A differential resonant sensor apparatus and method for detecting relative humidity in an ambient air. The apparatus generally includes a sensing loop, a reference loop and a mixer. A hydrophilic sensing layer can be deposited on a sensing resonant beam and a corresponding hydrophobic reference layer can be deposited on a reference resonant beam for detecting water vapor concentration in the ambient air. The hydrophobic reference layer possesses similar visco-elastic properties as the hydrophilic sensing layer with no water absorption properties. A differential reading electronic circuit may be interconnected with each resonant beam for signal processing. The absorbed humidity with respect to the sensing resonant beam changes the mechanical resonance frequency, which can be detected as a change in the electric resonance frequency of the associated electronic circuit.