How to choose a sleeping bag - an Alpkit guide

By Kenny Stocker

Down versus synthetic? Box-wall or stitch-Thru? Caravan or Mummy? Slant Wall or Trapezoid?

Oh my! There are so many decisions to make when you’re buying a sleeping bag, and although some bags are made to be versatile, there’s no one-size-fits-all that will suit every situation. This guide should help you through the process of buying a down sleeping bag.

Choosing the fill

We fill our sleeping bags with duck down, goose down, or synthetic fibres; this brings the choice down to down vs. synthetic.

  • Bulk: very compressible - packs down smaller than synthetic bags.

  • Weight: A higher warmth to weight ratio, down is warmer per gram than synthetic insulation.

  • Thermal properties: traps your body heat and keeps you warm more effectively than synthetic.

  • Comfort: down is good at wicking moisture, making it more comfortable in warmer weather.

  • Performance when wet: a good synthetic sleeping bag will retain some of its thermal properties when wet, it will also dry and recover much quicker.

  • Cost: Synthetic sleeping bags tend to be a lot cheaper than down sleeping bags.

  • Care: Synthetic sleeping bags tend to be easier to wash than down sleeping bags.

Our Conclusion

Down is our first choice for warmth, weight, bulk, and comfort. The question is, can you keep your sleeping bag dry? If yes, we would always choose down fill over synthetic. If no, synthetic gives you a bit of reassurance.

Fill quality

Not all down is made equal! (if only things were so simple…)

The best down comes from eider ducks and geese, but the best balance of price and performance comes from white goose down.

Fill power denotes how lofty down is (or how much space it takes up). The higher the fill power, the better the down. An average quality goose down would have a fill power of about 500, but things start to get more interesting at around 650; anything above 750 is pretty darn exceptional!

If you're likely to be in particulaly damp conditions, or potentially get your sleeping bag wet, a good synthetic bag like the Mountain Ghost is your best bet


There are two shapes to choose between: caravan and mummy.

  • Rectangle shaped bags with a zip down one side.

  • Really roomy, great if you want a natural sleeping position, and excellent if you’re sleeping over at your mate’s house.

  • Not so good if you’re trying to keep warm and it’s -10˚C out…

  • In the days of old, Arctic explorers would cut down their sleeping bags to fit the shape of their bodies; now we do it for you.

  • Cut to fit your body shape so doesn’t give you the room to move that a caravan sleeping bag does.

  • Less space means less airflow inside and retains your body heat.

  • Less excess sleeping bag means it's lighter and less bulky.

Our conclusion

Warmer, lighter, and more compact? We’d go for a mummy sleeping bag every time


Baffled by baffles?

Put simply, a sleeping bag is made of a shell fabric and a liner stuffed full of down. To stop the down from migrating around the inside of the bag and creating cold spots, you can choose between Stich-Through and Box Wall construction.

  • Stitch-through is the simplest construction technique, where the shell and liner are stitched together to form channels, preventing down migration.

  • Stitch-Through construction is simple and lightweight, however it creates colder spots at the seams, making it more suited to warmer weather bags.

Box Wall construction
  • For Box Wall construction, we make ‘baffles’ by sewing vertical walls of fabric between the liner and shell. This creates ‘boxes’ of down to prevent down migration and cold spots in the bag, it also avoids cold spots around the seams that are the typical of Stitch-through construction.

  • There are a few variations on Box Wall Construction which are significantly more expensive (ie. Trapezoid and Slant Wall construction). Opinion varies, but we value good quality down over fancy baffles.

Our Conclusion

If we’re off for a fast and light adventure in warm conditions, we’d opt for Stitch-Through, but as soon as the temperature starts to drop we’d be switching to a bag with Box Wall Construction.

Temperature Rating

Can you really put a blanket figure on how warm someone will be a sleeping bag?

The idea of a temperature rating is to give an accurate idea of the temperatures that you could use a sleeping bag in. It’s super handy, but we’d recommend taking temperature ratings with a pinch of salt. Why? Because there are so many variables to consider for an accurate indication of a sleeping bag’s performance. One of these variables is you! We all have different bodies, so we all have different requirements for a good night’s sleep!

