100% silk sleeping bag liner. The Silk Sleep Sack helps add 10-12 degrees to your sleeping bag keeping you warm on chilly nights. Plus it's ideal for use as a stand-alone sleeping sack when staying in hotels, hostels or anywhere you sleep. Silk is a natural and breathable material and provides a luxurious and comfortable sleeping environment. It also comes with a new twist — an attached waterproof stuff sack, it prolongs the life span of your liner and is great for storage. Compare the Silk Sleep Sack to others and you will see with our new features and outstanding price point why our liners are sure to impress.