Since most of energy in buildings is used for creation of healthy and comfortable indoor environment, lately an increased attention has been directed towards the optimization of operation of heating, ventilation and air conditioning (HVAC) systems. This paper presents graphic-analytical approach to investigate the impact of indoor temperature and relative humidity on energy efficiency. Thermodynamic analysis is performed using statistical data for outdoor air conditions in Latvia and recommended values of indoor air parameters for office buildings, prescribed by European standards in the field of indoor climate. The study shows that it is not economically viable to maintain optimal indoor air condition throughout the entire year, thus the allowance for deviation of temperature and humidity should be considered during certain periods, especially in summer. Results of this study could be further used for the improvement of building norms and regulations regarding the design and operation of HVAC systems.