One of the most common rating systems you will see in the shop is the EN ISO 23537 system. This is frequently misquoted so it is well worth double checking if people are speaking about Comfort, Limit or Extreme - they have very, very specific meanings, here it is quoted directly from BS EN ISO 23537-1:2016.

Comfort Temperature (Tcomf)

Lower limit of the comfort range, down to which a sleeping bag user with a relaxed posture, such as lying on their back, is globally in thermal equilibrium and just not feeling cold.

Limit Temperature (Tlim)

Lower limit at which a sleeping bag user with a curled up body posture is globally in thermal equilibrium and just not feeling cold.

Extreme Temperature (Text)

Very low temperature where the risk of health damage by hypothermia is possible. Note: This is a point of danger which can lead to death.

We don't think you will be camping out in a lab, but it is a useful sanity check. For us the most useful figure is the Limit Temperature, which more or less equates to our own recommended AK Sleep Limit. Don't be fooled by some people who mistakenly refer to the Limit temperature as a 'Comfort rating'.

AK Sleep Limit

We have created the AK Sleep Limit to use alongside the EN13537 measurement. This value is what the majority of customers will find to be the tipping point between a comfortable and restless night’s sleep.

TOG Rating

We get our sleeping bags tested at Leeds University. They give us a TOG rating that extrapolates in to a comfort temperature. The higher the TOG value, the warmer the bag. The comparison chart below shows the sleeping bag insulation required by an average person.

Season Night Time Temperature Thermal insulation
Summer 15ºC to 8ºC 3 to 5 TOG
Spring/Autumn 10ºC to 0ºC 5 to 8 TOG
Winter 3ºC to -10ºC 7 to 10 TOG
Mountain -5ºC to -20ºC 9 to 12 TOG
Polar/High Mountain -15ºC to -40ºC 11 to 16+ TOG

Source: University of Leeds

EN13537 ratings can be helpful, but no test results can quite measure up to experience and your choice of sleeping bag depends on personal preference and usages

Considerations when using EN13537

EN13537 is excellent for helping to compare sleeping bags, however you should not base your decision solely on the rating, as many people use their sleeping bags for conditions that are not taken into account by the tests.

For example, we all use different sleeping mats, sometime we even change out sleeping mats according to the seasons. EN13537 had to pick one sleeping mat to base its rating on which would cover summer and winter use. The EN13537 has created a scientific test that attempts to pin down a figure that is so hard to pin down.

EN13537 is highly useful, having set a precedent that encourages brands to give accurate indications of the temperatures that their sleeping bags are appropriate for, and preventing optimistic temperature ratings. However, there’s no substitute for experience, so we’ve compiled a list of considerations for choosing a sleeping bag.

For more information on EN13537, take a look at the

EOG EN 13537 Test Results

You are the radiator: two people in a two-man tent is warmer than one person in a two-man tent

A few pointers…

Whatever the sleeping bag’s temperature rating, it’s you that heats it up in the first place. The warmer you are, the more warmth there is for your sleeping bag to retain.

A few pointers for choosing a temperature rating:

  • Heavier people sleep warmer than lighter people.

  • Women sleep colder than men.

  • Younger people sleep warmer than older people.

  • Unfit people will sleep colder than fitter people.

  • If you have the heating on in your house during the summer months, you’ll probably need a warmer bag.

  • Experienced users can get away with less as they know the tricks of the trade.

  • Eating well, keeping hydrated and warming up before you go to bed with some star jumps will help to warm your bag.

  • Two people in a two-person tent is warmer than one person in a two-person tent.

  • Your sleeping mat is part of your sleeping system, there’s no point in having an excellent sleeping bag with a terrible mat separating you from the ground.

So… ready to decide?

A good quality sleeping bag will last, so before you buy any sleeping bag it is important to think of all the places that you might use it; not just that impending trip to Spitzbergen. The Cornish coast at the weekend or the yearly bash in the Cairngorms.

Consider all the conditions you might use it in; damp snowholes, stormy boatdecks, leaky tents and even the odd night on the sofa. Tough, isn't it? You want to use it everywhere!

So choose your insulation, choose your shape, choose the construction, and choose how much down you much down you want to put in it. And above all else: stay warm!

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