Heavy mist + down

Yes, a tarp will certainly have helped.

Mist/dew is not a real problem under a tarp because your body will generate enough warmth to keep the dew point well below the dew point of the outside air. In still air this can still be a problem, and, when it rains. The mist comming off leaves can happen anytime there is a lot of moisture because of rain, dew/condensation and plant respiration. This is the equivalent of rain as far as down is concerned. Water added to the bag at 100% humidity will disipate into the down. Enough water will saturate it.

1) Even on a clear night, use a tarp to hold the extra 10% of IR radiated out from your body/bag.
2) If it is damp outside, it will still be damp under a tarp. But, your body heat may keep you dryer than adding any more moisture to the system.
3) Using a tent will help with convective heat loss, but may add to the humidity levels. Use a LOT of ventilation.

I've occasionally camped when the dew raised or fog formed and coated everything with water. I think a tarp over me would have helped some, but I can think of a time it happened anyway. I use a lightweight water resistant bivy sack which helps more, but even it has saturated on me three times in the past 5 years which includes some thru-hikes. The sleeping quilt didn't saturate all the way through, but when I packed up and had to compress it some, you know the water soaks through. But usually, when that happens, it isn't because of rain and thus I have a chance to dry it out later in the day in the sun. It hasn't happened to me enough for me to be worried about it and I never felt in danger. If nothing else, I would have packed up in the dark and just started hiking to warm up.

I'm near the ocean almost all the time. Saturation without precipitation, been there! 'Scotch mist' sounds so nice, too, doesn't it? :)

This is why I use synthetics. I also use a Copper Spur in these conditions. The combination of low fly and half-fabric walls prevents a breeze of supersaturated air from hitting the sleeping bag directly. The fabric also seems to 'draw' the moisture. Better to have the wet on the inside of the fly than on my sleeping bag and other gear, especially in cold weather! Remember that we aren't talking about 'a little damp'. Under the right conditions, beads and eventually streams of water will form on pretty much any surface. Only airtight seals keep it out.

The Spur has good high venting too. So, when necessary, a beeswax tealight in a 3 oz minilantern really helps dry out the tent and its contents, even in prolonged 100% (plus!) humidity.

All that fog and mist is good for the skin, though, or so says my Newfoundland granny! :)

Thanks guys, that's good to hear. I really didn't want to have to go full synthetic for the one trip out of ten that I end up in the Misty Mountains again.

you were just only under some netting. right ?
with that in comparison, a tarp would have helped a Great Deal.
especially if you brought the windward edge to ground and/or got behind a nice clump of brush.
even if that stuff seems like it's not moving .. it is.
any sort of cover will knock most of it out of the air for you. and additionally, you'll "feel" vastly more protected.
fog normally = no bugs too !

a proper tent is a bit more protection, but with just a trap, you'll reap the lion's share of the good. i've currently got the tent from hell, and it can get dampish in there under some(rare) conditions.

long ago … used to sleep out all summer on the deck (very spoiled preppy community of Moraga*, CA) and many mornings the fog would roll in across the coast range and deeply soak my hammock setup. then the sun would come up, and you could watch it fight the fog back over the hills (yes.. spoiled Rotten ). when i switched to tarp (canvas) with one side completely open, all that soaked bedding nonsense went away. as well as it got me off the deck to another area away from the house, and that worked out well for all parties involved (me/parents).

* – Go Mats !