When you know you are going to battle the cold weather while you sleep, show the chill what defeat feels like with the Sea to Summit Thermolite Reactor Extreme Liner. This liner adds up to 25°F or 15°C to your sleeping bag set up, because it is designed with the 110g/m² Thermolite fabric. This piece is also lightweight, slightly stretchy, breathable and fits into its own stuff sack. On nights that the weather is mild, this piece can be used as a stand-alone bag.