The aim of this thesis is theoretical energy renovation of residential building in Ljubljana. The building was built in 1963 without additionally installed thermal insulation on buildings thermal envelope. Since the investor decided to renovate the buildings envelope, the question arised, if the installation of thermal insulation is required, or even sensible, eventhough it is, based on current legislation. The thesis is based on the regulation on energy efficiency in buildings and consideration of the three types of thermal renovation and their variations in the program KI Energija 2014. It proves the necessity of the installation of thermal insulation and the viability of individual prejudice. The payback period of the investment is based and determined on calculation of the net present value. Simulation analyses are made for three thicknesses of thermal insulation – determined by current regulations and its 50 % and 100 % increased thickness.