Libraries were seen as an education and cultural center. Cultural properties in libraries can deteriorate chemically, mechanically and biologically because of inadequate indoor climate (temperature, relative humidity and whose fluctuations) and substrates. The aim of the thesis is to improve indoor climate of the historic libraries to preserve paper based collections by preventive conservation approach. As a case study, Necip Paşa Library was selected. The Library, which houses 1147 valuable manuscripts, was built in 1827 in Tire-İzmir-Turkey and has no heating-cooling system except entrance zone. Preventive conservation approach was performed in three phases. As the first phase, the indoor microclimate of the Library was monitored for one year. Secondly, the chemical, mechanical and biological degradation risk analyses are carried out on the manuscripts based on the measurements. According to the measurements, only the chemical degradation is observed. Lastly, the library was modelled via DesignBuilder v4.2.054, and calibrated with respect to ASHRAE Guideline 14. To minimize chemical degradation risk, natural and mechanical ventilation strategies are integrated into DesignBuilder. The natural ventilation scenario did not influence on chemical degradation risk, while the latter diminishes the chemical degradation risks for some extent. The building had two main interventions: the addition of a semi-open entrance space and the closure of this space by wooden-framed windows. To evaluate the effect of these interventions, two different models are developed via DesignBuilder. The simulations of the models did not exhibit any difference on the indoor climate comparing with the existing construction. Evaluation of the scenarios and interventions result with that the mechanical ventilation system is the most effective solution for the Library. Keywords and Phrases: Library, Tire Necip Paşa Library, Degradation Risks, Paper Based Collections, DesignBuilder, Preventive Conservation, Calibration