I have camped for 28 days at a stretch with a -20 synthetic bag at 0 degree temps and found myself getting colder each night as
the bag lost it's loft from moisture and compression. We finally ended up adding a synthetic overbag on the next trips.
which is why ive said repeatedly over the years that if you have a synth jacket put it OVER your downbag when sleeping ...
and using down under synth is the "ideal" for of moisture management
in winter youre spending the fuel anyways to melt water, might as well do it overnight and warm up your bag ... but in the shoulder seasons you generally arent going to boil water every single night ... and in those temps a synth bag is much thinner than a -20F one
to put it simply a thin down layer will actually dry FASTER than a much thicker and heavier synth one simply due to the moisture content vs drying area ... however a synth layer of roughly the same weight will handle moisture better and provide some insulation when damp ... the trick is to have the INSIDE dry, which is not hard with a hawt nalgene
again to reiterate .... its is exceptionally hard, if not impossible to dry very damp sleep system at the limit of its temp rating without the sun ... once it gets that way you better pray you have excess insulation somewhere, enough fuel for MANY hawt nalgenes, a fire, or can get out of dodge quickly
a damp synth at its limit will still degrade, but slower, and it provides some insulation when damp (not wet) ... and is MUCH easier to dry out even without the sun
ideally you would want a second over layer ... something you "sacrifice" but can dry easily enough during the day when walking, or where it doesnt matter if its a bit damp ... ie an overbag or synth jacket
if im worried about condensation ... i put my jackets over my bag ... in the morning i can wear em till the heat dries em out