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Spotlight - equipment views and reviews from the AK team

Choosing a down sleeping bag

By Alpkit
01, Apr, 2012

Down versus Synthetic? Caravan or Mummy? Slant Wall or Trapezoid? The design team tells us what to consider when deciding which sleeping bag to buy


Choosing down or synthetic fill is probably the most important decision when buying a performance sleeping bag. Down has many advantages over synthetic insulation, but it is also more expensive and needs looking after. Saying that, well cared-for down will outlast a synthetic bag by many years. So how do you compare all the features and decide which sleeping bag is right for you? This small guide aims to help you through the process of buying a down sleeping bag.

Down vs Synthetic: Choosing the fill

We fill our sleeping bags with goose down, duck down or synthetic fibres. So the two options are down or synthetic. Down can be either goose down, eider down or duck down and is a natural material that's very good at keeping you warm, but if you can’t keep the bag dry it can't do its job. Can you keep the bag dry? If yes then go for down, if you can’t you should ask yourself why not?

When down is superior to synthetic

Down insulation packs down smaller and weighs less than the equivalent volume of synthetic fill required to provide the same level of insulation. It is very efficient at trapping air and keeping you warm in the cold, and since down is also good at wicking moisture it is more comfortable than synthetic in the warm.

When synthetic sleeping bags are better than down bags

What if your down sleeping bag gets wet? Uh oh! It's one thing to shiver through one uncomfortable night (you will probably laugh about it when you get home) but if it puts you in a life threatening situation 4 days from a hot shower, maybe a 'good' synthetic sleeping bag would be the better option. Synthetic bags are considerably bulkier and weigh more than down bags, but they do retain insulation when wet. Don't get us wrong, you won't be warm - the only thing that's warm and wet is a jacuzzi. All the same, a down bag used in tandem with a tarp would give you a significantly better night's sleep than a synthetic sleeping bag used out in the open.

If you're likely to get your sleeping bag wet, a good synthetic bag like the Mountain Ghost is your best bet

Down quality

Not all down is made equal. Down comes from different birds of different ages. So which is best? In general, the best down tends to come from the eider duck or geese, but in particular white goose down provides an excellent balance between price and performance. Down sleeping bags are categorised by their fill power, this number is a measure of the loft, or the amount of space a certain weight of down occupies. Although this value is measured differently around the world, an average quality goose down would be about 500 fill power. For our purposes, things start getting interesting at about 650 fill power and anything above 750 fill power is exceptional.

Sleeping bag shape

Sleeping bags come in two shapes; Caravan and Mummy.

Caravan style bags are what you used as a kid. They are oblong with a zip along the side and bottom, they are really roomy, and you can open them up like a duvet. Don't let anyone tell you different, this is the best shaped bag for a decent night's kip (after all they don't make mummy shaped beds). The only problem is if you want a caravan bag to keep you warm at -40C, you would need a caravan to move it.

The roomier and more comfortable the space inside the sleeping bag, the bulkier and heavier it becomes. This is where the development of the mummy shape came from. In the old days Artic explorers and mountain adventures found that if they trimmed the shape of a square shaped bag to fit shape of their bodies, they not only reduced the weight and pack size of the bag, but they would be warmer as well. The mummy shape (yes they really do resemble the shape of an Egyptian Mummy) is universally recognised as the shape of the modern sleeping bag. By trimming of all that excess space, the bag becomes closer fitting and reduces the flow of air inside that can suck out all that heat you have been collecting.


We often get asked about the sizing of our sleeping bags, the most common question being from someone that is 6ft 3" asking if they will fit in one.

Well, we determined the maximum size our bags using our mate Steve. He is 6ft 2 and has spent quite a few nights in a sleeping bag, so we value his opinion. We put Steve in one of our regular bags and asked him if was comfortable. He said he was but he wouldn't want the bag any smaller. Build wise he is no rugby player, but I wouldn't say he was skinny.

There are plenty of people a little bigger than this who say they have fitted in, but they'll be putting up with a little discomfort; whether the bags a little tight across the chest or is snug and possibly tight on the head. There is no way we can describe this, so we will stick with our 6ft 2in measure and hope that if you are on the cusp you will be able to draw your on conclusions. However if we were 6ft 3" we would go for the next size up.. the long.

The next consideration is width. The PipeDream has a trim cut for the very best performance. If you are a 44" chest or larger, the bag with will feel tight, remembering of course that this isn't a quilt and any mummy bag will feel constricting until you get used to it. If the PipeDream sounds too tight for you, opt for the SkyeHigh series which has a roomier cut than most.

Sleeping bag construction

The construction of a down bag is not rocket science. You stitch together a shell, a liner and then stuff the goose down in the middle. All you then need to do is sew strips of fabric in to stop the down just falling to bottom or sides of the bag. This method creates little 'boxes' of down and is known as Box Wall Construction (BWC).

There are several derivates of BWC but the two most commonly used are Slant Wall and Trapezoid construction. These methods are more expensive than standard BWC since the box walls are cunningly slanted. An awful lot is spoken about these construction techniques,and how they are significantly better (warmer) than standard BWC, but our advice is that you would be better spending your money on the best quality fill, rather than poor fill and marginally better construction. If you spend more on the bit that nature did, you won't go far wrong.

Seasonal greetings - staying warm all year round

The old fashioned way of grading sleeping bags was based on a season rating. You go in to the shop tell the sales guy when you are going to use the bag and you walk out the shop a happy bunny. 1 season = Summer 2 season = Late Spring, early Autumn 3 season = Spring, Summer and Autumn 4 season = Winter 5 Season = Some where really really cold

Well it was a easy system to understand but somewhat open to abuse. Today we all have to be a bit more careful and there are now two systems emerging as the most commonly used in the UK, we won’t attempt to explain the systems as the guys from Ajungilak have produced a fantastic document which if you want to know more is well worth reading. The 'Sleep well' technical document is essential for all gear freaks.

