The THERMOLITE Reactor Extreme is the warmest backpacking knit sleeping bag liner Sea to Summit offers. The hollow fiber yarns work just like the hollow-core fur of a polar bear in that they trap heat effectively. When the mercury drops you'll appreciate the mummy shape and drawcord hood with mini cord lock attached. Depending on your environment and sleep conditions, this liner can increase your sleeping bag warmth by up to 25F.
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