Use a sleeping pad. These lightweight pads are designed to prevent conductive heat loss to the cold ground, but they also provide another layer of defense between your bag and the ground, keeping your sleeping bag cleaner than if you were laying right on the floor of your tent. For car campers, we recommend the comfortable Therm-a-Rest LuxuryMap Sleeping Pad, which is also easy to inflate, deflate, roll, and store. For backpackers, we suggest the Therm-a-Rest NeoAir XLite NXT—it’s lightweight, warm, and comfortable, and it packs down to the size of a 1-liter Nalgene bottle.
