The THERMOLITE® Reactor™ Compact sleeping bag liner is the ideal choice for gram counters under 6 feet. It essentially uses a combination of two weights of THERMOLITE® fabric, with the thicker more insulative being strategically placed across the torso and in the feet area. The hollow core fibers work like the fur of a polar bear in that they are incredibly insulating and trap heat effectively. This sleeping bag liner keeps your sleeping bag clean and is super easy to wash. It will bump up the warmth of your sleeping bag temperature rating considerably, depending on your environment and conditions.