With the increased energy usage and its affect on the environment the European Union (EU) has issued climate and environment goals which will oversee countries energy saving potential. These goals focus on several areas including the energy usage within buildings. The purpose of this thesis is to categorize and analyze the energy saving potential for buildings constructed 1945 or earlier. The building stock in focus consists of 73 apartment buildings in Vasastaden, Linköping. The buildings were all chosen with Boverket’s database within building characteristics GRIPEN. The method consisted of stocktaking via location visits, categorizing and performing simulations on the buildings. The fictive buildings were based on the categorized building stock. They were modelled and simulated with the help of IDA Indoor Climate and Energy (IDA ICE). With the results of the energy usage a greater understanding of energy saving potential was obtained. The result of the categorization displayed that typical buildings in Vasastaden are buildings out of brick, fully detached (no adjoining walls), specifically with 3 floors. The fictive buildings have an calculated energy usage of 102-144 kWh/m2,year and have an energy saving potential within ranges of 17-41% compared to the building regulations set by Boverket (BBR). The building stock obtained an calculated energy usage of 804-3 515 MWh/m2,year. GRIPEN displayed several inaccuracies compared with the collected data from the location visits which proves that more frequent updates in databases such as GRIPEN are of high significance.