Sea to Summit Reactor Thermolite Mummy bag liner. I used once but then decided to go a different way with my insulation system in my hammock. The manufacturer says "Thermolite® insulation is a hollow-core fiber that provides extraordinary warmth for its weight and remains extremely breathable. It's very comfortable, more compact and warmer for its weight than fleece liners.Adds up to 15°F degrees of warmth to a sleeping bag, or can be used on its own as the ideal warm-weather bag. $35.00 Free Shipping