What Our Gearhead(R) Experts Are Saying: "This liner is insanely warm for the weight. The grid-pattern Thermolite fleece traps heat really well, and the shoulder zips plus drawcord footbox make it easy to regulate temps. I've used it to push a lighter bag deep into shoulder season-and as a standalone in warm weather. Super versatile." We reach for the Sea to Summit Reactor Fleeceweight Liner when temperatures dip and our sleep system needs serious reinforcement. Built from hollow-core Thermolite EcoMade fibers arranged in a heat-trapping grid pattern, it's the warmest liner in the Reactor series and the foundation of a dependable cold-weather setup. The soft fleece interior feels instantly cozy, helping us settle into camp even on frosty nights. Thoughtful temperature-regulation features make it adaptable across seasons. Shoulder zippers allow venting without shedding warmth, and the drawcord footbox can open for airflow or close tightly on the coldest nights. Integrated design ensures seamless compatibility with Sea to Summit bags using the Free-Flow Zip system, while HeiQ Fresh technology keeps the liner feeling fresh through extended use. Warm, versatile, and remarkably comfortable, it's the upgrade we rely on for winter camping or late-season backpacking.