I still don't understand why they would use both the inner coating and the PU-coating unless they're afraid of durability issues (the argument they use against eVENT)
You are probably right in saying this. I've heard that as well.
I know one of the benefits of having a PU component is in the lamination and taping processes. Because PU adheres so well to the adhesive, Gore most likely thinks that the sacrifice in breathability is made up for in its durability. We discussed this at length at our Westcomb sales meeting.
The explanation I got from the material engineers is that instead of relying on the PU layer for lamination (which makes building a Goretex jacket much easier), Westcomb actually forces the glue through the pores of the membrane (again 80% of the ePTFE membrane is air) at a much higher pressure than is needed for Goretex allowing for the tape to adhere to the face material. This requires the person applying the tape to go much slower and be much more precise. This might also mean why there are fewer companies willing to use eVent. By taping the jackets this way, although it takes much more time to produce, they have found that the seams are just as strong and the tape to be as permanently bonded and wear-resistant as Gore taped seams. Westcomb has also not seen any delamination issues throughout their extensive lab and field testing.
One thing Westcomb has tried to do is narrow their seam tape, but they can only go so far. The narrowest they have been able to use without compromising durability is 11mm. Arcteryx uses 9mm because of the Goretex's PU membrane. This does give credence to the fact that the PU membrane does allow for a better surface adhesion.Here's
an interesting article which explains the new process eVent is using in its application of the hydrophobic, oleophobic polymer. This will actually increase the breathability and performance of the membrane. Innovation is good. <img src="/forums/images/graemlins/smirk.gif" alt="" />