> "New synthetic insulations, escecially Climashield, are the lightweighter's dream" (posted on another thread) and seeing that BMW's entire Cocoon line is synthetic.
>The only benefit I'm aware of is that wet down does no good, and synthetic is safer in this regard.
...I'm the one who posted that. As Roger said, you've pretty much summed it up. The reason why synthetics are a lightweighter's dream is that they allow you to wear/bring as much as *all* of your daytime clothing to bed if you want to/have to. And they allow you to do it every night, even if you're damp after the day. That includes your damp base layers, your soaked socks, and your jacket that has absorbed a bunch of sweat during a rest break. By morning, your body heat will generally have dried your duds and you can carry on. Many people choose synthetic insulation because some or all of that moisture may wind up in your sleeping bag/quilt.
The issue is the condensation point. Your body heat will drive vapor from your skin (and damp clothes) outwards through your layers of insulation. At some point, though, this water vapour will cool down enough to condense, and become liquid water droplets.
If the vapour makes it all the way out through your base layer, jacket, sleeping quilt, and bivy *before* it condenses, life is good. You are drying your sleep system with body heat, and there's no concern.
What commonly happens, however, is that cool and/or damp ambient conditions experienced at night will make that vapour condense *before* it has exited your sleep system. This Condensation Point (aka Dew Point) may be located *inside* of your bivy, or it may even be inside of your sleeping quilt. This is a frequent occurrence, especially in climates like the Pacific Northwest and New England, and especially during the shoulder seasons.
If this condensation happens in or on synthetic insulation, no-harm-no-foul. It may feel clammy, but it will still keep you almost as warm while you wait for morning. If, however, you use down insulation? You are in for a difficult night. Your down will start to gain weight and collapse, and will very quickly become into a sloppy mess. Once it's a wet sloppy mess, it holds *no* heat and you might have to start thinking about getting a fire going. (Hopefully it's not raining or snowing. :)
"Well I am careful to keep all of my clothes and gear dry."
So is almost everyone. But as a lightweight backpacker, you probably have fewer options to help you do this. You probably don't have an extra jacket, or a full change of clothes to wear to bed.
Also, your skin gives off moisture all night long. This is called Insensible Perspiration, and it's a process whereby your skin tries to maintain a certain minimum humidity on its' surface. Your wicking base layers are constantly taking this humidity *away* from your skin, and your skin is constantly perspiring at a low level to replace this lost moisture. If it's cold/damp enough, all of that moisture winds up in your insulation. (More than a pint a night, apparently.) Some Arctic explorers report sleeping bags weighing 60lbs by the end of their trip because insensible perspiration has been freezing in the insulation for weeks or months: it's a real and potentially dangerous phenomenon depending on your circumstances.
Since the dew point is generally not *immediately* next to your skin, some people have success wearing down insulating gear inside of a synthetic bag. That way you have synthetic insulation at the condensation point and down where the moisture is not likely to condense. This same down layer, however, will likely be worn during the day -- not practical on rainy+humid expeditions!
There's also the issue of external moisture. The shelters involved in lightweight backpacking are often more prone to condensation than the big double-walled tents of traditional backpacking. If it's cool and rainy outside, this can lead to sleeping in a home-made fogbank all night. Some users have reported actually watching a down sleeping bag collapse over the course of a night.
Finally, many lightweighters are involved in more remote/extreme trekking. A fall in the river, a leaking platypus, or a down bag that becomes soaked during a day or two of humid/rainy camping is no big deal when you're 10 miles from the car. But imagine being 100 or more miles from civilization, at 2 a.m., possibly solo, with a failed insulation system and a bad case of the shivers. Such a scenario is why many experienced lightweight backpackers opt for synthetic insulation pieces over down ones.