Dan, do you know why goretex would make their paclite membranes out of PU instead of PTFE? Cost? Weight?
As Roger described, GoreTex arrived at a useful product with their PU/PTFE/PU sandwich (or maybe it’s just PU/PTFE/Nylon). However, this product is hardly better than regular PU in terms of breathability (it’s only advantage) and if offers a lot of disadvantages because it’s more complex, harder to make and expensive.
If you have a sandwich that is 5 microns PU + 20 microns PTFE + 5 microns PU then it’s 30 microns thick and 2/3 PTFE and 1/3 PU. Since the breathability always declines with thickness but about twice as fast for PU, this 20 microns of PTFE hurts breathability about as much as 10 microns of PU. So this 30 micron sandwich is roughly equal in breathability to a 20 micron layer of just PU. Don’t put much weight in those specific numbers, but just realize that a competitor can offer that same thickness in a PU product that is simpler and cheaper but marginally less breathable, or offer a bit thinner PU competitor that breathes just as well, but not quite as durable – the latter of which most people don’t even realize because most outdoor gear is hardly used.
So the point here is that much of the advantage of PTFE was lost when Gore realized they needed to coat it with PU, and the vast majority of competitors have realized that it’s simply not worth the hassle/complexity/cost to even bother including PTFE in a sandwich when you could just use PU. That’s why so many other membranes (e.g. North Face HyVent, Patagonia H2NO etc) are all PU. The only differentiating factors are how thick, how well protected and how well it’s all bonded together.
Gore also realized that there is very little upside to this complexity, so for a long time now (decades?) they’ve only used the sandwich approach for their highest end membranes. The rest are just PU like everyone else. So “Gore Tex Pro” is the sandwich and almost everything else (e.g. Gore Tex Active) is PU. My perception is that Gore doesn’t want to gamble with low durability as much as many of their competitors, so their PU membranes are generally a bit thicker and thus less breathable, which is is the right approach IMO, even though high breathability specs do sell jackets. With that said, GT does claim their PU membranes (Gore Tex Active) are quite a bit more breathable than their sandwiches (Gore Tex Pro) per the website, so their PU membranes are much thinner.
As a corollary to this, when you see really high breathability claims for a PU membrane (e.g. Pertex Shield+) that should raise a red flag because the manufacturer is either (1) lying or (2) using a super thin membrane. The only way to get high breathability with PU is to make it super thin. So I’d much rather by a PU membrane claiming 10,000 HH than 30,000 HH because the latter is likely not to last very long and then I’m much worse off with rain soaking through than I am with a less breathable coat. Unfortunately the market is focused on the unattainable goal of breathability, so manufacturers are catering to this.