For sleeping bag temperatures the EN13537 is the independent standard is used by nearly all reputable sleeping bag manufacturers and it produces two key ratings, the first is the 'Comfort Rating' which is the lowest outside air temperature at which a standard Woman can sleep comfortably in the bag. The second rating is the 'Low Comfort Limit' and is the lowest outside air temperature at which a standard Man can sleep comfortably in the bag. Differences in physiology between men and women produce these different ratings.
Both these ratings assume the sleeper wears a baselayer, hat, socks and insulating pad. In theory Men should be interested in the Low Comfort Limit, and Women the Comfort rating. However one component of being warm is our metabolism, some of use will 'sleep warm' and others will 'sleep cold' - a knowledge of our own tendency will also inform our decision. Therefore if you are a man and the lowest temperature you expect to use the bag in is 0C then look for bags with a 0C Comfort Limit, but if you are a cold sleeper you may want to go for a -2C or even -5C bag.
Also there are other factors that will affect how warm a bag we need such as... will we be in a two skin tent or single skin? Mountain hut or bivy bag? Are we prepared to wear more clothing when needed? What about a sleeping bag liner? How good is our sleeping pad? Etc.. For a fuller discussion on EN Ratings and other factors... READ MORE...
The filling provides the insulation and stops you getting cold. Nearly all the bags we sell are filled with good quality 'down', this is because it beats any synthetic insulation hands down when it comes to keeping us warm for a given weight of filling and also packs down a lot smaller. Given the better performance of down sleeping bags versus synthetic equivalents why would anyone choose to buy a synthetic bag?
Most people will therefore choose to buy a Down filled bag. Down is graded into various qualities, higher quality down is more effective at keeping you warm, so you need less of it resulting in a lighter sleeping bag. Sleeping bag construction also plays a part in how effective the down is because if the down doesn't have space to expand into (or 'Loft') it will not be as effective. You could simply be guided by the EN ratings as to how effective a particular bag is, or you could look more deeply into it - for a fuller discussion of Down, Fill Power and Hydrophobic Down...
The insulation in the bag keeps you warm by trapping a layer of still air around your body, thus reducing heat loss by Convection. In an effort to eliminate all possible space, choose a sleeping bag with an appropriate length for you. Therm-a-Rest bags come in three sizes: small (5 ft. 6 in.), regular (6 ft.), and long (6 ft. 6 in.). Pick the size closest to your measured height.
It is also easier to trap the warm air in if there is less volume of air to keep still, this explains the popularity of Mummy shaped bags which tend to be warmer for a given weight because there's less air to keep still, but that's no good if you feel restricted and can't sleep because of it. Larger bags or rectangular ones may be more comfortable for you and it may be worth the extra weight if it means you get a good night’s sleep. Step 3 is about deciding if you need the most efficient Mummy style bag or something larger that you feel more comfortable in.
However, you may simply wish for a more spacious, comfortable shape to your bag so you can move around more. If so, there are rectangular and semi-rectangular shaped bags. These shapes generally add weight and are not as focused on warmth so suit a more casual 2-3 season use for family camping trips or fair weather backpacking trips.
Another popular alternative is the use of a quilt instead of a traditional bag. With a lot less material needed there is an instant weight saving and in mild conditions you may not want the full body wrap style of a traditional sleeping bag. Paired with the right mat for insulation from below quilts can be used year round with a few adaptions to your system.
Features may not make your sleeping bag warmer per se, but they may make it easier to get in and out of, easier to regulate temperature or protect you against draughts and moisture, all important considerations.