Housing sector in Latvia similarly like in other EU countries is one of the biggest energy consumers. On average energy consumption for space heating in Latvia apartment buildings is very high 193 kWh/(m2a). Energy efficiency performance of building can be significantly improved during building renovation. Housing stock in Latvia needs to be renovated and one of the most important questions during renovation is cost optimality of energy efficiency measures. The work conducted during this study deals with question on cost optimal energy performance level in residential buildings. During this study optimization model has been developed and tested. Comparing measurement data with calculated developed model has been validated. Case study of typical soviet time apartment building is analyzed and presented in this article. The building analyzed represents typical soviet type apartment building. The optimization work conducted during this study allows us define cost optimal energy performance level in case of apartment buildings renovation. As we can see from results obtained the optimizations curve is very flat. As the global costs depends from different factors like discount rate, energy inflation rate and other factors it is more appropriate to show optimal energy performance range no single one wining renovation alternative. And best renovations alternative should be found during design stage. From the results we can conclude that optimal energy performance range for space heating is from 30 to 75 kWh/(m2a).