Sleeping Bags 101

8 things to consider when buying a sleeping bag

A sleeping bag has one function and that is to keep you warm. As such it is a comparatively simple piece of equipment, but people sometimes find it difficult to choose the right sleeping bag – temperature rating, weight, materials, shape and price are all variables that will impact on which sleeping bag you choose.

There is also not one sleeping bag that can do it all. If you have the budget you may purchase 2 or 3 sleeping bags for different conditions, whereas new/budget hikers might rather opt for a good “all-rounder” and use other means at their disposal to increase the sleeping bag’s versatility.

1. Understanding temperature ratings

In the 1980’s, sleeping bag temperature ratings mostly referred to the seasons – anything from a “summer” bag to 4-seasons. Nowadays it is more specific and scientific, with reputable manufacturers publishing the actual temperatures at which their sleeping bags can function for an “average” person.

There are some attempts to standardise the ratings, but there are unfortunately also some problems in this regard as ratings are sometimes just not realistic. The applicability of ratings will also be influenced by personal biological factors such as size, circulation, metabolism and energy levels.

Two types of ratings are usually shown, being the “comfort” and “extreme” temperatures, with some manufacturers also showing a “transition” or “limit” rating.

Comfort and transition/limit rating: The comfort temperature is the one at which one should have a cosy night’s sleep in the bag and is the temperature that should align with the expected low-temperatures on your hikes. The transition rating is the temperature at which one will still be fine, but you are “transitioning” into temperatures where you will start to feel cold and will have to use additional insulation (like warm clothes) – as a guideline this should align with the coldest temperatures at which you expect to use the sleeping bag.

Extreme rating: The extreme rating is, in theory, the coldest temperature at which one might survive. This has little practical application as it could mean that you will lose all your fingers and toes, but just not die. The extreme rating is also the one where one finds the most unrealistic figures… I have seen bags where I am pretty sure you would be dead (or wishing you were) if used at the extreme rating. In my opinion the best thing you can do with the extreme rating is to ignore it, as it might give you a false idea of what the sleeping bag is suitable for.

As a guide, if a transition/limit rating is not shown, we suggest that the coldest practical rating for a bag is around 4 to 6 degrees below its comfort rating.

2. Down vs Synthetic Insulation

The sleeping bag’s “fill” is the insulating material inside the bag which is the determinant of its temperature rating. Down (goose or duck) and synthetic insulation are the main fill types. One might also find bags with a mix of down and feathers, and for practical purposes that can be considered as low quality down.

The main characteristics of down vs synthetic insulation are as follows:

Down Synthetic
Lighter weight for the same temperature rating Machine washable and quick drying
Highly compressible Insulates better when wet*
Durable Cheaper

*Contrary to popular opinion, down still insulates when wet, but synthetic insulation tends to retain its thermal properties better than down when wet though.

With down, the manufacturer will also publish the quality or “fill power”, which is a measure of the loft or “fluffiness” of the down. The higher the fill power, the more air the down can trap, and thus the more insulating ability.

Fill power ranges from about 300 for feathers to around 900 for the highest quality goose down. For hiking purposes quality down will start at around 600-fill power, with higher fill powers resulting in weight savings, but exponential price increases.

When buying down products, keep an eye out for companies that are Responsible Down Standard (RDS) certified, which safeguards the welfare of geese and ducks that provide the down, by ensuring against cruel practices such as live plucking and force feeding.

3. Design

The following designs are used:

  • Rectangular: plenty of room for both legs and arms to stretch out (used more for camping than hiking), but warmth is sacrificed for roominess.
  • Hybrid/Semi-rectangular: aka a “modified mummy”, this designation covers a variety of shapes, all of which offer a compromise between warmth and roominess.
  • Mummy: this bag style follows the contours of your body and has a snug fit for maximum warmth.

There is a pay-off between weight and comfort in these designs – all things equal a mummy design will be warmer and lighter but less roomy. For hiking purposes, one will either use a hybrid or a mummy-style bag, with the latter becoming the better choice at extreme temperatures.

Rectangular, hybrid and mummy-shaped bags from local manufacturers.

4. There is no magic – only design and materials

One can get a pretty good idea of a sleeping bag’s functional warmth by looking at the design, the fill (type and quality) and the weight thereof. There simply is no magic by which a lightweight bag can be functional in extreme temperatures – compare your bag’s fill (type, quality and weight) to that of similarly rated bags of reputable manufacturers and see if it all “adds up”. If you venture into sub-zero temperatures, accept that your sleeping bag will weigh more and do not get bamboozled by unrealistic ratings.

5. Versatility and features

Rectangular and hybrid bags might have features like an all-around zip (turning the bag into a blanket); the ability to join bags together and continuous channels which allows you to move the filling around to change the warmth of the bag – read each bag’s specifications to understand what it can and cannot do. These come at a cost of thermal efficiency though and it is recommended to gravitate towards the mummy-shape at sub-zero temperatures.

