Today’s sleeping bags offer a huge and growing variety of features and designs. Even the least kit savvy person could point out that they come in two varieties; synthetic or down insulated, but, even though this may be one of the most important choices to make, it’s by no means the last. For ultralight touring in particular there are many variables that can add or reduce weight to your bag beyond the filling type and quantity. Here is a breakdown of of what I think are the most salient.
Good sleeping bags will come in a range of sizes, usually at least short, medium or long. It makes a lot of sense to get the bag that’s right for you. If you’re tall, not having enough leg room can be very frustrating but more importantly, if your feet are pushed against the bottom of the bag you’ll compress the down or synthetic insulation which will let heat escape leading to cold feet. If you’re short then you have the good fortune not to need the extra weight of a medium or long bag. You might also find you will warm the sleeping bag faster as there are less air pockets to heat.
Down or Synthetic
One of the big decisions is choosing between synthetic or down insulation. Down offers the highest warmth to weight ratio. It’s also very compressible, saving you space. However the performance of down rapidly decreases at it becomes damper – if your down sleeping bag actually becomes wet then it will be almost useless at keeping you warm. Recently there has been lots of hype around treated down that stays drier and thus warmer in damp conditions. The down feathers are covered in a durable water repellent (DWR) which essentially makes them hydrophobic. There is a small weight/warmth penalty for the hydrophobic down as well as extra cost but you can bet that within a few years the competition of different treatments and brands will bring the prices tumbling down. No pun intended. The treated bag might be colder, compared to a non treated bag when totally dry, but after a few consecutive nights of camping, the non treated bag will have become damp and lost their edge, from then on the treated down will be warmer. That is, until the normal bag is 100% dry again and its insulative effectiveness is fully restored.
Synthetics on the other hand are very durable, resistant to moisture and perform adequately when damp, but don’t compress well and have a poorer warmth to weight ratio. I once had a full bottle of methylated spirits leak into my old synthetic sleeping bag, it was totally soaked. I lay the bag outside for the night and slept in my clothes. It was a miserable nights sleep but the next day my sleeping bag was perfect. It didn’t even smell. The meths had just evaporated. I can’t say that down would have been any worse off soaked in meths, but I wouldn’t like to try it! Synthetic bags are pretty tough.
If you’re not sure which way to go know that, most people, including myself would advise a down sleeping bag over a synthetic one. In practice sleeping bags are not hard to keep clean and dry and, I think, that the benefits of down outweigh the disadvantages when compared to synthetics.
Down comes in different fill power ratings. The fill power of down is measured as volume (cm³) to weight (g). So an example would be a fill rating of 500cm³/g which means 1 gram of this down will fill a space of 500cm³ under laboratory conditions. The more air space the down occupies the more air can be trapped by it and this ability to trap air correlates to the insulating performance of the down. In theory, the higher the fill rating the warmer the sleeping bag will feel.
Fill Rating Isn’t Everything!
Fill power is only the start of the equation as to how warm a sleeping bag is, a better way to get an accurate measurement is by following the European temperature rating standard ‘EN 13537’. The tests are conducted using realistic conditions in independent laboratory settings and take into account the sleeping bag as a finished product – its size shape, and build quality will all play a part in its rating, not just its filling. EN 13537 is the most accurate way of comparing one bag against another. Be careful of companies that don’t show EN 13537 ratings. In particular many American companies show their own, similar looking, ratings resulting from non-standardised tests and can’t be compared to other sleeping bags from other manufacturers.
Within fabrics you typically have Ripstop and non-Ripstop nylons. Ripstop fabrics use heavy threads woven into the fabrics in a grid format, this greatly improves the abrasion and tear resistance of the fabrics whilst adding marginal weight. Some of the strongest and lightest nylon fabrics made at the moment are produced under the Pertex brand such as the Pertex quantum line. It helps if the outer shell of the sleeping bag has a DWR coating, especially if the down is not treated. Many companies use their own fabrics, or rebranded third party fabrics. If you can find it, fabric weight will tell you something about how it will feel and how strong and light it will be. For example, Pertex Quantum GL fabric weighs 25g / m2.
The shape of a sleeping bag is a more personal choice. The more snug the fit the less the bag will weigh. Similarly, you may opt for a roomy toe box or not, a hood or not and even a zip, or not. Some of the most minimalist sleeping bags are just a hood-less, zip-less tapered sack that you climb into from the top.
Hoods and zips are both relatively heavy optional components of sleeping bags; even having a reduced length zip can save precious grams. An often overlooked feature that adds to the weight, whilst also adding to the warmth is a draft excluder. A sleeping bag’s draft excluder is a tube of insulated fabric, often using the same insulation as the rest of the bag, that runs down the inside of the sleeping bag behind the zip. It stops air and heat transfer through the zip. Obviously, without a zip you would have no draft excluder so you’re saving twice by eliminating that feature and opting for a climb-in sack design.
Finally you should look at baffles. Baffles are the walls that separate the down in a sleeping bag into compartments. There are many different techniques to increase the down’s efficiency but as a general rule, the more baffles you have (presuming they are well placed) the more efficient and thus warmer the bag will be. Look for a design that keeps the down positioned on the top of the sleeping bag and stops it slipping to the side or clumping up in sections with the least amount of baffles. That should give you the best balance of warmth and light weight.