How do membrane-based boots breathe?
Yesterday I was shopping for breathable waterproof boots* and this is what the salesperson explained to me. The shoe material does NOT matter. The damp air from the foot goes through the membrane (Gore-Tex) to an intermediate "pocket" between the membrane and the outside shell of the shoe. Once the shoe is removed, the dampness begins flowing back through the membrane and into the air, thus drying the shoe.
Is this correct? Do shoes not breathe through the outer layer at all?
* - I call 'boots' any shoes, that are high enough to support the ankle, but mainly leather ones
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The salesperson explained that leather keeps much warmer, but has no effect on breathability or water proof-ness.
Well he's not really correct there, so breathability is based on the concept that moisture will pass from a high saturation of moisture (next to your skin) to a low saturation of moisture (the outside). The temperature is also important, the high saturation area needs ot be a higher temperature to the outer (that's what the DWR helps with in a goretex jacket)
The Goretex material facilitates these process with pores that that allow water (in the correct state) out but not in. So if the water is trapped inside to the goretex layer (i.e by the outer fabric) this will obviously lower the ability of the moisture to pass though the goretex layer and therefore make the boot less breathable.
Leather is not as breathable as fabric. Air/moisture, etc. does not pass though leather as well as fabric. It is breathable but not as breathable.
If there was a
"pocket" between the membrane and the outside shell,
which I don't think it does, this is going to become saturated fast, and if it's saturated your boot won't breath. This is what a vapour barrier is but that works totally different to a breathable shoe.
So I would suggest that breathability is going to be better in fabric boots than leather ones. That said, he is correct that leather is warmer, so it's a a balancing act, warmth vs breathability. How much this is effected is debatable but it will have an effect.
He is correct that the outer has little input into the water proofness of a gore text boot, that's what the goretex layer does. Obviously there are differences between goretex boots.
TL;DR: he was making it up...
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Goretex does two things well.
It's a very light completely windproof layer.
Properly cared for it's waterproof.
There are many places it's used where it's completely inappropriate and just adds cost. In my experience boots are one of those places.
Goretex only "breathes" when there is a significant difference in the moisture content on one side of the barrier verse the other. That's why it breathes best in cold dry conditions. If it's 100% humidity (i.e. raining ) it's not going to breathe much if at all, if the DWR fails and the jacket wets out, it doesn't breathe at all.
Goretex in a boot doesn't have much going for it. It's generally put in as a "sock" layer behind the inner lining of the boot. This means the layer behind it is not exposed to the cold dry air, but whatever the outside of the boot is. At best the moisture vapor will just go through the barrier and condense on the inside of the boot. All the Goretex is doing is slowing down the drying time of the boot and acting as a vapor barrier to keep your feet wet longer.
In terms of keeping outside moisture out, by the time it gets to the Goretex layer it's too late. The boot is wet and as it drys out, the moisture vapor can go IN just as well as out.
You can get all of the advantages of Goretex in a boot when it works well by getting Goretex socks. These are vapor barrier socks made with Goretex, you can use them when it makes sense and not use them the 90% of the time when Goretex in a boot makes no sense. Here's one example.
http://www.rei.com/product/875635/showers-pass-crosspoint-waterproof-bike-socks-mens
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