Being a member of Marmot's Ultralight series of sleeping bags, the Hydrogen 30 Down Bag is one of the first options we recommend for the ounce-counting backpacker that still appreciates a good night's sleep after a long day on the trail. To keep the Hydrogen warm in the face of condensation, Marmot used Pertex Microlight fabric to save weight, increase breathability, and protect the down insulation from light moisture. Better yet, the insulation consists of 800-fill down that's been finished with Marmot's Down Defender treatment to help prevent it from clumping and ultimately losing insulating properties should it happen to get wet.