4:45 p.m. on January 7, 2014 (EST)
I think the moisture issue with down bags and the resulting loss of loft is an issue that is largely exhagerated. Yes, it is true that as down gets wet it will clump and lose its ability to loft resulting in it not providing as much insulating ability.
IMO this is mainly only an issue in the Winter, and shoulder seasons when the temps are dipping below freezing. Aka when the insulating ability is absolutely critical.
So, yes it can happen. But lets look at all the facts here. In order for down to completely lose all ability to insulate it has to literally be drenched, as in soaking wet, as in you decide to submerge it in a river etc. If you take proper precautions and keep your down bag properly protected the likely hood of this actually happening is very very slim. Keep your bag in a waterproof stuff sack, wraped in a garbage bag, etc.
So what's the real threat to this taking place? Its moisture from the air/humidity, and moisture from you. It is true that after a period of a few days the down will begin losing a little loft. This isnt as dramatic as it sounds. We are talking about a degree or two of insulating ability lost about every 2-3 days. So if your out on a week or two long trip it can have a substantial impact over the course of the trip. If your only out for a weekend or a few days, it will make NO difference what so ever that you would be able to detect without some serious scientific testing.
Ok so, what can we do about it? There are a couple of things that all but completely mitigate this from occuring.
1) When you wake up and get out of your bag, immediately ball up your bag and give it a good bear hug and squeeze all of the air out of your bag. Do it a couple times. This literally takes maybe 30 seconds. What this is doing is getting all of that moist air from the night(from your body mainly) out of the bag while its still warm before it has a chance to condense inside of the bag/on the down.
2) If you take a rest break during the day, and or once you get to camp take out your bag and lay it out to let it air out. This works even better in a sunny spot. But anywhere will help, if its windy or sunny or both it works best. I personally rarely lay my bag out during the day on a rest break, unless its a reallly length break. I usually just immediatly hang my hammock as soon as i get to camp and hang my bags on it to get some air.
If you do either of those with any frequency the issue is negligible.
I too live on the East coast where there is frequently high humidity. I do those two steps and have absolutely zero problems with my down bags or quilts.
Synthetics:
They dont loose loft due to moisture, so are inherantly warmer in conditions where they would be exposed to it. But if its wet, it will still be cold and uncomfortable, and pretty much useless...despite what anyone says. Try it one time and you'll quickly see what I mean. I spent many a night in the field in the military with a wet or damp synthetic sleeping bag.
Now, if I was going to be backpacking in a rain forest, like the PNW for example. I would use a synthetic bag. But for most of the country its really not as big of an issue as people make it out to be.
To me the benefits of down make up for this potential issue, and with a little preventative measures its really no issue at all/ very minor.
So from all that just remember:
Keep your down bag sealed in some waterproof fashion in your pack, and air out your bag when you get the opportunity and your down bag or quilt will always keep you nice and warm with its lofty goodness.