diffusion through solid membranes

[HVACK]

I am in need of a bit of help regarding the diffusion of water vapor and oxygen through a thermoplastic membrane. In this case, the membrane is lexan with some butyl rubber components.

I am calculating the diffusion of water vapor and oxygen through these membranes using Fick's law. I know the vapor pressure and partial pressure of these constituants as well as the temperature, atmospheric pressure, and permeability constants. My question is this:

I have a slight atmospheric pressure differential across the membrane. Should pressure differential not be a consideration in addition to the vapor pressure and partial pressure of the constituants on either side of the membrane?

Thanks in advance for your help.

HVACK