You’re exactly right聴sleeping bags need to breathe. If they don’t, then moisture from the occupant can condense between the inner and outer shells聴where the insulation is doing its thing聴and dampen the down.
Swallow
On the other hand, too much moisture from the outside can do the same thing. Hence, a continual search by bag makers for a good shell material that will keep the insulation dry, yet breathe well enough to ensure that moisture isn’t trapped inside. Gore’s DryLoft has been a long-popular product along these lines. Think of it as essentially “Gore-Tex Lite”: not heavy enough to keep a bag dry under hard rain, but enough to shed flakes of snow, a little dripping from the tent fly as you get in and out of the tent, that sort of thing. Feathered Friends uses, among other things, eVENT, a waterproof-breathable fabric that is somewhat similar to Gore-Tex chemically but that has a more open pore structure. It’s available in their 20-degree Swallow bag, for instance, for $355 versus $270 for the standard nylon-shell bag (www.featheredfriends.com).
My own experience with eVENT聴in a bicycling jacket聴has been extremely favorable. It breathes well and is waterproof. So while I haven’t tried a bag made with it, my belief is that it probably works pretty well, minimizing condensation while shedding extraneous water.
The question is: Do you really need it? I’ll concede that keeping a down bag dry on a wet, weeklong camping trip can be a challenge. But in that sort of weather you’re better off with a synthetic-fill bag anyway, so the moisture issue is taken off the table. Otherwise, good camp craft can keep a down bag dry under most conditions. And, many good shell materials today breathe very well and offer good water-resistant characteristics. I’ve long thought the best shell material is a polyester microfiber, which is what Western Mountaineering, Marmot, and other companies use in several of their bags. This fabric is very breathable, windproof, and because polyester absorbs less water than nylon, inherently water-shedding. For most conditions, that would be my first choice.