Research aims are to explain the impact of the built environment on the efficiency of sustainability of historical buildings. The most vital factor of sustainability in hot and semi-dry climates is the thermal efficiency of inner spaces. The conventional building materials used in historic buildings in Mosul have high thermal efficiency, but it works with the surrounding built environment as an integrated thermal system. This surrounding built environment could be exposed to dramatic changes within the efforts of reconstruction of Mosul city. Al-Aghawat mosque was selected as a case study due to its surrounding urban context and original conventional building materials. This research adopts ENVImet analysis method. Physiologically Equivalent Temperature (PET) index was assumed to be the indicator for thermal comfort in hot and semi-dry weather. ENVI-met program applied for simulation of two cases at the hottest day in the summer of 2019-2020 through climatic data. The first case estimates the air temperature in the inner and open spaces of the mosque with its original surrounding context and then checks the air temperature of the same spaces after removing the surrounding context. The results reveal changes in thermal efficiency within the two cases. In order to conserve the thermal efficiency of the historic buildings, the researchers recommended adopting a conservation strategy of the surrounding context.