Sleeping Bag Selection: Temperature Ratings, Fill, and Fit
Your sleeping bag is the most critical comfort item on any camping trip. Too warm and you overheat and sweat. Too cold and you shiver all night, destroying your energy for the next day. Understanding temperature ratings, fill types, and how personal factors affect warmth lets you choose a bag that actually matches your needs rather than one that matches a marketing claim. This guide covers the science and practical considerations behind sleeping bag selection.
Understanding Temperature Ratings
Sleeping bag temperature ratings indicate the lowest temperature at which the bag keeps an average sleeper comfortable. The EN/ISO 23537 standard tests bags on a heated mannequin and produces three ratings: Comfort (temperature at which a cold sleeper stays warm), Lower Limit (for a warm sleeper), and Extreme (survival only, not comfortable). Most manufacturers advertise the Lower Limit, which represents the optimistic end.
Individual variation is significant. Cold sleepers, women, thin individuals, and exhausted or dehydrated sleepers need a bag rated 10-15 degrees lower than the expected overnight temperature. Warm sleepers and well-fed, well-hydrated individuals can get away with a bag rated closer to the expected low. When in doubt, buy a bag rated colder than you think you need. You can always unzip a warm bag, but you cannot make a cold bag warmer.
Down vs Synthetic Fill
Down fill (goose or duck) offers the best warmth-to-weight ratio. A 20-degree down bag weighs 2-3 pounds, compresses to the size of a football, and lasts 10-20 years with care. Fill power (600-900+) measures loft quality: higher fill power means more warmth per ounce. The drawback is that down loses insulation when wet and takes hours to dry. Hydrophobic down treatments help but do not eliminate this vulnerability.
Synthetic fill (Climashield, PrimaLoft) retains insulation when wet, dries quickly, and costs 30-50 percent less than comparable down. The trade-off is more weight (a 20-degree synthetic bag weighs 3-4 pounds), more packed volume, and shorter lifespan (5-8 years before insulation degrades). Synthetic is the better choice for consistently wet conditions and tight budgets.
- Down: lightest, most compressible, longest-lasting, loses warmth when wet
- Synthetic: heavier, bulkier, insulates when wet, cheaper, shorter lifespan
- Hydrophobic down: compromise, water-resistant treatment, costs more than regular down
Bag Shape: Mummy, Rectangular, and Quilt
Mummy bags taper from shoulders to feet, minimizing dead air space and maximizing thermal efficiency. They are the lightest and warmest option per ounce. The trade-off is restricted movement, which some sleepers find claustrophobic.
Rectangular bags offer more room to move and can unzip fully as a blanket. They are heavier and less thermally efficient because of the extra volume of air inside. They suit car camping where weight is not a concern. Quilts (backpacking quilts) eliminate the back insulation (which compresses under body weight anyway) and save 20-30 percent of weight compared to mummy bags. They require a well-insulated sleeping pad to work.
Sizing and Features
A bag that is too long wastes insulation heating empty space at the foot. A bag that is too short compresses insulation at the feet and shoulders. Most bags come in Regular (up to 6 feet) and Long (up to 6 feet 6 inches). Some brands offer Short and Wide options. A hood with a drawstring keeps head heat from escaping (you lose 10 percent of body heat through your head).
A draft collar around the neck and a draft tube behind the zipper prevent cold air from entering through the bag opening and zipper. Stash pockets inside the bag keep your phone and headlamp warm and accessible. A two-way zipper lets you vent from the bottom on warm nights without opening the top.
Care and Longevity
Wash sleeping bags infrequently — only when they smell or lose loft. Use a front-loading washer (top-loaders can damage baffles) with a down-specific soap (Nikwax Down Wash) for down or a mild detergent for synthetic. Tumble dry on low heat with clean tennis balls to break up clumps and restore loft.
Use a sleeping bag liner to extend the time between washes. Liners add 5-15 degrees of warmth, keep body oils and dirt off the insulation, and are easy to wash frequently. A silk or synthetic liner weighs 3-8 ounces and costs $20-50. It is the cheapest way to extend both the warmth and the lifespan of your bag.
Frequently Asked Questions
What temperature rating sleeping bag do I need?
Buy a bag rated 10-15 degrees below the coldest temperature you expect to encounter. For three-season backpacking (spring through fall), a 20-degree bag covers most conditions. For summer-only camping, a 40-degree bag saves weight. For winter camping, a 0-degree or colder bag is necessary.
Is a down or synthetic sleeping bag better?
Down is better for weight-conscious backpackers in dry conditions. Synthetic is better for wet climates, budget-conscious buyers, and situations where the bag may get wet (kayak camping, rainy Pacific Northwest). If you can keep your bag dry, down provides the best performance per ounce.
How long does a sleeping bag last?
A quality down sleeping bag lasts 10-20 years with proper care (stored uncompressed, washed infrequently with appropriate soap). A synthetic bag lasts 5-8 years before the insulation compresses and loses effectiveness. Liners extend the useful life of both types.
Can I make a sleeping bag warmer?
Yes. Add a liner (5-15 degrees of warmth for 3-8 ounces). Use a better sleeping pad (ground insulation matters as much as bag insulation). Wear a hat and dry base layers. Eat a high-calorie snack before bed (digestion generates heat). In extreme cold, fill a water bottle with hot water and put it in the bag.