Some useful features which are found in mummy-shaped, and most hybrid bags, are:

  • Cowl – insulated hood which wraps around the head
  • Drawstrings – that pull the sleeping bag tight over the chest and neck and around one’s head to keep warm air from escaping.
  • Zipper baffle – an insulating “tube” of down-filled material that hangs over the zipper, ensuring that the bag does not form a cold area along the zipper.
  • 3-D foot-box – the zipper stops at the side of the bag near the foot, and the bag is shaped so that your feet are boxed-in. This results in warmer feet and more functional length.

6. Sleeping bag liner/inner

Liners/inners are very useful, but it is important to understand their limitations. A liner is a lightweight (i.e. thin) material sheet that is stitched together on the sides and foot-end, lining your sleeping bag to keep it clean. Especially down bags are difficult/expensive to wash and washing reduces its loft over time, so you want to keep the bag as clean as possible for as long as possible. A normal lightweight liner will add, at best, 1 or 2 degrees to a sleeping bag’s temperature rating.

A liner made of insulating material will add more warmth and is a great way to increase your sleeping bags temperature range. However, it’s alarming to see claims on liners indicating far more degrees than what they are realistically capable of, which could get people in serious trouble if they rely on them to survive freezing temperatures. I recommend that you “feel” the liner’s material and imagine wearing a t-shirt of the same material, estimating how much heat it adds. Liners are often marketed with the rating of “up to x degrees”, which is reason for caution. Generally, I’ve found that the realistic functional warmth added will be a maximum of 50% of the “up to” rating.

7. Increase your temperature range

South Africa generally has mild temperatures and it may be overkill to buy a minus-10 degree bag for an occasional high-altitude hike in winter. Our winter temperatures, even in the mountains, rarely goes below minus-5 degrees, but you have to be prepared to endure colder temperatures. Some ways in which you can manage extreme temperatures:

  • Sleep in a tent (with a buddy): A tent – especially a four-season tent – will retain some body heat and will be warmer than sleeping out in the open or in a cave. Sharing a tent with a few friends can up the temperature inside by a degree or two.
  • Sleep in warm clothing: There’s a popular myth that the warmest sleep in a down sleeping bag is naked. It may be the fastest way to heat up (and direct body-on-body heat is the best way to save a life if someone is hypothermic), but you will most certainly sleep warmer with some clothes on. Thick socks, a beanie and thermal underwear will add a couple of degrees. Add some sweatpants and a down jacket to that, and you can increase the range by a considerable amount.
  • Use a thermal liner (as discussed above): There’s also the (weight inefficient) options of adding a fleece/down blanket (preferably inside the bag) or doubling-up on sleeping bags.
  • Insulation from below: This does not so much add to your sleeping bag’s temperature range, but it ensures that the sleeping bag is effective within that range. You body weight will compress the sleeping bag’s fill below you, effectively nullifying the loft and the bag’s insular capability from below. At freezing temperatures, the ground will become ice cold – it is therefore paramount to have decent insulation to sleep on. The old-school gaper pad’s closed-cell foam is an effective insulator, and if you prefer the comfort of a blow-up mattress, modern inflatable hiking mattresses have insulating foam inside the mattress (pioneered by the popular brand Thermarest). Air-only blow up mattresses do not insulate properly and should not be used in very cold conditions.
  • Re-filling an old bag: Your bag’s fill does lose some of its loft over time, and eventually a bag’s functional temperature will fall well below those it was tested at. Some manufactures will add fill (especially down) to your old bag to spruce-up the temperature range. A complete re-fill is also possible, but the cost may be prohibitive.
This 15+ year old Capestorm Dragonfly bag used to have a transition rating of minus 8 degrees. Over the years the loft has decreased, and it is now only good for the Drakensberg’s warmer months and will need a down top-up to again be functional as a winter-bag at high altitude.

8. Sleeping bag size and weight

It is important to look at the sleeping bag’s dimensions and find a size as close as possible to your body (allowing for some room to move); optimising both warmth and weight.

You should also be aware that a sleeping bag with a footbox has a longer functional length than one without it, as the footbox is shaped around your feet, whereas a flat sleeping bag will be “raised” by your feet, reducing the usable length. The best is to try and find a store that stocks the sleeping bag you are interested in and actually get inside the bag to feel the fit.

For the weight conscious there are bags made from ultra-light outer-fabrics, light-weight zips and high fill-power down. These bags will typically be more expensive and less durable than bags of heavier materials, and one might have to balance the weight of the bag and the weight of your wallet. In recent years we have seen ultra-lightweight down sleeping bags come to market at a reasonable price, especially with cheaper duck-down.


Weather in the mountains can change without warning and in extreme cases staying warm can be a matter of life and death. Make informed decisions and use the right sleeping bag for the job.

About Willem Boshoff 25 Articles
Willem is an actuary by profession and an adventurer at heart. He spends as much time as possible outdoors - camping, hiking, mountain biking, surfing and rock climbing are his activities of choice – and he enjoys reading and writing. He has hiked and trekked in the Himalayas, Andes, Patagonia, Alps, Corsica and done the Camino Portugues, and thinks locally the Cederberg and Drakensberg offers some of the best wilderness-hiking experiences in the world. He is also passionate about conservation and sustainability. He lives in Cape Town.


  1. Worth mentioning that the harvesting of down is extremely painful and cruel to the animals involved.

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