“Overbag” fill material for winter

DriDown and similar products work by reducing the contact area between liquid water droplets and the treated surface. This prevents absorption.

Unfortunately the problem with sleeping bags is moisture vapor condensing once it's already penetrated through the down. The DWR will still help a little, but not likely enough to eliminate the problem. If you want to avoid it all together, VBLs are your friend. To that end Warmlite has an anecdote about a polar expedition using synth bags that eventually turned into solid blocks of ice and were just as useless as a soaked down bag for warmth.

However using a thin synth top bag would give the benefit of faster drying times during the day, of course this is assuming you can actually keep the dew/freeze point inside the synth which may be hard in really cold weather.

I don't usually hike that much below freezing, but if I did, I would follow the advice from everyone I know who does. You must use VBL.

VBL clothing or a liner doesn't weigh much for what it does.

This is especially true on long hikes below freezing as your bag/quilt(s) will probably not get a chance to dry out and the moisture/ice will build up night after night.

Hi Eric,

MEC sell this overbag which weighs just over a pound.

http://www.mec.ca/AST/ShopMEC/SleepingBags/Overbags/PRD~5021-089/mec-amulet-ultralight-overbag.jsp

On my last winter trip I notcied a build up of moisture in the chest area of my bag and found layering a Synethic jacket over my chest helped a
Lot and the bag was dry in the morning, the same thing could act like an overbag.

I doubt you'll have much luck getting the frost out of the bag. The only real option is to get it to melt and evaporate. Towards that end, if you make an overbag,use black fabric for maximum solar heat gain when the sun is shining. If the sun does not shine, well at least your down bag is dry! And you could wear the overbag around camp as a robe and maybe get it to dry some that way.

How many days are your trips usually? For a few nights the moisture buildup is not a big deal usually. Longer than that I'd go VBL for real winter. I do my snow camping in the spring, so sunny days are the norm and I find I can keep up with the moisture by airing out the bag whenever I get the chance.