What filaments allow air to pass but not water vapor? In some situations it useful to have a material in which air passes but water, and water vapor, does not. Printing cell membranes or cases for something that must be immune to humidity changes (like something that holds a highly oxidative metal) are two examples. 

Teflon (PTFE) is a material that has this property. But PTFE is horrendous to 3D print.

Silicone is another material that has this property. 3D printing silicone can be done with a syringe mechanism. But there are also thermoplastic rubbers (e.g. TPU, TPE) that are designed to mimic Silicone.

Are there any filaments on the market which naturally form holes with the porous diameter for this feature? Is there any way to manipulate the diameter of this hole, even starting from pellets? Are some filaments more likely than others to have this feature?
It is difficult enough to make 3D-printed objects water-tight (unless you paint them). Making them permeable to air but not water vapour is too big an ask, I think.