This is another item that has massive variation in price and quality. Quality in bags is defined by reference to its weight, warmth, compressed size, and materials used in its construction. There is a lot to go into around shape and construction of a sleeping bag but I will not go into that here. It can be summarised by saying that wider bags with zippers around the whole bag are more comfortable, but heavier; tapered or 'mummy' shaped bags are less comfortable but are lighter and more efficient. Your main considerations will usually revolve around weight and warmth.
Both of these are determined by the quality and amount of the material used to ‘fill’ the sleeping bags. All types of fill are designed to compress well, and then ‘loft’ up to fill the bag when taken out of the pack. Fill on good quality bags is always either:
Down (fluff from under the feathers of poultry, usually ducks or geese) – lighter relative to the warmth it provides than synthetic down, also more durable, more compressible and will last longer than synthetic bags. The downside is it is much harder to clean, generally more expensive, and will take a long time to dry, as well as losing its ability to insulate faster than synthetic fill when wet.
Synthetic imitation down – still quite light, faster drying and will maintain its loft when wet, easier to wash, and generally cheaper. Downsides include that it is not as warm for its weight as down, and not as compressible or as durable.
Basically, the more insulation is used, the warmer a bag will be, but also the heavier it will be. The higher the quality of the insulation, the lighter it will be at a certain temperature.
You do not always want the warmest sleeping bag, a sleeping bag that is too warm is uncomfortable to sleep in. Sleeping bags generally have a temperature rating on them. You want a sleeping bag with a temperature rating ideally just below what you are expecting to encounter. That being said, people that get cold easily might want something much warmer than what is expected. But, as a general guide, for standard trips which are conducted in three season weather (late Spring-Early Autumn), you will want something that is rated to around -5 – 2 degrees Celsius. For Summer you could get away with something less warm, but you want a bag that can see you through as many seasons as possible.
Once you have decided on the temperature you want, you need to decide how much weight you want to carry, and by extension how much you want to spend. In my opinion, at the above temperature range for a man, anything below 1kg is pretty good.
Sleeping bags tend to have reasonably good sale prices, so shop around one you know the temperature and weight you want. They are also pretty similar between brands in terms of materials used and construction techniques, so find the best price you can!
You want a bag that is not too tight on you, you need room to move. Not only is this more comfortable, but if a bag is so tight its compressing the baffles, it will reduce the warmth of the bag. The thing that makes a bag warm is the thickness of the baffles, if you compress them it loses its ability to insulate as effectively.
In the above section on sleeping bags, I refer to the quality and loft of down. Down’s quality is largely measured in terms of its ‘loft’. Loft is the ability of down to go from a compressed state to filling a certain amount of space. The amount of space down can fill is determined by its quality; more specifically it is determined by the size of the bird it comes from.
The quality of the fill is more often a question when looking at down sleeping bags, as synthetic fills tend not to vary in the level of loft they can achieve. When looking at down sleeping bags or for that matter down jackets in outdoor stores, you will often come across tags saying something along the lines of “650 loft grey goose down”. Salespeople will often make a massive fuss over these figures, but don’t pay too much attention. The “650 loft” is a measure of the quality of the down. Higher quality down will fill more space than lower quality down. What this means practically is that it takes less higher quality down to fill a sleeping bag of a certain size than lower quality down. So the bag with higher quality down will weigh less, but usually cost more.
How good are the various lofts?
550-650 loft is the standard down used in sleeping bags, this is slightly more than most synthetic insulations, which usually have a loft of around 500.
700-800 is pretty good, and is what most brands use for their high end brands.
800-850 is what most premium brands use for the top of the line sleeping bags.
850+ is rare, very expensive and not used by many companies; it is usually only used by small garage manufacturers.
Common myths and things salespeople will say that are NOT true:
The quality of the down makes the bag warmer - NOT TRUE – it will only make it lighter at the same temperature. So a bag rated to -4 using 600 loft down will be heavier than one rated to -4 that uses 800 loft down.
Goose down is better than duck down – this is very misleading. Geese grow larger than ducks, so the fluff under the feathers is bigger, so hence has a higher loft. Therefor higher levels of loft (700+) usually comes from geese. But that doesn’t mean that goose down is worse than duck, 600 loft is still 600 loft whether it comes from a goose, a duck or a rabbit. It’s like the old joke: “what’s heavier, a ton of bricks or a ton of feathers? Answer is neither, they both weigh a ton!”. The same can be said of when brands talk about grey or white down; it makes no difference to performance.
Down gets wrecked when it gets wet – again not wholly true – it just takes a while to dry and can be a bit tedious to clean, though you will only need to clean it very rarely.
Most brands will list a three-tier temperature range on their sleeping bags. Unlike some outdoor tests for garments, like breathability, temperature tests for sleeping bags are uniform across manufacturers. I won’t go into detail about what that test is here; if your interested look up EN 13537 tests. Basically, the test usually gives three ratings:
Comfort Limit: The lowest temperature a standard woman can sleep comfortably.
Lower Limit: The lowest temperature a standard man can sleep curled up and be comfortable.
Extreme Limit: The lowest temperature a standard woman will not die at.
The problem with these ratings is that there is no such thing as a standard woman or standard man. So often you won’t know if they are accurate until you actually use the bag. But the ratings are useful in comparing different bags relative to each other. If you have used a bag that has been rated, then you know whether you need something warmer or not, but if you haven’t used one it gets trickier. In this situation, my advice is that if you aren’t a person that gets particularly cold easily, the ratings can be relatively accurate. If you are buying for a girl, use the comfort limit rating, if you are a guy use the lower limit rating as your guide. If you know you get cold easily, buy a bag with a rating well below the temperatures you expect to encounter.