Sleeping Bags 101

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 two or three sleeping bags for different conditions, whereas new or budget hikers might rather opt for a good “all-rounder” and use other means at their disposal to increase the sleeping bag’s versatility.

Understanding temperature ratings

In the 1980s, 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).

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. 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.

Down vs Synthetic Insulation

The sleeping bag’s “fill” is the insulating material inside the bag which determines its temperature rating. Down (goose or duck) and synthetic insulation are the main fill types.

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

Design

The main sleeping bag designs are:

• Rectangular: Plenty of room to stretch out but warmth is sacrificed for space.
• Hybrid/Semi-rectangular: A compromise between warmth and roominess.
• Mummy: Follows body contours and provides maximum warmth.

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

There is no magic – only design and materials

You can estimate a sleeping bag’s warmth by considering its design, fill type, fill quality, and weight. Lightweight bags can’t perform in extreme cold — higher insulation means higher weight. Compare specifications to trusted manufacturers before buying.

Versatility and features

Rectangular and hybrid bags may have features like all-around zips or the ability to join bags together. These options add flexibility but slightly reduce thermal efficiency.

Useful features in mummy or hybrid bags include:

• Cowl: An insulated hood for head warmth.
• Drawstrings: Tighten around the chest and neck to retain heat.
• Zipper baffle: Prevents cold spots along the zip.
• 3-D foot-box: Provides warmth and comfort for feet.

Sleeping bag liner/inner

Liners help keep your sleeping bag clean and can slightly increase warmth. Lightweight liners add around 1–2°C; insulated ones add more but never as much as advertised. Choose realistic ratings to stay safe in cold conditions.

Increase your temperature range

South Africa’s mild climate means extreme sleeping bags are often unnecessary. However, be prepared for colder nights by following these tips:

• Sleep in a tent: A tent traps heat and can raise temperatures by a few degrees.
• Wear warm clothing: Thermal layers, socks, and beanies significantly boost warmth.
• Use a liner: Adds insulation and keeps your bag clean.
• Insulate from below: Use foam or insulated pads to avoid heat loss to the ground.
• Refill old bags: Restoring loft improves performance, especially for down bags.

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.

Sleeping bag size and weight

Choose a bag close to your body size — this optimises both warmth and weight. Try before you buy where possible. Lightweight materials and higher fill power down reduce weight but can increase cost and reduce durability.

Conclusion

Weather in the mountains can change quickly, and staying warm can be a matter of life and death. Make informed decisions and use the right sleeping bag for the job.