The topic of this final thesis is "Thermal bridges in building reconstruction". In order to achieve an energy-efficient building during the reconstruction, it is necessary to know the internal and external physical processes that take place in the building in order to be able to choose the attached material for the reconstruction. "Thermal bridge is a smaller area in the envelope of the heated part of the building through which the heat flow is increased due to a change in the material, thickness or geometry of the construction part of the building. Thermal bridges represent weak points in the construction of the building from both an energy and hygiene point of view." [1] Thermal bridges can most often occur at the joints of structural elements, such as places on the balcony panel, places where doors and windows are installed, threads, hoops, joints of external and internal walls, joints of roof and wall, joints of rafters with reinforced concrete cerclages and wooden branches. Attention should be paid to the design of the building and proper construction, the building should be as compact and simple as possible, without many bends, a large number of different balconies and the like. You should also think about the choice of materials, not only construction but also insulation, which plays a big role in the whole story with thermal bridges. We also have to be careful when choosing the thickness of the material, because not all materials are suitable for certain places on the structure, so the insulation is chosen incorrectly at the place of insulation, although the use of layer thickness can lead to the formation of thermal bridges on the building structure