable interventions with specific attention to its heritage value. First, indoor and outdoor microclimate of Salepçioğlu Mosque was monitored from October 2014 to September 2015. The physical model of mosque was created via dynamic simulation modelling tool, DesignBuilder v4.2. The model was calibrated by comparing simulated and measured indoor air temperature within hourly error ranges defined by ASHRAE Guideline 14. Whole-year thermal comfort analysis was conducted on monthly basis by using adaptive thermal comfort model. The adaptive comfort analysis of monitoring campaign shows that the Mosque does not satisfy acceptable comfort levels. Different scenarios were applied to better comfort levels. The best improvement is obtained with underfloor heating by which discomfort hours drop into 1369 hours by 31.34%, while it was 3760 hours by 86.08% in the baseline model.