Sea to summit reactor thermolite liner the reactor is made from the lightest weight thermolite that dupont has yet approved thermolite is a hollow core fiber that provides extraordinary warmth for its weight and remains extremely breathable adds up to 15 degrees of warmth to a sleeping bag or used alone is the ideal warm weather bag lighter weight more packable than fleece adds up to 15 degrees of warmth to a sleeping bag mummy shape with a box foot