The present study is devoted to efficiency evaluation of a combined indirect evaporative – compressor cooling system under various outdoor air humidity conditions of temperate climate. The investigated system is located in the recently restored historical building, The Art Museum Riga Bourse, which was initially built in the middle of the 19th century. The indirect adiabatic chiller supplies cooled fluid to the conventional cooling system, consisting of ventilation cooling coils and fan-coil units on separated loop. Using the data, acquired by BACnet BMS controllers and experimental data logging system, we have analyzed the cooling plant operation efficiency dependence of outdoor air humidity for a period of four month. The saved each minute data have been exported as CSV files, recalculated to each hour average values and analyzed.