Originally Posted by toddhunter
I'm still trying to get it. I understand how it works. The VBL traps warmth, humidity and sweat near the body, keeping the insulation outside the VBL dry. But the part that is not explained well is the venting process, which apparently is required when you feel the (inevitable) perspiration. It sounds like as long as you manually vent at the moment you feel the sweat, this will keep you comfortable. I'm trying to picture this as convenient or comfortable.
That is pretty much it, I think you have got it. But you won't necessarily have " (inevitable) perspiration", if by that you mean "sweat". The body produces that insensible perspiration to keep the skin moist, or so I have read. If that moisture disappears into the air and off of your skin, it can not only produce evaporative cooling ( same principle as an air conditioner or swamp cooler), but once it is gone your body just produces more. Then more cooling and even dehydration is contributed to.
However, inside a vapor barrier the humidity level will quickly be near 100%, and the vapor can not escape. So at that point, since the skins humidity level is at a maximum, your body stops producing this insensible perspiration. So if you maintain a just warm enough- but not necessarily cool/cold temp, you will not produce any more moisture. So you will not sweat, unless you actually overheat. It will feel a little ( or a lot) humid, but not necessarily wet.
Unless you actually go a little further and over heat a bit. Then your body tries to cool things down by producing sensible perspiration, or liquid, aka sweat. If this happens, you have used your VB at less than expert level. Then again, if you were having a hard time staying warm, and now a little sweat is your problem, it may not really be that much of a problem. And however much sweat you produce- due to overheating- it does not enter your insulation, but stays inside your VB. Not the most comfortable situation, but maybe more comfortable than shivering and better than wet insulation. I actually suspect that people using breathable insulation sweat a lot more than they realize when they are toasty warm, And their insulation is happy to soak it up so they don't even know that they have sweated. The sweat will be uncomfortable and very obvious, but contained and kept from the insulation, inside a VB. Any hint of actual sweat ( as opposed to just very high humidity or a muggy/clammy feeling) is a sign to vent. Maybe not to vent the VB, but to vent the insulation that is on top of the VB.
Just for the sake of experimentation, I put on a VB shirt about a half hour ago, at room temp. For about 15 minutes I have felt definitely too warm for comfort, and feel like I am on the verge of sweating. But I know from experience that if I sat outside at 60 or 65F, with a VB and no insulation, I would not sweat but would probably be warm enough. You should try such an experiment yourself, maybe with a bread sack used as a sock over a very thin layer of socks. But just remember the VB will make you warmer, so if you are already warm enough without the VB, you might sweat with it.
After a little over 1/2 hour, mostly sitting in my recliner, I took the VB off. I instantly felt a dramatic cooling ( but felt no such thing when I took my cotton T short off to put the VB on), but I found no signs of sweat. Not even on my back which was pressing into my recliner. But if the room had been even 2 or 3 degrees warmer, I'm sure I would have been sweating. But I might still take a little sweating over shivering cold and/or having any body moisture, maybe even outright sweat, getting into my insulation night after night. All depends on how cold it is and how long I'm going to be out.
I don't think it will ever be as comfortable as a breathable system, assuming you are warm enough in that system. But it can have dramatic warmth benefits for the weight and thickness, as well as keep your bags loft from getting a little lower every day under the right conditions. My rough guess of the temp difference between my cotton t shirt at room temp vs no t shirt and my VB shirt is 10 to 20F. From uncomfortably warm with the VB, to almost a little too cool with the VB gone and cotton T shirt back on.