When you are planning on camping in zero degree temperatures, you want to make sure that you will stay warm while you sleep. Other than your tent, arguably the most important piece of nighttime camping equipment is your sleeping bag.
Most sleeping bags come with a temperature rating, however, these ratings are guidelines, not guarantees. Because of this, it can be helpful to know what features to look for when picking out what zero degree sleeping bag is right for you.
There are two main insulation types: down, and synthetic. Some sleeping bags are also a combination of down and synthetic materials; this helps offset the downsides to both types. As in most things, there are pros and cons to each.
Down sleeping bags are pricier, but they last much longer. Down is best for cold, dry conditions, as it doesn’t insulate when it gets wet. You can get “water-resistant” down, but while it will hold up better in damp moisture, water-resistant is not the same as water-proof and you will still be cold if you’re sleeping in heavy rain.
The biggest advantage to synthetic sleeping bags is that they insulate even when wet. Synthetic bags are also considerably cheaper. However, they tend to be a little less warm and breakdown over time, leaving you with a sleeping bag that won’t keep you as warm as it once did. Synthetic bags are also bulkier and therefore harder to pack.
Sleeping bags work by trapping air around your body. When your body heat warms this air, it keeps you toasty. Fill power measures the ability of down to trap heat. According to REI, “it is calculated by how many cubic inches 1 ounce of down can fill in a testing device.” The higher the fill power, the more air the down can trap.
The fill power of a sleeping bag doesn’t change its warmth. Two bags with the same temperature rating can have different fill power. What it does affect it the bag’s weight. A bag with 800 fill power is going to be lighter than a bag with 600 fill power because the 800 fill power bag is made of higher quality down that traps air better and therefore needs less fill for the same amount of warmth.
Sleeping bags come in several different styles: rectangular, semi-rectangular, mummy, and double-wide. Mummy sleeping bags are made to fit close to your body’s contours, thus trapping air better and offering more warmth.
If the close-fit of a mummy bag makes you feel claustrophobic, a semi-rectangular bag is your next best choice for warmth. These bags fit closer than a rectangular bag will, but offer a bit more wiggle room than a mummy bag.
If you are really looking to hold in the heat, consider getting a mummy or semi-rectangular bag with a hood. The hood will have a drawstring around the opening so you can close it tight around your face like a snug hat. This will keep you from losing heat through the top of your head.
Though not part of your sleeping bag, a sleeping pad impacts more than just comfort — it also greatly affects how warm your sleep situation will be. Temperature ratings on sleeping bags are given with use of a pad, not without, so skipping this piece of equipment will land you zipped up in a bag that’s less warm than you expected.
The ground is cold, sometimes colder than the air, and you will be losing a lot of heat to it. On top of this is another, more mechanical problem. Remember how trapped air equals heat? When you lay on your sleeping bag, you squash out all of the heat-trapping air, thus losing warmth where you need it most. A pad puts an insulating layer between you and the ground, and helps counteract the effect.
There can be little doubt that a warm sleeping bag is one of your best layers of defense when camping in zero degree temperatures. By keeping these sleeping bag features in mind, you should be well on your way to making an informed choice about which bag will be best for you and your adventure.