When we need a little extra warmth on cool fall camping trips, we pack the Sea To Summit Reactor Fleece Mummy + Drawcord Sleeping Bag Liner. The cozy liner gives us a 10% thermal boost to our 3-season sleeping bag, helping retain more heat while we camp at higher elevations. The drawcord design of the footbox allows us to customize our ventilation, offering extra breathability on milder nights and trapping heat when nighttime temperatures plummet.