The dissertation present four multi-apartment buildings, which were built in 1959, 1984, 1988 and 2014. For each building I calculated the heat balance in its initial condition and looked into effect of period on the energy efficiency of buildings. I also descriped legislation and policies. Later I carried out analysis of the suggested measures, which contribute to reducing the heat for heating and cooling. In order to reduce the required heat for heating, i have implemented a combination of measures that help buildings to meet the requirements of PURES 2010. The work was mainly based on the use of the TOST program. The program is used to calculate the energy balance of the buildings according to the Rules od efficient energy use in buildings, taking into account SIST EN ISO 13790 an TSG-1:004:2010. The quality of the thermal protection of the buildings envelope describes the thermal transientity of the structural assemblies, which I checked using the program TEDI. The essential goal of dissertation was the calculation of the heat balance of buildings in their initial condition and a comparison of the results obtained before and after the hypothetical renovation of the buildings. It was expected that it will be the oldest building (1959) the energy most expensive. In comparison to a new building (2014) consumes six times more warmth for heating calculated per unit of useful surface. The rest of the building consumes two to three times more warmt compared to a new building. Minimum thermic isolation, which the building already have, greatly reduces the need for the necessary heat, which makes the building much more energy efficient.