The down sleeping bag facts
07 November 2008
Note: Many of these articles are very old, and although the technical information is still relevant the equipment mentioned may not be (for example a Stormy cooker was state of that art in 1995, but not in 2021).
Two years ago I wrote an article entitled ‘beyond car camping’ that dealt with down insulation versus synthetic insulation. The piece was aimed at climbers tackling routes where they couldn’t guarantee that their sleeping bags wouldn’t get wet. Synthetics won out in the article and many climbers afterwards seemed to think that I was saying that down was dead.
Well, to reset the balance I’ve decided to do justice to down and dedicate the whole of this month’s gear notes to our friend the feather, plus take a look at the stand-out down pits on the market that show just what is possible in this medium.
The down in your sleeping bag isn’t just the feathers of any old dead bird. The feathers come from the smaller clusters found on the under plumage of waterfowl like geese and ducks. Land fowl like chickens aren’t used as the quality is too poor - although saying that, I expect you might find quite a few chicken feathers in your budget market stall ‘puffer’ jacket.
This natural fibre, while highly variable in nature, provides more warmth per ounce than any other material. Down’s unique structure is responsible for its tremendous ability to trap warm air, with its filaments divided and sub-divided to reach out into its surroundings, creating dead air space that slows the movement of warm air.
By its very nature, a down-filled sleeping bag is as much as 35% lighter than its synthetic counterpart and is infinitely more compressible, providing the most warmth with the least bulk. Down is also known to have the greatest longevity and it will outlast any other insulation by three to five times, making it the most economical choice after its initial investment. Down also helps the sleeping bag drape luxuriously over the body and ensures good coverage and warm contact. The bigger the down cluster the better the insulation and loft. The size of the cluster is determined by several factors.
The bigger the bird the bigger the down cluster, so that’s why big geese make better down than smaller ducks. The climate in which the bird is raised also has an effect, with thicker clusters being found on birds that need more insulation (unfortunately Siberian geese are rare). Perhaps it’s for this reason that Vietnamese and Caribbean down isn’t as highly prized as more northerly grown down. Most down is a by-product of the food industry and so the size of a down cluster is determined by when the bird gets the chop.
China is the chief source of down and in the last 20 years, they have gone from just supplying the raw product to also manufacturing the majority of the world’s down products. This has led to a dramatic drop in price forcing the home grown down manufacturers to move their focus - like many western industries - from bread and butter bags to the more specialized end of the market. Most of the 500-fill to 600-fill down comes from Chinese geese, with the birds being killed at only 12 weeks (weighing 2.7kg to 3.6kg).
Europe is the second-biggest source of down, with its birds generally being raised to an older age than Chinese birds, meaning most high quality, high-fill down comes from Europe. The Euro birds get the chop at 16 to 20 weeks (6.4kg to 7.3kg), producing a higher grade down of between 650-fill and 700-fill. The highest quality down comes from very old birds of over two years old (9kg). Some of these birds are ‘guard geese’, used to protect the other farmyard animals and are highly prized, meaning this down is not cheap. Much of this down is exported from the former Eastern Bloc, meaning down quality has improved since the fall of the Iron Curtain. Perhaps this is one of the reasons why the Poles climbed all those 8,000m peaks in winter? (Well that, and being absolutely nails.)
Not all birds need to die to keep you warm though. Some Hungarian geese are plucked while still alive, while the platinum premier down comes from the living eider duck. This cliff-dwelling bird is a protected species and so the down must be collected by hand from its empty nests. Personally, I’m not sure who I’d rather not be - a Hungarian goose or an eiderdown collector?
One problem associated with down coming from the food industry is the increasing early age that the birds are processed at, making good quality down from older birds harder to come by. This is an increasing problem with all the natural materials used by the outdoor industry (including leather), as intensive farming lowers the life span of the merchandise. Once you’ve got the raw material the next step is preparing it so that it can be used to make your sleeping bag.
