The research presents the calculations for the heat losses through the envelope of the apartment building. The calculations are provided for different levels of insulation of enclosing structures (i.e. wall, windows, roof etc) according to three normative documents: Set of rules 50.13330.2012 (rus), Construction Norms and Regulations 23-02-2003 (rus) and National Building Code of Finland, Part D3 (fin). The impact of level of thermal protection of external enclosure structures on the values of heat losses, the operating costs and the costs of fuel and energy resources during the heating period (for 10, 30 and 50 years of upkeep of buildings) on the example of the one habitable apartment building is analyzed. Calculation of payback periods of the energy saving actions directed on increase of level of thermal protection of external enclosure structures of buildings is given. Economic efficiency of investments in energy saving measures is analyzed.