Condensation control is tough. Keeping rain and puddles out is much easier.
Originally Posted by
Carbonfiberboy
In the second case, I've not heard of a high altitude climbing expedition that used anything but down bags. These expeditions last for weeks and the bags do fine. My experience is that water vapor does not condense in the down. I've been using down bags for 50 years and never had a wet bag.
Yes, weight is very important with climbing trips, so it's no surprise that they'll use down insulation.
Water vapor will certainly condense within the insulation. That is to say, before it exits the outer fabric. It can be next to impossible to notice without weighing the bag. Sometimes you get lucky and the water vapor will condense after leaving the outer layer and leave a layer of frost on the surface of the bag, but that doesn't always happen. It usually doesn't take too long to dry out though.
Originally Posted by
Carbonfiberboy
Another thing one can do wrong is to pitch one's tent in a hollow before a rainstorm. One can also use too thin a pad or a half pad that allows the bag to rest on a wet tent floor following mistake #1. I've done both those things a couple of times and bailed
myself out with a PackTowl. I just pack the bag wet and it dries out fine the next night.
That's a pretty bad mistake. It happens though, especially when we're too exhausted to think clearly. Using a lightweight bivy or an inflatable mattress can help with this.
I hate bringing up the bivy because setting up a shelter in a puddle is a bad thing, even with a tent that has a bathtub floor. But if you have the bivy, at least it'll help if things get this bad. The bivy will also help with condensation by moving the condensation gradient out of the bag and into the bivy where it will condense. There may be frost or puddling inside the bivy, but the former isn't a problem and the latter shouldn't be a problem with water resistant and waterproof shells.
The inflatable mattress is nice for getting off the wet ground. Hopefully it's nothing more than condensation.
Originally Posted by
Carbonfiberboy
Another thing one can do with a down bag is to bring wet clothes into the bag and have them dry by morning. I normally do this after riding or hiking in the rain. It always works as long as one doesn't bring in way too much stuff. But that's a good case for having a bag that's a little warmer than the absolute minimum.
This could be done with any type of insulation, at least if a vapor barrier isn't used. I do agree with having a little more insulation than absolutely necessary, although for me it's usually because I occasionally let my body temperature drop too much before crawling into my bag, so it's nice to have extra insulation to warm back up faster.
Originally Posted by
staehpj1
Not soaking the bag with sweat is a matter of not using too much insulation or too little ventilation.
I mostly agree. Good technique should prevent too much internal condensation. Some will inevitably happen occasionally, especially when sleeping in heavy fog like when sleeping on the shore of a lake. The shore of a lake, while idyllic, is a terrible place to camp because of condensation. There are a couple ways to prevent condensation in insulation, but I'm not sure they warrant much discussion. 1. vapor barrier. 2. warm tent.
Originally Posted by
staehpj1
I have not had a problem keeping my down bag dry enough even when using a bivy. Sometimes there would be a bit of water or ice on the surface, but it brushed right off (the bag has a DWR shell).
That's what I mention directly above. With a bivy, it can be warm enough within the insulation that the water vapor won't condense until it leaves the shell of the sleeping bag. If it's warm enough, the water vapor will also pass through the bivy, but if it's cold enough, it'll condense between the outer shell of the sleeping bag and the bivy. This is a good thing, and quite effective when it's cold enough for that vapor to turn into ice because all you have to do to get rid of it is flip the bivy inside-out in the morning and shake out the ice.