The long and complex process of plucking, washing, drying, dedusting, sorting and blending, has a huge effect on the final quality of the down. The down is wet-plucked by processors who rip the feathers off the bird in the initial food preparation stage. Dry-plucking is now uncommon but it’s far gentler to the down. The plumage is then thoroughly washed to get rid of all the dirt. If this isn’t done correctly it can ruin the delicate structure of the down. In Europe plumage is steam-dried immediately after wet plucking, whereas in China it is traditionally spread out to air-dry in the open - a method which calls for extra treatment later in the cleaning process.
Eiderdown is hand-gathered from the ducks’ nests and contains many foreign substances, twigs, etc. The peculiar nature of the down makes it impossible to blow through a sorter, so these impurities have to be picked out by hand. A long and costly process (eiderdown twig plucker?).
Once clean the down is carefully dried and sorted. Sorting involves the plumage being blown down a chamber full of cubicles, with the heaviest material (feathers) falling into the first cubicle and the lightest (the down) landing in the further cubicles. Throughout these processes, the down is dedusted to remove any undue particles.
The final process is blending the different specific mixtures of different downs or of down and feather (80/20, 60/40 etc). It is also a useful way of ensuring consistent quality throughout a batch of down by mixing it thoroughly after sorting and before bagging. To ensure consistency between winter and summer down from the same source some suppliers store down for months and blend it throughout the year.
Roughly speaking fill power is a measure of how much volume is taken up by a given quantity of down. The higher the fill power the loftier the material, loft meaning the amount of air held within the material, which translates into insulation (still air is a great insulator remember).
So if we were to compare the fill power of one pound of potato peelings with one pound of cotton wool balls, the cotton wool would win as it would take up a much larger volume, (something worth remembering if you’re forced to make your own sleeping bag out of household items).
Firstly there is no such thing as a ‘World Standard Fill Power Test’. Each country uses its own system, UK, USA, Japan, Sweden, Germany, etc. The basic elements of the tests are, however, common to them all. Down fill power is determined by the number of cubic inches a small amount of down occupies. The down is fluffed up by an air blower, then kept at a controlled temperature and relative humidity (21°C and 65% humidity to prevent static from giving a false reading) for a minimum of three to five days. Then a small sample is taken, which varies between 20g, 30g or 1oz depending on the country.
This down is then placed into a clear tube (24cm x 60cm in the US) and agitated. A weight is then placed on to the down and allowed to settle. The poorer, denser feathers will compact more than the finer, loftier feathers and so a measurement can be made to find the cubic inches per square ounce (i.e. 500, 600, 800 etc). The test is only accurate to within 5%, so your bag may not be quite as lofty as it appears unless the tag claims a minimum fill power. The problem is that because of the size of the tube, the sample of down and lid weight vary from country to country; one man’s 800-fill will be another man’s 650-fill. The US test quoted by most US companies gives a result 4% higher than the traditional UK ‘Lorch’ test. Most serious outdoor down manufacturers do extensive in-house batch testing to make sure what they say on the swing tag is what’s stuffed in your bag. LONG TERM FILL POWER?
Down processing technology has increased in sophistication over the last decade allowing processes that can increase the fill power of cheaper down. This gives the customer a budget bag with a seemingly expensive fill rating. The problem is that this bag will soon lose this added loft, leaving you with a lacklustre and disappointing sleeping bag. I know two sponsored climbers who recently found themselves stuck together high on a mountain in sleeping bags of apparent equal weight and fill. One bag was made in the UK and lofted so well it looked as if the occupant had a bad case of gas, while the other ‘top’ range bag made in the Far East resembled an old sock.
The lesson is that if you want the best then buy premier bags from the companies that have a good reputation and steer clear of the meat and gravy ‘festival’ and car camping bags manufacturers making bags by the tens of thousands. Remember a good bag shouldn’t be cheap, as there are no real bargains in this high-quality market, but good down will take a lifetime’s abuse, dry cleaning and stuffing.
