The purpose of this thesis is comparison of calculated and in situ measured values of heat transfer coefficient. The heat transfer coefficients are measured on different types of concrete walls considering their thickness, type of wall elements (full concrete wall or hollow concrete block) and thermal isolation. The most important objective while designing energy efficient building, energy retrofitting existing buildings and conducting energy certification is assessment of thermal characteristics of external walls. The heat transfer coefficient is characteristic which describes thermal characteristics of building elements. A method of calculation heat transfer coefficient of opaque structure is given by HRN EN ISO 6946:2008. By applying this procedure it is possible to get reliable results. In some cases it is not possible to apply recommended procedure (unknown layers of construction material, unknown thickness and density of layers), therefore it is necessary to carry out in situ measurements. In this thesis basic concepts related to building physics are described. An overview of the legislative framework for the implementation of energy efficiency in buildings, especially the parts related to heat transfer coefficient, energy audits of building and energy certification is presented in this thesis. In situ measurements described in this thesis were carried out on concrete walls. This thesis also gives historical evolvent of concrete construction in the Republic of Croatia so it is possible to analyse period of time when concrete buildings were constructed and their thermal characteristics. The last part of thesis gives an overview of the procedure and equipment used for in situ measurements. Comparison of results was made, pointing out the possible mistakes which can be made during measurements which could lead to a discrepancy between the calculated and measured values of heat transfer coefficient.