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If you look at modern Gore-Tex very very closely, you'll see a layered structure like the one below.
Outside (X% humidity, cold)
-------------------------------------------------------------
ePTFE layer (hydrophobic and porous)
-------------------------------------------------------------
polyurethane layer (hydrophyllic and wicking)
-------------------------------------------------------------
Inside (Y% humidity, warm)
Water vapor molecules that hit the hydrophyllic layer are wicked into it. They will then re-evaporate on the other side, into the pores of the ePTFE layer. When the outside fabric and air are dry (less than 100% humidity), the molecules can just stay gaseous after passing through the pores, and float away. This is why Gore-Tex works so well in dry conditions.
When X and Y reach 100% humidity, liquid water starts to condense on the outside surface, and the hydrophyllic layer on the inside becomes saturated. However the inside and outside waters do not touch each other; they are still separated by the dead air space in the ePTFE membrane. It acts as an insulator, which preserves a temperature gradient. Gore-Tex seems thin to our hands, but to a water molecule it's quite thick!
To simplify the drawing:
wet (outside)
--------------
dry air (ePTFE)
--------------
wet (polyurethane)
--------------
wet (inside)
Because of vapor pressure, both bodies of water will be evaporating into the dead air space, as well as reabsorbing water vapor from it. However because the water on the inside is warmer, it has a higher vapor pressure, and so it evaporates more quickly. Thus over time there is a net loss of water molecules from the inside to the outside--breathability! Even under 100% humidity conditions, as long as there is a thermal gradient.
Now it's not going to be nearly as fast as when the outside air is dry, because the temperature gradient is not going to be nearly as big. DWR treatments enhance the effect by keeping the outer fabric from wetting out, which increases the thickness of the dead air insulation. This makes the garment feel warmer and lighter too.
This isn't a static system though. When you first get dunked into very cold water, the entire garment will drop in temp and you might get some condensation on the inside. But over time your body will drive heat into the inside system, reestablishing the gradient and breathability, and pushing that condensation out.
As I mentioned in my other post, the clue that gas pressure is not the key is the fact that our tops don't inflate before they start breathing. Put a balloon over the whistle of a tea kettle and see what happens--that's water vapor gas pressure, and it's really powerful (it drove a whole industrial revolution). But Gore-Tex breathes without inflating.
Reference:
http://www.freepatentsonline.com/4194041.html |
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Yeah, I'm weighing in again New 
BoaterTalk 
