The present study is devoted to the evaluation of efficiency of the combined indirect evaporative – compressor water cooling system under various outdoor air humidity conditions of temperate climate. This is a building- based study, which represents the results of the analytical research, conducted in the recently restored 19th century historical building, The Art Museum Riga Bourse. Indirect adiabatic water chiller is equipped with a compressor, and supplies cooled fluid to the conventional air conditioning system, consisting of ventilation cooling coils and fan-coil units on the separated loop. Using the data acquired by the data logging system, we have analyzed the dependence of the cooling plant operation efficiency on outdoor air humidity for the cooling system operation period of the year 2012.