First off, sorry for the TMI. 😬
I often sweat a fair bit while sleeping, even when not too warm. Sometimes not too much, but sometimes enough that sheets are wet enough to get a few drops out if I wring them.
My old synthetic bag has seen better days and I'm planning to replace it this year and was considering spending the extra for down after seeing the weight and size difference.
My questions is, if I go with down and sweat in the bag, is that moisture going to kill the loft and render the bag worthless for the night?
Thinking I might need to stay with synthetic but I'm hopeful for down and figured I'd ask.
You might want to consider a bag liner to help keep your new bag clean(er). Then there's less issue with the sweat negatively affecting the down. Although, I would think the salt from the sweat would, over time, be more damaging to the fabric.
There are silk liners that would be great in Summer, too.
I got a liner a while back for the r-value but haven't needed or used it.
I think my worry with it is that on some nights it might not be enough since it's pretty thin.
The question still lingering in my mind is if a bunch of moisture in the bag would render the bag worthless or if the waterproofing I've heard some treat down with now would be enough to keep it lofty and useable.
I would certainly go with a bag liner to absorb moisture and consider wearing some sort of clothing to absorb moisture as well. Water and down do not play well together. Definitely purchase a bag with treated down.
Down loses its insulation qualities when it gets wet.
My concern goes beyond sweat. If it rains and water enters your tent, the bag will get wet. If it's humid outside as it often is after a rainfall, then it will take a long time to get that bag dry again. So unless size and weight are really important, I'd look for a synthetic bag.
If you tend to sleep warm then perhaps you can save size and weight by getting a bag that's meant for relatively warmer temperatures. On occasion if you need more warmth you can also wear a fleece jacket inside the bag.
Synthetics also lose value when wetted, but dry much more readily. A "waterproof" bivvy sack, encasing the bag, is useful in humid/wet conditions and allows use of the ag without a tent or tarp. I have awoken to puddles of water on my bivvy sack with no impairment of the down, but also experienced loss of R value in a rain/snow storm, temperature right at 32F, humidity about 180% (!). Bags do dry out eventually....
To answer your specific question, moisture in your bag, of whatever origin, does not render the bag "worthless." although the R value is reduced. In a way, that is advantageous, since a wet bag ensures an early start in the morning...(speaking from experience)
@aarond you've gotten great advice from several of our Superusers already! Just a few additional thoughts that line up with that advice:
Hope this helps!
Great advice from JenK. I log 30 to 50 nights spent in the wilderness every year. I will use down bags only in dry environments. I also can perspire a lot at night and find that the down bag is full of moisture in the morning even in very dry conditions.
One tip is to keep the zipper open at your feet 4 to 6 inches. I use a Big Agnes bag where there is a sleeve for my pad. I've noticed that having a collar on your bag allows you to keep the head opening larger for more moisture escape as well. It seems most of my moisture comes from my shoulders, neck and head area. I wear a balaclava and try to keep my head from being totally wrapped up in the sleeping bag.
Down is lighter, and I primarily travel in places where I can keep my bag dry and can air it out every morning.
There are lots of advantages to synthetic bags and you should choose for your own needs. For most of my backpacking career I had three sleeping bags, and various liners and covers to choose from.
The play is the thing. Just get out there.