Key takeaways
- A sleeping bag traps body heat and is rated by the lowest temperature it keeps you comfortable at.
- Down is warmest for its weight and most packable; synthetic is cheaper and keeps insulating when wet.
- Mummy bags maximize warmth-for-weight; rectangular bags trade warmth for roominess.
- A bag only handles the air — you still need a sleeping pad's R-value to block heat loss to the ground.
How temperature ratings work
A sleeping bag keeps you warm by trapping a thick layer of still air in its insulation. Its rating estimates the lowest temperature it’ll keep you comfortable: standardized EN/ISO testing gives a comfort rating and a lower limit rating. Choose by the comfort rating and add a margin for cold sleepers, wind, and altitude.
Down vs synthetic
Down offers the best warmth-for-weight and packability and rates its quality by fill power, but loses warmth when wet. Synthetic keeps insulating when damp, dries fast, and costs less, at the price of more weight and bulk.
Shapes
A mummy bag tapers to the body with a hood for maximum warmth-for-weight; a rectangular bag is roomier but colder for the weight. The ultralight alternative is a quilt — see sleeping bag vs quilt.
For three-season backpacking, a common choice is a 20°F (-7°C) comfort-rated down mummy bag on a sleeping pad of around R 4 — the bag handles the air, the pad blocks the cold ground.
The bottom line
A sleeping bag is the heart of your sleep system, and the right one balances temperature rating, fill type, and shape against your trips and budget. Remember that the bag only handles the air around you — pair it with an adequately rated sleeping pad, or you'll still sleep cold from the ground up.
Frequently asked questions
How do sleeping bag temperature ratings work?
Standardized EN/ISO ratings give a 'comfort' rating (the lowest temperature a cold sleeper stays comfortable) and a 'limit' rating (for a warm sleeper). Choose by the comfort rating and leave a margin, since people sleep differently.
Down or synthetic sleeping bag?
Down is warmer for its weight, packs smaller, and lasts longer but fails when wet and costs more; synthetic keeps insulating when damp, dries faster, and is cheaper but is heavier and bulkier. Down suits dry, weight-conscious trips; synthetic suits wet conditions and budgets.
Sleeping bag or quilt?
A bag fully surrounds you with a hood and zipper for maximum warmth; a quilt drops the crushed-flat underside and often the hood to save weight, attaching to the pad. Bags are warmer and cozier; quilts are lighter and favoured by ultralight backpackers.
Sources
- Sleep systems — The Mountaineers
- Choosing a sleeping bag — American Hiking Society