Under the pressure of energy shortage, energy saving has become one of the most important topics. The world isseeking different ways to follow the sustainable development concept and to solve the energy shortage crisis.This thesis is based on the idea of improving energy efficiency in the building industry which is one of thebiggest energy consumption industries. The aim of this paper is to simulate a renovation of an existing oldbuilding in Sweden according to the concept of building a Swedish Passive House and to see how much energycould be saved after the renovation. The target building Taljstenen 7A-C was built in 1960 in Uppsala and itbelongs to the housing company Uppsalahem. The target building is facing extensive renovation due to its age.An energy consumption model of the present building was built by the software VIP-Energy after measurementsand calculations. Based on the model, three important improvements are made in a simulative renovation process.The three improvements are insulating building envelope, installing a new FTX ventilation system with highefficient heat recovery system and installing solar collectors for hot top water and space heating. The resultsshow a significant reduction of energy consumption of the renovated building compared to the original onewhich is from 516MWh per year decreased to 371MWh. Although the renovated building did not completelyfulfil the Passive House Standard in Sweden, it still has improved to be a low energy building. The purpose ofsaving energy can be achieved.