Sleeping bags are categorized by fill, shell, size, shape, internal space, weight, compatibility and temperature rating. The required temperature rating depends on the time of year the bag will be used, the elevation and the local climate and environment. Be aware that weather in the Rockies can change overnight, and it is not uncommon to encounter temperatures below freezing combined with snow even in July and August. Therefore, you should consider a bag that is suitable for subzero temperatures even if you only intend to use it in the summer.
Of the several types of fill, down is the best choice. The rest are all synthetic imitations of down. The superb features of down are that it has high loft (the higher the loft, the warmer you will be), it is lightweight and has great compressibility. The only drawback is that when down gets wet it becomes useless. It will no longer keep you warm, it loses its loft and gets heavy and it takes forever to dry. You might as well pull up your tent and head home.
The synthetics are an adequate, less costly alternative to down, and of course, some are better than others.
The main advantage of synthetics over down is that when they get wet, they will dry out in a reasonable time, allowing the journey to continue. Synthetics do not have the loft or compressibility that down has, though, and they are slightly heavier. Hollofil, Hollofil 2 and Quallofil, though considered a technological alternative to down, are the least-regarded synthetic fills. These bags are bulky, heavy and do not compress well, but they are inexpensive. Primaloft, Primaloft 2, Liteloft, Microloft and Thermolite Extreme are better performers but still they are not the best. Although they are quite compressible, they will not stand up very well in the long run.
Polarguard, Polarguard 3D and Polarguard HV are the top synthetic fills and currently hold 75 per cent of the market. They are very compressible, have good loft, which they maintain longer than the others, and they are very durable. They are also lighter than the other synthetic fills.
When purchasing a sleeping bag, choose it for size as well. There are mainly three adult sizes: small, medium and large. The bag should leave room to pull the hood around your head while leaving a few inches at the bottom for leg movement, but not too much legroom. All empty space in the bag must be warmed with body heat, and wasted space means wasted heat. Spending the night warming up dead space in the bottom of a sleeping bag can make for a very long, cold night. Consequently, when it comes to retaining warmth, a mummy bag is far superior to a rectangular bag. A common problem with some campers, however, is that they feel claustrophobic in a mummy bag, so if you have difficulty in elevators, you will probably have trouble in a mummy bag. To respond to this problem, some mummy bags are outfitted with elastic waist and leg bands to allow for expansion.
Another relevant feature is a double-ended zipper (having a zipper pull at both the top and the bottom of the bag), as this allows partial unzipping along the bottom to permit cooling and movement. There are good rectangular bags available for those who just cannot tolerate a mummy bag.