I got an email the other day from a Sierra mountaineer who was wondering what the pros and cons were of getting a down sleeping bag with a GoreTex Shell versus microfiber. GoreTex, of course, is the famous waterproof-breathable membrane that dominates the outdoor clothing industry. Microfiber, or its relatives, refers to the standard nylon or polyester sleeping bag exterior.
Microfiber shells are usually woven as tightly as possible and DWR treated to make them water-resistant. Since down sleeping bags become useless when they get wet, it would seem to make sense to give them all GoreTex shells. So why make microfiber down bags?
As anyone who's glanced at a tag in a ski shop knows, GoreTex is expensive. Add Gore to your bag, and you can expect to pay a significant premium. Along with the extra cost comes extra weight, and (somewhat) reduced packability. So, the Gore-Tex option is not without its liabilities. But perhaps the more salient question is: does it keep your bag drier?
That answer probably depends on a variety of factors, the most important of which is the climate and conditions you'll be facing in the backcountry. I imagine a GoreTex bag would be a better choice in truly wet conditions like the mountains of the Pacific Northwest. Then again, if it's really that sloppy-wet, shouldn't you be using a synthetic bag?
Another complication concerns water vapor from a surprising source: your body! I've read estimates suggesting your body gives off a liter or more of water overnight, which means you're basically dumping a Nalgene's worth of H20 into your sleeping bag every time you use it.
While GoreTex is breathable, it's not nearly as breathable as a good Microfiber shell. A GoreTex bag is naturally going to trap more of that moisture inside your bag, rather than allow it to pass harmlessly through. So, while GoreTex may be helpfull in preventing your sleeping bag from getting wet from external sources (rain, snow, spilled soup), it seems likely it will make it more difficult to dry out your bag when it does get wet.
For these reasons, I tend to tilt toward getting a MF bag myself, rather than Gore. If I am especially worried about external moisture, I'll pair my down bag with a good bivy, like Black Diamond's Winter Bivy Sack. This seems to me to be the best solution, as it preserves the versatility of a MF bag while conferring the advantages of GoreTex, if needed. Also of note: some bags exist with Event™ or Epic™ exteriors, perhaps offering a better blend of waterproof-breathable performance.
Still, someone out there must be using a Gore bag, down or synthetic. Maybe you can add a different perspective?