The Sea To Summit THERMOLITE Reactor Extreme Liner is Sea to SummitUs warmest knitted sleeping bag liner. If your adventures take you to colder climates, the Reactor Extreme will add plenty of warmth to a sleeping bag. Plus in warmer weather, it can be used as a Tstand aloneU sleeping bag. The hollow core fibers of the THERMOLITE fabric work like a polar bear's fur and, along with the knitted structure, trap plenty of insulating air. Super soft and amazingly stretchy, the Reactor Extreme uses a heavier version of THERMOLITE to provide extra warmth for an incredibly comfortable nightUs sleep. Use one to build your cooler-weather sleep system and to help keep your sleeping bag clean too.