* 400-fill is low-grade insulation that is used in the budget sleeping bag market and for bedding. * 500-fill is your entry-level performance fill and is used by most suppliers in their cheaper bags. These bags are usually made in China close to the down supply so as to keep the cost low. These bags can be bought for half the price you used to get them for when they were made in the UK. * 600-fill is good quality down, but this quality of down requires work by the supplier to find and maintain this level making it more expensive. * 700-fill is a rare and expensive product. Very few goose downs achieve a higher fill-power than this and for duck down it is the very top limit. * 800-fill, although bandied around in catalogues as if it grows on trees, a true 800-fill is extremely rare and expensive and only comes from one or two sources. * 900-fill is the stuff of marketing departments, overeager catalogue copy writers and legends.
Now the down is ready to be placed into the sleeping bag, but you should not confuse a mega-fill as a sign that the bag is well-made or an efficient insulator. It is now that the skill of the manufacturer comes into play in order to squeeze the most performance out of the down used.
The whole point of high quality down is to provide warmth for the least weight (otherwise you might as well use feathers - much cheaper and just as warm if you put enough in). To maximize the effectiveness of the finest fill requires good design. Down, with the highest fill power in the world, will not provide the best weight/warmth package if the shell uses inferior materials or poor construction. And such is the price of the best downs that they are an expensive folly unless matched by design and craftsmanship of an equally high order. DOWN CONTROL
Firstly, it is not the down that is keeping you warm at night, it is the air trapped within its complex structure. For maximum performance this combined structure of thousands of feathers must remain stable and to do this you need down control. This is done via the use of baffling throughout the bag and is also used to control the distribution of down throughout the bag. Poor baffling allows the down to shift and leaves gaps and cold spots, causing the down to bunch up and lose its efficiency.
Baffles are generally made in two ways. The simplest is a stitch-through baffle where the down is held between the inner and outer shell. This is cheap to do and saves weight, but the stitching line is an insulation weak point. The most common method is to join the shells via strips of netting, thereby forming boxes into which the down can then be stuffed. This is highly efficient and creates a good insulation block around the sleeper devoid of cold spots. The real test of a manufacturer is how they construct these baffles. If baffles are too large, down can move around across a larger space. Smaller baffles keep the down within a smaller area but add weight and complexity. A good way to achieve down control is to fill the baffles densely enough so that the friction within the baffle prevents movement. Sufficient density can be seen when the shell fabric firmly bulges between baffle seams.
Designers like Rab Carrington and Pete Hutchinson have pioneered other forms of baffling like cross-baffling to reduce gravity shift and trapezoid baffles that eliminate cold spots. Mountain Equipment’s excellent elasticized inner on their Extreme bags is a very effective way of achieving effective thermal contact.
Good quality bags can be split into two camps. Firstly there are those that are used with an emphasis on protection, with the shell - and often sections of the inner - using highly water-resistant fabrics. These bags are intended for general purpose use, be it camping, climbing or expeditions. Shell materials like DriShell, DriLite, Endurance, Conduit and Epic are just about waterproof and very tough and go a long way to keeping their delicate insulation healthy, taking some of the stress out of using a down bag - revolutionizing the down market. These materials are more expensive and heavier than plain unproofed fabrics but the weight penalty and cost is worth it if your bag stays dry.
The second category of bags is the ultra-light variety, where the shell materials are chosen for their gossamer-like weight. These fabrics can shave hundreds of grams off the total shell weight and also help the bag to loft and drape to the down’s maximum advantage. The drawbacks are that they are non-water resistant (they are coated with DWR but this is only cosmetic) and are far more fragile than full weight fabrics. Another drawback is that they are less down proof so you may get some feathers poking through the weave. If this happens don’t pull them out as this will make a small hole. Instead, pull the feathers back through into the bag. For the weight-obsessed mountaineers, this is a sacrifice worth taking in order to shave a couple of hundreds of grams off their backs and all that is required is a little more care.
* Neck baffles reduce the effect of convection drastically, stopping warm air from escaping when shifting in the bag. The complexity and weight of the baffles depend on the intended use and temperature range of the bag. Some ultra light bags don’t feature a neck baffle and depend instead on just the draw cord around the face. * Zips add weight to the bag, both due to their own weight and the weight of the baffling and anti-snag tape, seams etc. By losing the zip, or reducing its length you can save a great deal of weight. The drawbacks are the bag is harder to vent, but this isn’t a problem on ‘summer’ weight bags.