The work herein presented investigates the sensing properties of a graphene-based Schottky diode, operating in real environmental conditions, that is at room temperature and in presence of humidity. The role of water in the field of gas sensors for air quality monitoring has always been a crucial point: the water is always present in the environment and in most cases acts as interfering molecule, by altering or masking the effects of the other analytes. In this work, reporting device employed for NH3 detection, we show that the adsorption kinetics of water takes place on completely different time scale so that the ammonia contribution can be distinguished. The hetero-junction consists of graphene sheets obtained by Liquid Phase Exfoliation and deposited onto a Si cathode by drop casting. Cross interference effect between water vapour and NH3 was studied by exposing the diode at several Relative Humidity values ranging from 20% up to 70%. All tests were performed in air at 500sccm total flow at room temperature. A current drift due to the water is mainly visible whereas the effect of the pollutant is revealed as an abrupt decrease of the diode current.