The Sea to Summit THERMOLITE. Reactor Compact Plus Liner is the ideal sleeping bag liner choice for gram counters under six feet who want to add warmth to their sleep system. The world's first 'hybrid' sleeping bag liner uses a combination of two weights of THERMOLITE fabric, with the warmer fabric strategically positioned across the torso and in the footbox. The THERMOLITE fibers work like the hollow structure of a polar bear's fur, which traps extra insulating air. The Reactor Compact Plus keeps your sleeping bag clean, plus it's super easy to wash.