Choosing a down sleeping bag

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, if looked after properly 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

The two options are down or synthetic. Down can be either goose down, eider down or duck down, is a natural material and is very good at keeping you warm, but if you can’t keep the bag dry then it cannot do it’s job well. Can you keep the bag dry? if this a yes then go for down, if you can’t then 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 uh now you didn't really want to do that did you. It is 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 then maybe a 'good' synthetic sleeping bag would be the better option. Synthetic bags are considerably bulkier and weigh more than a down bag but they do retain insulation when wet. A down bag used in tandem with a tarp would give a significantly better nights sleep than a synthetic sleeping bag used out in the open.

Other options do exist, such as the hybrid dualmax system used by Marmot and developed with Tung Sang, one of the leading sleeping bag factories in the world. Although this looks like being a perfect sleep solution, down is still the lightest most compressible and resilient filling available.

Down quality

Not all down is the same, it can come from different birds and it can come from birds of different ages. So what is the best down? 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 about 750 fill power is exceptional.

Sleeping bag shape

Sleeping bags come in two shapes; Caravan and Mummy. Caravan styles 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 nights 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 then 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 more in keeping with the shape of their bodies, then not only could they reduce 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 the bag that can suck out all that heat you have been collecting.

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 some what 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.

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: Univ 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 that’s it choose your insulation, choose your shape and then choose how much down goes in your shape. Stay warm.