The ‘library,’ as a building type, is the visiting space of researchers and storage space of wisdom of past and present thinkers. Historic libraries host unique pieces of cultural heritage such as books, manuscripts, documents, maps, artifacts, paintings, sculptures, and frescos from the previous generations. The thesis aims to reduce energy consumption and improve occupants’ comfort while paying attention to degradation risks of paper-based collections in the historic İzmir National Library Building, Türkiye. Thus, it examines building retrofit actions with different impact criteria and conducts the optimization tool to define the most optimal ones. First, three degradation risk assessment procedures were determined for the paperbased collections of the İzmir National Library Building. Within the scope of the thesis, the library building was monitored between 01.10.2016 - 01.10.2017. According to the results of the monitoring process, mechanical, chemical and biological degradation risk analyzes were carried out. The digital simulation model of İzmir National Library was prepared in DesignBuilder v. 7.0.0.102 software and calibrated monthly for a year via hourly air temperature data. Three retrofit optimization scenarios, i.e. neutral, low and high risk impact criteria, were defined by grouping different design variables such as glazing type, exterior door material, pitched roof floor construction, exterior wall construction, heating set point and cooling set point of HVAC system. The objective of optimization was reducing energy consumption and discomfort hours at the same time. The simulation results showed that all three retrofit actions reduced energy consumption and discomfort hours. Scenario 3 was the most successful among other scenarios, providing energy savings by 26.6% and reducing discomfort hours by 80.3%, according to the base case. Besides, there was no significant change in the degradation risk analyzes according to the base case.