The Reactor Fleece Liner from Sea To Summit crafted from hollow-core Thermolite Pro fibers arranged in a grid pattern, serves as the cornerstone of any high-quality cold weather sleep system. Designed to offer maximum warmth, this liner features shoulder zippers and a drawcord footbox for temperature regulation, seamlessly integrating with Sea to Summit sleeping bags equipped with the Free Flow-Zip system. Versatile and adaptable, the Reactor Fleece Liner ensures comfort across various seasons and can even double as a standalone sleeping bag in warmer weather. With its drawcord footbox providing ample ventilation, users can also enjoy the flexibility of moving around camp while still wearing the liner, if necessary. Thermal boost to retain heat and warmth: The Reactor liner acts as the core of your sleep system, extending the performance of your sleeping bag and sleeping mat in the coldest of conditions Hollow-core Thermolite EcoMade fibres: Utilized in a grid-pattern fleece for maximu