With its 800-fill goose down insulation and waterproof shell, the Sierra BTU -5 degrees F four-season sleeping bag is perfect for your next frigid expedition. Goose down is a natural renewable insulation that provides a higher warmth-to-weight ratio than other materials. As a result, goose down bags are not only warm, but also extremely compressible and lightweight. To prevent the fill from getting wet, Sierra added a Drizone waterproof/breathable shell, which allows moisture to escape from the bag while keeping water and dew out. This is a crucial feature for goose down bags, which perform poorly when damp. And hikers will love the BTU's Funk! Just turn the bag inside out, leave it to sit in the sun, and walk away. The liner will break down dirt and oils to produce a cleaner, fresher bag--a must for multi-day treks.