Indirect evaporative cooling (IEC) is an interesting passive cooling technique in which water is sprayed over the return air in the secondary channel of a heat exchanger. By combined heat and moisture transport this air is cools down while at the same time fresh outdoor air is cooled indirectly by passing over the other path of the heat exchanger. To assess the performance of this technique a 1D stationary model of a wet surface heat exchanger was developed in order to investigate the physical processes occurring in both airstreams and to calculate the outlet conditions. Validation against experiments in an installation in operation shows that modelling results agree well with measured data. The system’s effectiveness proves to be independent of the inlet conditions of both airstreams. A correlation between the model effectiveness and the air flow rate is proposed which can serve as an easy assessment of the thermal performance of the technique. By means of sensitivity analysis the parameters influencing the outlet conditions of the heat exchanger are investigated. The model is used as a new type in the dynamic simulation program Trnsys in order to assess energy savings and comfort in buildings cooled by indirect evaporative cooling, alternatively combined with other techniques.