A photonic crystal fiber (PCF) long-period grating (LPG) humidity sensor has been developed with high sensitivity and selectivity for nondestructive detection of moisture ingression into structures that can potentially lead to corrosion. We have proposed two types of nanofilms to be coated on the surface of air channels in the grating region of the fiber using electrostatic self-assembly deposition processing. The primary nanofilm does not affect LPG properties such as resonance wavelength or transmission intensity which can impact sensing characteristics; however it increases the sensitivity by changing the refractive index of the surrounding material. The secondary nanofilm is used for selectively adsorbing analyte molecules of interest. The experimental results reveal that, compared to the conventional fiber LPGs and exterior nanofilm-coated PCF-LPG, the interior nanofilm-coated PCF-LPG humidity sensors have higher resonance intensity change of 0.00022%/10-3dBm at relative humidity (RH) of 38% and average wavelength shift of 0.0007%/pm in range of 22% to 29%. The proposed sensor shows excellent thermal stability as well.