It's trendy to go fast and light, but many adventures still demand overnight stays in the mountains. Whether you're bagging winter Munros or big walling in Yosemite, choosing the right bag will make the difference between snoozing and shivering.
Down or synthetic?
Insulation materials used in sleeping bags fall into two categories: synthetic and down. Despite advances in synthetic insulation, down still can't be beaten for its outstanding warmth-to-weight ratio, and the best down bags are almost twice as warm (or half the weight) as their synthetic counterparts. But, before you leap to the conclusion that down is always the perfect choice, let's look at the other factors.
A quick browse in any outdoor shop will confirm one point: a good down bag costs at least twice as much as a synthetic bag with equivalent temperature ratings. But, as down maintains its insulating properties (loft) for at least twice as long as any synthetic filling, taken over the usable lifespan of a bag, these costs equal out. On the other hand, anyone who's spent the night in a wet down bag will be quick to confirm that it isn't an experience worth repeating.
A down bag will lose almost all of its insulating properties when wet, and will take a long time to dry out and become effective again. Synthetic bags retain a much higher proportion of their insulating value and, as they absorb much less moisture, will dry out much faster. For a guaranteed good night's sleep you've got two options: choose a down bag but take great care to keep it dry, perhaps opting for a water-resistant shell, or spend far less on a synthetic bag, accepting its penalties in weight, bulk and longevity.
Synthetic bags
Synthetic insulation is produced by the petrochemical industry. There are many competing fill materials, all claiming better warmth or greater useful lifetime than their rivals, but they all work by trapping air between polymer fibres. Some materials trap more air by using extremely fine microfibers, or hollow fibres. These insulating fibres are usually thermally bonded to a backing fabric to hold them together, which is then attached to the shell of the bag. Well- designed bags will overlap, or offset, insulation layers so that cold spots are not created; look for terms such as 'shingle' or 'double-offset layer' which describe such construction.
Down bags
Down is a natural material, grown as part of the feathery plumage of birds such as geese and ducks which live part of the year in a cold environment. Down has a different structure to normal feathers, becoming almost spherical to trap as much air as possible between its filaments. Most natural materials can show wide variations in quality and performance – and down is no exception. This makes the sourcing, handling and grading of the down an important part of making a good bag.
Once the down has been selected, washed and treated, it needs a bag to go into. But getting the most out of the down, and ensuring that no nasty cold spots develop during the night, is where good design comes in. The very best bags aim to get exactly the right amount of down in each compartment of the bag and hold it in place with internal walls or baffles. There needs to be enough space for the down to 'loft up' as it traps air, but not too much, as this allows the down to migrate and leave areas without insulation. Some areas of the body need greater insulation than others. The end result is a complex 3D architecture of panels and seams, all designed to keep you toasty warm.
Testing and standards
Buying a sleeping bag used to be a real lottery. Season ratings were used, and it became the industry norm to greatly overstate the performance of synthetic bags in an attempt to go head-to-head against down bags. The introduction of a European standard (EN 13537) has levelled the playing field a little. The standard is by no means perfect, but it has stifled some of the more outrageous claims, allowing synthetic bags to be sold on their true merits of low initial cost and warmth when wet.
The warmth of the bag is measured using a heated manikin, with temperature sensors on its 'skin', placed into the sleeping bag. A climate chamber sets the external temperature, and the difference between the chamber temperature and the skin temperature gives a measure of the bag's insulation. From this the following is derived:
Upper limit: temperature at which a standard man will sleep without perspiring, assuming hood and zips are open and arms are outside the bag.
Comfort: a standard woman will be able to sleep in a relaxed position.
Lower limit: a standard man will be able to sleep curled up.
Extreme: a standard woman will survive six hours without hypothermia (but may suffer a cold injury).
When choosing a bag, use the comfort and lower limit temperatures as a guide. These are only a guide, because human factors come into play. Women generally feel the cold more, as does anyone who is exhausted or hungry. It will also feel colder if it's windy or humid. A good sleeping mat makes a big difference if the ground is cold, and the list goes on. Wearing a set of thermal underwear, socks and a hat at night will add surprising amounts of warmth, so always plan on having these with you.
Dan Middleton is the BMC Technical Officer.
Expert Q&A
This issue's expert is Neil McAdie. He's been relying on down sleeping bags for the last 35 years. For the last decade, he's also worked for Rab, the specialist down jacket and sleeping bag manufacturer.
Q. What does fill power mean?
A. Fill power is a measure of the lofting ability of down. A given weight of down with a higher fill power will loft to a greater volume (and so trap more warm air) than the same weight of down with a lower fill power. For activities where weight is important, choose a high fill-power bag.
Q. Many down fills are 90% down and 10% feathers. Can you get 100% down?
A. Yes. However, it's the fill power, not the ratio, that determines performance.
Q. Is down produced ethically?
A. At Rab, all of our goose down is sourced from free-range farms in Eastern Europe. We use Eastern European down as the birds are raised to an older age than in China, and the biggest down clusters come from more mature birds that have developed in a cold environment. The down is a by-product of the meat industry; it's not live-plucked.
Q. Do I need a weather-resistant shell?
A. A weather-resistant shell (at Rab we use Pertex Endurance) will help protect the down from external moisture – be that condensation in tents, a dripping snowcave or spilt tea. This can make a big difference, especially when winter camping and mountaineering. However, any water-resistant sleeping bag shell will slightly increase the weight and reduce the breathability of the bag; more frequent airing is required to maintain the loft on multi-day trips.
Q. Where can I get my bag cleaned?
A. Synthetic fill bags can be easily washed in a large washing machine or by hand. You can also wash your own down bag, but handling a heavy, wet down bag risks damaging the fragile internal baffles. We recommend professional cleaning (try www.elitecleaningandaftercare.co.uk). Using a lightweight silk sleeping bag liner also minimises the need for cleaning. Repairs to Rab bags are carried out at the Rab factory in Derbyshire (the chances are your vintage bag may be repaired by someone who first made it back in the 80s!). For repairs to other brands, try specialist repair companies such as Mountaineering Designs or Lancashire Sports Repairs.
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