Selecting a sleeping bag

Usage (Summer vs Spring/Fall vs Winter vs Mountaineering)

• Temperature ratings are imprecise:  It is best to take the temperature ratings assigned by manufacturers as a rough guide.  There is no industry standard method of determining temperature ratings of sleeping bags.  While you can assume that one manufacturer's 20^o F sleeping bag will keep you warmer than that same manufacturer's 30^o F sleeping bag, you cannot assume that one manufacturer's 20^o F sleeping bag will keep you warmer than another manufacturer's 30^o F sleeping bag.
• People and situations vary: We all have different metabolisms and degrees of cold tolerance.  Two people can be side by side in identical sleeping bags while one is "a bit too warm" and the other is "freezing."  Multiple factors can affect how warm a person feels while they sleep: amount and type of clothing; sleeping pad insulating capacity; tent insulation and vetilation; type, quantity and warmth of food/liquids recently consumed; level of exertion during the day; wind; humidity; etc.  
• Consider a temperature range:  Given the imprecision of manufacturers' ratings and given the wide variation in people's comfort levels, it is advisable to consider bags within a range of temperature ratings.  Moontrail has divided sleeping bags into the following four classes based on manufacturers' ratings and bag design:
  • Summer camping: 40^o to 50^o F (4^o to 10^o C)
  • Spring/Fall camping: 15^o to 35^o F (-9^o to 2^o C)
  • Winter: -5^o to 10^o F (-20 to -12^o C)
  • Mountaineering: -40^o to -10^o F (-40^o to -23^o C)

Shape

• Mummy: Very space and weight efficient because their tapered shape reduces the amount of fabric and fill that must be packed and carried.  The lack of dead space allows these sleeping bags to warm up more quickly and keep you warmer in colder temperatures than does a bag with a less tapered cut.  Some people find the cocoon-like feel comforting, others find it claustrophobic.
• Rectangular: The whole point of a sleeping bag is to let you sleep comfortably.  If a mummy is too constraining, a few more ounces of carrying weight and a few more cubic inches of sacrificed pack space is a reasonable price to pay for a good night's sleep.  Rectangular bags also lend themselves to a wider range of uses: extra bed padding in a cheap hotel room, a quilt at home, a guest bed for your hardier friends.  However, because rectangular bags are more difficult to warm up and bulkier, they are best suited for summer camping and car-camping.
• Semi-rectangular: A cross between a mummy and a rectangular bag.  You could say "the best of both worlds" or "the worst of both worlds" (no compromise is perfect).  A semi-rectangular sleeping bag sacrifices some thermal efficiency and pack size for extra wiggle room.  Though some mummies are designed so they can be coupled, semi-rectangular bags often provide a more natural fit when two people wish to zip their bags together and share body heat.

Fill (insulation material)

• Performance: Down provides the best insulation for weight and volume (ie, it is light and packs small).  However, down is a very poor insulator if it gets wet and down is relatively expensive.  Polarguard 3D, Primaloft and Thermolite are all high-end symthetic fills.  The fiber manufacturers each claim benefits for their particular formulation.  Published studies by disinterested parties using objective tests are lacking (if you know of any valid studies comparing the various fills, please let us know).  Primaloft is known especially for its ability to retain insulating capabilities when wet.
• Price: As you would expect, the higher performing fills are more expensive.  If you don't mind a slight weight and volume penalty, bags using less expensive fills can be a bargain for camping or backpacking in mild-to-moderate weather.
• Fill Ratings: The rating a down bag is given describes the lofting capabilities of the down used.  The number is the amount of volume one ounce of down will fill in cubic inches.  For example: a bag with down rated at 600 will fill 600 cubic inches, while one with 800 rated down will fill 800 cubic inches. 
  A higher fill rated bag will generally retain it's loft longer than one with a lower fill rating.  In addition, a higher fill rated bag will compress more and be lighter than a lower rated fill bag in the same temperature category.

Size

• Length: The lengths listed in the tables are from the bottom of the bag to the top of the "body" portion of the sleeping bag (ie, the hood is excluded from the measure).  Measure your height when standing from the floor to your chin, plus give yourself a couple extra inches of space (if your feet are pressed against the bottom of the sleeping bag, it could be difficult to keep them warm during the night).  If you will be placing a few items (eg, the next day's clothes, water bottle, camera) at the foot of the sleeping bag, add a few more inches to your toe-to-chin height.  There is no point in buy a bag much longer than your toe-to-chin height because the extra space just increases weight, pack-space and cost while requiring you to warm a larger amount of dead space in your bag.
• Width: If you do not mind a tight fit, you can save weight and pack-space with a relatively narrow bag.  Because narrower bags require less fill, they are often less expensive than a comparable, wider bag.

Features

• Hood: As much as 25% of a person's body heat is lost through the top of the head.  Insulated hoods are a standard feature on most sleeping bags (except summer bags).  Often tightened with a draw string.
• Multi-sectioned hood: Hoods fashioned from multiple panels provide a better fit than single panel hoods.
• Velcro closure tab: Often found at top of zippers and on draft collars to ensure that the zipper or collar does not open when you move around in the sleeping bag.
• Insulated draft collar: Helps keep the warm air in and the cold air out by forming a collar around your neck - often tightened with a drawstring.
• Staggered seams: Help prevent cold spot by ensuring that seams of the inner liner do not line up with seams on the outer liner.
• Zipper draft tube: Runs the length of the sleeping bag inside the zipper.  Keeps cold air from seeping in through the zipper teeth.

Care of the sleeping bag

• Storage sacks:  Do not leave your sleeping bag in its stuff sack for long periods of time.  Sleeping bags can loose their loft and their insulating abilities if maintained in a compressed state for too long.  It is best to store a sleeping bag in a large, breathable sack.
• Bag liners: A bag liner helps keep your sleeping bag clean by absorbing your perspiration and the grit from the previous day's hike.  A clean sleeping bag is warmer (oils don't make good insulators) and loftier.

Tips for staying warm

• Stay dry: If wet, dry yourself off before getting into the bag.  In the sleeping bag, adjust the hood tightness and zipper height to prevent sweating.  Do not breath into the sleeping bag which causes condensation within the bag.  The insulation value of all fills decreases when they are wet.
• Use a sleeping pad: A pad not only increases your comfort, but provides an important layer of insulation between you and the heat robbing ground.
• Wear long underwear: Long underwear keeps you warmer by retaining heat near your body, keeps your sleeping bag drier, and helps keep your bag clean.
• Wear a hat: Even if your sleeping bag has a hood, a hat provides a layer of insulation next to your skin that retains heat.
• Eat and drink well: Your body needs plenty of calories and fluids to maintain a warm state in a cold environment.
• Loft the bag: Lay the sleeping bag out at least an hour before bedtime.  This allows the insulation to expand which maximizes its insulating ability.