I have no idea where you're getting this info, but every source I have says otherwise. These materials do not rely on air permeability for their breathability. Schoeller's WB-400 was designed for softshell stretch wovens and relies on it's small amount of air permeability for it's breathability. In fact, what makes this Schoeller material unique is that it has a slight bit of cfm something that no other membrane has been able to accomplish. The only thing close would be Polartec WindPro which it's cfm is around 20 or so. I stand by what I said and have not doubts. I'm certain that eVENT is completely windproof recording 0 cfm. I will have to respectfully disagree.
And I'm afraid I have to disagree with you
I have the air-permeability numbers right in front of me.
and take a look at: http://www.outdoorsmagic.com/news/article/mps/UAN/3921/V/2/SP/
I'm not saying air permeability is really big; just in the range of 0,25 to 0,4 CFM. Not much but noticable.
Other air permeability figures (offical figures from Polartec):
powershield 6 to 10
powershield O2 40
By the way, WB-400 is similar to Windstopper in that both are a laminate. The difference, again, is that WB-400 has a slight bit of air permeability where Windstopper has none.
For windstopper, go to the windstopper website where you will see that Gore claims an air permeability of no more 5L/m˛.s (or equivalent in imperial units) (watch the movie)
The reason why eVENT performs well is not because of air permeability. eVENT does well because it doesn't have a solid PU membrane blocking the moisture from escaping. It has nothing to do with how much air can pass through. The pores are simply too small. A way of demonstrating this is to take a piece of eVENT or Windstopper and try to blow through it. I've done this countless times in demos and have never felt a thing. If eVENT allowed a slight breeze to pass I should be able to feel my breath which I don't.
see the test as performed in the Outdoorsmagic link.
The fact that eVENT has no solid PU-layer allows it to use the porous structure of the PTFE layer to its full potential. The slight air permeability helps. When I put my sleeping bag in my dry bag with eVENT bottom, I can definitely feel the air being pushed through the eVENT layer, not the seams.
I would also dispute that the 1st generation Gore was more breathable than eVENT. I would say they had similar characteristics, but I doubt one was better than the other in this regard.
First generation Gore-tex used a bare PTFE layer. eVENT uses the PTFE layer but coated with a tiny layer of a fluorocarbon based acrylate or methacrylate. This makes the pores a tiny bit smaller and thus also the breathabilty. Ever seen the famous Gibson graph in which a number of membranes and coatings were compared to PTFE? In fact, minimising the thickness of the coating layer seemed to be one of the challenges in creating the eVENT membrane. Putting the coating inside was not the problem, just chemistry and using the right solvent (IPA), finding a stable chemical emulsion and a polymerisation process which didn't clog the pores was the real challenge. Of course, now they use CO2 as a solvent.
If Windstopper has the same ePTFE characteristics as eVENT, they they should perform similarly.
And I know people who have used both who say they do have similar characteristics. But you can actually really compare both if all factors except the membranes are kept constant.