Yeah, i can attest to this. Especially in temps below 10deg F, always find a thin layer of frost on the top shell of my down bags, and sometimes some sections are frozen stiff if I don't use a VBL. Synthetics seem to show it more on the outside from my experience, which really makes me wonder how much is trapped inside the down.

I wouldn't want a VBL top quilt for anything above consistent less-than-20ish temps, but I definitely want a cuben UQ, which I would use under pretty much any circumstances depending on fill.

Quote Originally Posted by bigwig View Post
Wow. This is the first semi-rude post I've seen on these forums; usually they are so pleasant. Anyway, not all the moisture is from sweat, but the average person sweats about 1-1.5 liters of water a day. A liter weighs 1 kilogram. If, as most people, you sweat more at night than in the day (at rest) and you've spent about 8 hours in your bag, almost a pound isn't unreasonable. Even more moisture is lost through breathing and the water saturation around your face can add quite a lot to it. I'm a big guy and I do sweat a lot.

The only way that your bag weighs the same in the morning as when you go to sleep, if you're not using a VBL, is if your body heat is keeping the water molecules heated enough that they are pushed all the way through the insulation on your bag before it freezes (some evaporation can occur directly from ice, but not much). If that is happening, as you say, to 100% of the water that you've sweated, either the air is incredibly dry around you, or your sleeping bag is doing an absolutely horrible job of insulating. Either way, you would have a ton of evaporative heat loss; you'd be cold. So, either you have only weighed your bag when it was warm and dry that night, you need a new scale, or you haven't actually weighed. You may be the only person that doesn't have to worry about airing out your bag in the morning or when you get home.

Really, this shows that you don't know much about how VBLs work. A higher humidity next to your skin slows down sweating; it doesn't speed it up. Plus, with wpb gear the rate that the moisture comes off the wpb material would be slowed, resulting in less sweat in your bag. Wpb gear isn't a true VBL but, in my exerience, it still helps. I do wear true vbl socks (your feet are one of the major sweat producers in your body).

Anyway, you most certainly don't have to use it, and since the humidity around your skin is more humid with it some people find it uncomfortable. There's just no doubt, though, that it will keep you warmer, especially in very cold temperatures. I don't feel strongly about it one way or the other, as opposed to your "I strongly disagree." I was just trying to point out that there are most certainly benefits to it.