The Silverthorne uses a 2-layer construction to help eliminate cold spots and keep you warm. There is also a zipper baffle to help seal in your body heat. It is composed of a 100% cotton flannel liner and rugged cotton canvas outer. When it's time to store or transport the bag, it conveniently rolls up with the roll-up straps so it's easy to carry and doesn't take up too much room. Two Silverthorne bags can be zipped together to create a double bag. 4251809 Silverthorne +5deg 4900027 Rectangle Liner - PolyCotton 4900029 Rectangle Liner - Brushed Polyester