But, there were problems with this approach. For one thing, it's exceedingly difficult to make a waterproof bag. Why? Because of all the seams that are required to secure the baffles (if a down bag) or batting (if synthetic). Every needle hole is a potential avenue for moisture to get through. True, those seams could be taped, but doing so is prohibitively expensive. So right off the bat you've defeated the purpose of using waterproof materials.
The other problem is that it's really important the bag "breathes" well. If the outer shell is too effective at blocking moisture, even if configured with a waterproof-breathable material, moisture from your body will become trapped between the bag's inner and outer shells. That's bad, as moisture reduces the insulating ability of the down or even synthetic fill. In cold conditions that moisture can even freeze. So to an extent, a bag that's waterproof will almost inevitably prove less warm than one that isn't.
Still, the notion of a waterproof bag has appeal. After all, for ultralight camping, it could eliminate the need for a tent or bivy bag. Raining? No problemjust zip up the bag, pull the hood over your head, and dream away. And, in fact, a few manufacturers are tackling this issue. Exped, a European gear-maker that entered the U.S. market three years ago, is marketing a line of down-filled bags that employ a waterproof-breathable shell fabric and electronically "welded" seams. The three-season version is called the Ibis Waterbloc; it's rated to just below 20 degrees and costs $399 (www.exped.com).
For now, that's about it. None of the big-market bag makers are working this end of the market right now, although that may change. My own belief is that the added expense of a waterproof bag such as the Ibis, which is a good $150 more than most decent-quality 20-degree bags, adds little in the way of extra versatility.