The THERMOLITE Reactor Compact sleeping bag liner is the ideal choice for gram counters under 6 feet who want to add warmth to their sleep system. The worlds 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 hollow core THERMOLITE fibers work like the hollow fur of a polar bear to trap extra insulating air. The Reactor Compact Plus keeps your sleeping bag clean and is super easy to wash. It will bump up the warmth of your sleeping bag considerably, depending on your metabolism, your sleeping bag and sleeping pad, and the environment.