Indirect evaporative cooling is a sustainable and energy efficient method to supply cooling energy to buildings. However, the main constraint that limits the use of evaporative coolers is the ultimate temperature of the process, which is the wet bulb temperature of ambient air. In this paper, a model for the heat and mass transfer process in an indirect evaporative cooler is described. The model is validated against experimental measurements. A method is presented to produce air at a sub-wet bulb temperature by a single stage regenerative indirect evaporative cooling. The main concept lies in manipulation of the air flow by branching the working air from the product air, which is indirectly pre-cooled, before it is finally cooled and delivered. It is concluded that by using the proposed method, indirect evaporative cooling is capable of cooling air to temperatures lower than the ambient wet bulb temperature. The ultimate temperature for such a process would be the dew point temperature of the ambient air.