In this study on humidifiers for polymer electrolyte membrane (PEM) fuel cell application, the experimental outcome of two air-to-air planar membrane humidifiers with three different internal flow patterns including cross, parallel and counter flows are investigated under isothermal and insulated boundary conditions. At all temperatures and flow rates, the conditions of higher performance, corresponding to highest water recovery ratio (WRR) and lowest dew point approach temperatures (DPAT), are encountered in the counter flow case, in contrary to the cross flow configuration. The insulation condition with dry inlet temperature at 30 degrees C and wet inlet temperature at 60 degrees C has a higher WRR index compared to isothermal condition at 60 degrees C but is lower than isothermal condition at 30 degrees C. The DPAT in humidifier with insulation condition is approximately equal to that obtained in isothermal condition at 60 degrees C but is much higher than what results in isothermal condition at 30 degrees C. It can be deduced that the temperature of the wet side inlet plays a key role in the humidifier performance. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.