Sleeping bags in kangri

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

You are the Radiator

The wisest word on sleeping bags and their ratings is that it is you who warms the bag, not the other way round. If you do not generate enough warmth to keep you warm then you will feel cold.

A few pointers:

  • Heavier people sleep warmer than slighter 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 then buy a warmer bag,
  • Experienced users can get away with less, as they know the tricks of the trade,
  • Eat well, keep hydrated and warm up before you go to bed. Star jumps are good,
  • Don't forget that the mat you are sleeping on is a very important part of your sleep system. If you sleeping on snow or frozen ground then you need to be thinking of both a closed cell foam mat for insulation and a self inflating mat for comfort.

Temperature rating

If ever there was a mind soup of a problem rating: the sleeping bag rating has got to be number one: there is just so much to take into consideration to give an accurate indication of the performance of a bag at any given temperature. EN13537 is the European Standard for testing sleeping bag ratings that was intended as a cure for all this, the idea being that every bag manufacturer would test their sleeping bags to the same criteria so that we'd be able to compare like-for-like. Great in theory, but as soon you have a committee developing the standard it's bound to hit trouble. A test that tries to take so many things into account has to compromise somewhere.

Comfort Temperature (T comf)
Lower limit of the comfort range down to which a sleeping bag user with a relaxed posture (such as lying on the back) is globally in thermal equilibrium and just not feeling cold (related to standard woman and in standard conditions of use).

Limit Temperature (T lim)
Lower limit at which a sleeping bag user with a rolled-up body posture is globally in thermal equilibrium and just not feeling cold (related to standard man and in standard conditions of use).

Pipedream 400

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

So what is so wrong with EN13537

EN13537 is a standard test which helps you compare sleeping bags but it isn't a good relevant in deciding whether a specific bag is appropriate for you in the conditions you will use it in.  It's a bit like the quoted fuel consumption on a car - performance could be better or worse depending on your driving style and road conditions.

Most of our customers are using their bags in conditions that the EN standard does not take into account, this applies particularly to our bags designed for winter use. That is not to say they are doing anything wrong, it is just that most of us don't sleep in a lab. It is also important to consider the standard in full, for example EN 13537 expands on the commonly cited definition of Limit Temperature by describing the user to be "in a situation of FIGHTING against cold" which is probably not what most people would expect from some brands description of this limit as the lowest temperature at which a standard adult can have a comfortable nights sleep!

The standard had to pick a sleeping mat to base it's rating on. If they had picked a summer sleeping mat then winter bags would be unfairly compromised as the manakin would of felt colder sooner than they would of had they used a suitable winter mat. Equally, if they had used a sleep mat suitable for winter use then the summer bag has an unfair advantage as you wouldn't really be using this mat/sleeping bag combination in the real world. In the end the test sleep mat they chose reduces winter bags rating and increases the summer bags rating, this is no bad thing as it errs on the side of safety for many users.

Having said all that, the biggest problem with the standard is they tried to create a scientific test that attempts to pin down a figure that is so hard to pin down. In the old days you walked into your specialist outdoor shop, and it would go something like this.

Punter: "I am after a sleeping bag.. a warm one."
Shop dude: "3 season or 4 season."
Punter: "erm.. I'm going to Scotland in winter."
Shop dude: "I'd go for a 3/4 season, should be good down to -10c."
Shop dude: ".. or you could go for this one, it's a warm 3 season, I've used it down to -10 with a decent mat, I was only just a little bit chilly. Do you tend to sleep warm or cold.. ?"

With EN13537 you just get figures when what you really need is advice. Plenty of people can get away with a 0c bag at -10c and most of us have frozen our %^$ off in -10c bag at 0c.

Brands are now torn into either following the herd and testing (at some expense) to EN13537 or plucking a figure out of the sky. If EN13537 has helped out it has meant that almost every bag sold in the market comes with some form of rating, and has certainly discouraged some of the more historically optimistic labelling. Remember the Moon Bag? I am sure I was told it was good to around freezing, which it was if you liked near death experiences.

Although EN13537 is not a legal requirement, it formed a benchmark to test against. But like all tests, it should be taken with a pinch of salt; in 2006 the European Outdoors Group (EOG) published the results of its tests designed to demonstrate the success of EN13537. Unfortunately what it did show was that the standard was unreliable.

EOG EN 13537 Test Results

The most important thing to remember is that any test is not a substitute for advice from people that have used the bags, and learning from your own experiences to find what's best for you. If you buy a bag from one of the large outdoor chains they haven't always used the bags themselves and they rely on the data given to them by the manufacturer. We use our bags ourselves, as do our friends and our sponsored climbers when we can drag them out of bed. More importantly we get a lot of feedback from our customers on the trips they go on. So if you are planning an ascent of Kilimanjaro or doing the coast to coast then we have a pretty good idea what will work and what won't. We do back our experiences with scientific testing, for this we use Leeds University against the BS4785-1984 Standard, this give us a rating in m2 K/W. If we times the result by ten we get the TOG rating, and if you put your TOG rating in to calculater (hopefully a old school Texas Instruments one with the Light Emitting Diodes) then out pops a comfort rating wich is more or less in tune with bags tested under EN13537.

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

Before you buy any sleeping bag it is important to think where you might use it; not just the impending trip to Spitzbergen. A good quality bag will last so think of all those extra trips, the Cornish coast at the weekend or the yearly bash in the Cairngorms. Take into account all the conditions you think 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 and then choose how much down goes in your shape. Stay warm.

Alpkit down sleeping bags

Alpkit synthetic sleeping bags

Glossary of technical terms

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