The thesis presents the results of energy performance certificates before and after the renovation of one- and multi-dwelling residential buildings, based on two selected analyses. The first analysis examines the impact of insulation thickness changes (or its addition where missing), while the second analysis is focused on the impact of building orientation changes on the annual heat consumption necessary for heating the building QNH. The first analysis assumes a state, where TI of maximum thickness of 20 cm is added only to the exterior walls of building envelope relative to the outside. Based on the results of the change of the annual heat consumption required for heating the building QNH, it is shown that the implemented structural measures in the present case have three times greater impact on the results of one-dwelling residential buildings than on the results of multi-dwelling residential buildings. On average, savings on heating costs are in the present case five times greater in one-dwelling residential buildings. In the second analysis, the buildings’ orientation is being changed by 90°, 180° and 270° in relation to the existing situation. Based on the changes of the QNH value, it is concluded whether the building is optimally oriented or not. It is shown that most of the buildings are still not optimally oriented, despite knowledge of the effects of solar radiation gain. On average, savings on heating costs in the present case related to the changes in building orientation are twice as great in multi-dwelling residential buildings than in one-dwelling residential buildings.