mems based capacitive type humidity sensor

Hello
I am working on capacitive type humidity sensor in which I've to model diffusion of humidity in polymide layer that will change the permittivity of polymide hence changing the capacitance between electrodes in which polymide layer was sandwiched but I don't know which physics to use i'm new to comsol . I guess it's gonna be a complex structure so I decided to model diffusion in polymide first but again i don't know which physics to use and which material to use for humidity( should i use a material named "HumidAir" in COMSOL 4.3 ? would i be able to change it's parameters ?)

Thanks.