In an indirect evaporative cooling (IEC) installation the extracted air is cooled by means of adiabatichumidification. By passing over an air/air heat exchanger this air cools down the supply air. A clearinteraction can be observed between the relative humidity of the extracted air and the thermal comfortrealized in the building. To be able to predict the performances of this technique in an accurate way, agood knowledge of the indoor relative humidity is thus important.This paper presents the results of measurements carried out in the summer of 2006 in a non-residentialbuilding at the Belgian coast which makes use of indirect evaporative cooling. An evaluation of theindoor summer comfort is made and the interaction between the thermal performance and the indoorhumidity is investigated. Furthermore, dynamic simulations based on the systems effectiveness wereperformed using the multizone building simulation program TRNSYS, in order to evaluate differentparameters affecting the performance of IEC.