Built for serious cold-weather performance, the Sea to Summit Reactor Fleece Sleeping Bag Liner is the warmest liner in the Reactor series. Constructed with grid-patterned hollow-core Thermolite Pro fibers, this fleece liner adds substantial insulation to your sleep system-ideal as a core component for extreme conditions or as a lightweight standalone bag in warm climates. The mummy-shaped design maximizes heat retention, while shoulder zippers and a drawcord footbox give you versatile temperature regulation. Whether you&';re layering up for alpine expeditions or sleeping under the stars during milder nights, this liner keeps you comfortable and adaptable. Plus, it&';s treated with HeiQ Fresh odor control technology to stay fresh even after extended use. Aligns perfectly with Sea to Summit&';s Free Flow Zip system, so you can enjoy seamless integration when used inside compatible sleeping bags. It&';s the go-to option for adventurers who demand both warmth and flexibility from their sleep gear. Features: Maximum Warmth and InsulationMade with hollow-core Thermolite Pro fleece arranged in a heat-retentive grid pattern for ultimate cold-weather protection. Versatile Design for Any SeasonFunctions as a thermal liner in cold environments or a lightweight standalone sleeping bag in warmer temperatures. Drawcord FootboxAdjusts for ventilation or mobility-tighten for warmth or loosen to walk around camp without removing the liner. Dual Shoulder ZippersAllow for convenient arm access and customizable airflow while keeping your core warm. HeiQ Fresh Odor ControlBio-based odor technology helps the liner stay fresher for longer during multi-day or backcountry trips. Compatible with Free Flow Zip SystemDesigned to align with Sea to Summit&';s sleeping bags for optimized integration and thermal efficiency. Efficient Mummy ShapeContoured to reduce dead air space and maximize body heat retention during frigid nights. Compact and LightweightPacks down small into the included storage case for easy transport in any backpack or duffel. Lifetime GuaranteeBacked by Sea to Summit&';s commitment to long-term quality and performance.