MaxwellRainCoat

Goretex (invented by the Gores père et fils and Rowena Taylor) famously allows air and water vapour to pass through, but not liquid water.  It is a waterproof fabric that breathes.  Its trick is to sandwich a thin film of PTFE punctured by micropores in between two reinforcing sheets of conventional cloth.  The micropores allow air and vapour through, but are too small for a liquid to pass because of surface tension.

But it doesn't have a directionality.  The ideal garment would behave like Gortex, but only allow air and water vapour to pass out, not in.  That would keep you warm as well as dry, while still allowing a healthy flow of air.

The simplest one-way valve is a flap valve.  This uses the fluid medium that it is regulating as the source of power to prevent flow in one direction while allowing it in the other.  It ought to be possible to make micropores with additional microflaps over one end all out of the same material.  The flaps would not have to be perfect - we are after an aggregate effect (rather like Velcro, where not every loop catches a hook, but it still works fine).   A fabric made of such a material would allow gases to pass one way through it, but not the other and be exceptionally comfortable to wear.

Of course, if you can make the flaps small and light enough, right down at the atomic scale, then you have constructed a Maxwell's Demon...