Sleeping bag Fabrics & Fill

A feature of all sleeping bags (and down clothing too) is that the fabric does not absolutely stop every last bit of down from escaping. You will find the odd little feather floating around your tent in the morning. Some people get upset about this, but i

The outer cover of the sleeping bag is called the shell. It has to hold the down inside while being as light as possible and breathing out water vapour really well. This usually requires a really fine tight-weave synthetic fabric, and there are not too many manufacturers in the world who can make this well. The brand ""Pertex"" is very well known one here: it came from Perseverence Mills in the UK (now owned by Mitsui in Japan). They were arguably the first firm to produce such specialised fabric for sleeping bags. There are some very fine Japanese fabrics as well, but I know less about them.

A feature of all sleeping bags (and down clothing too) is that the fabric does not absolutely stop every last bit of down from escaping. You will find the odd little feather floating around your tent in the morning. Some people get upset about this, but it really does not matter. There are probably billions of downy feathers inside the bag: the loss of a few makes no difference at all. Yes, it could be prevented, but only by doubling or tripling the weight of the fabric. Anyhow, what gets out usually comes out the stitching holes, not the fabric, unless it was actually a feather rather than a down plumule.

The Hazards of DriLoft and other coated fabrics

For some reason (mainly associated with some serious marketing efforts as opposed to any logic imho) some manufacturers have switched to a Gore-Tex fabric called DriLoft for the foot of the bag, or even the entire outer shell. This fabric is supposed to be as light as Pertex and breath as well as Pertex but be waterproof as well. There are other membrane fabrics used these days too. The logic is that you can get drips of water from the inside of your tent in very cold weather, or from your snow cave, and these are going to make your bag very wet. Too many drips certainly can impact the warmth of a bag, although by how much is another question. But the DriLoft fabric is heavier than the light Pertex fabrics and much dearer, so there is a definite trade-off. The trade-off gets worse when you consider what happens on the inside of the bag. Gore claim their DriLoft fabric breathes moisture as well as many ordinary fabrics, but this is under their chosen test conditions. These just could be optimised to suit their fabric of course.

In practice, DriLoft does not work so well under most conditions. Because it has a membrane it does not allow air to pass through, while ordinary fabrics do. So ordinary fabrics will allow far more real breathing when you take into account your thrashing around inside the bag. In addition, the distinct barrier the Gore membrane creates can encourage far more condensation on the inside surface of the cold outer shell, and once this starts to happen the breathability of DriLoft goes right down for technical reasons. If this happens to ordinary fabric the moisture wicks through to the surface and evaporates off fairly quickly. In fact, it is quite possible for some of this water to turn into ice on the inside of your bag, especially if the air on the outside is way below zero. Coated fabrics like DriLoft have been found to be moreprone to this than uncoated fabrics: they get an ice build-up just inside the outer shell more easily and more often. The extreme version of this is experienced in the Antarctic, where sleeping bags used to be rigid with ice after a while.

Is it worth the extra cost and weight to have the DriLoft (or other brand) fabric? Not really, not unless you are going into extended snow cave camping. Even then, a good water-repellent coating is more effective. You may find that New Zealand sleeping bags cater more for snow caves: good bags I'm sure, but not as suitable for Australian conditions.

If you do have a sleeping bag with one of these waterproof shells, you will find it quite hard to pack. The air gets caught inside the bag and can't blow out through the fabric. The simple solution is to turn the bag inside out before you pack it, and/or to pack it from the foot region first. And make sure you air it inside out as often as possible.


Sleeping Bag Fillings

The filling in a good sleeping bag is either down, which in the good Australia brands ranges from good to very good, or a synthetic such as Qualofill or Polarguard. The filling in cheaper sleeping bags is usually a cheaper synthetic, and once again ygwypf. We won't mention the venerable kapok from the middle of last century, let alone the old Army blanket with blanket pins (ah, memories).

The thing to understand about the filling is that the material used (be it down or synthetic) does not provide the warmth. It is the air trapped in the filling which keeps you warm. So you want a filling material which traps the most air for the least weight. Down is still the best material for this: those birds have been at it for a long time. But some of the latest synthetics are not bad, and they do have some advantages.


Down filling

What is 'down' really? I will paraphrase from an excellent discourse by Ted Ripley-Duggan in a review of a Bask sleeping bag at Backpacker gear Test in this and the next section.

Down is feathers, right? Well, no. Down is the undercoating of waterfowl, as distinct from feathers. Each fragment of down is called a plumule, and grows from a single quill point, rather than a shaft, as a feather does. For sleeping bags, those waterfowl may be geese (gray or white) or ducks. Chickens need not apply. Traditionally, goose down has been preferred for sleeping bags, but there's a battle-royal between advocates of duck down and goose down that goes back a good thirty years to my knowledge (see Backpacker, issue number 2 for a fierce debate, and that's Summer 1973), and no doubt considerable longer than that! Suffice it to say that both can be excellent, if correctly prepared.


Down grown on cold-climate birds tends to be better than that of birds in warmer climates. Other factors enter into the equation, though, including the diet of the animals, the de-pluming method, etc etc almost ad infinitum. Also, most down is not ""undercoat"" alone; there's an admixture of feather, measured by weight. Bask offers a number of such blends; that used in the present bag consists of 85% grey goose down and 15% goose feather. This provides a fill power [or loft] of 650 in this case.

The better quality down comes from the more mature birds. The down on very young birds has not had a chance to reach its full stregth, and will degrade more quickly. However, there are only so many wild birds available, and most down comes from ducks and geese grown commercially. They are usually 'harvested' before they are fully mature - for food. The down is a by-product. Some down comes from Eastern Europe, but a lot of it comes from Asia and especially China. They eat a lot of duck there. But the best down is obtained from the nest of the wild (protected) Eider duck after it has hatched its babies each year. This is 'Eider down', and it is rare and very expensive.

Two things can reduce effectiveness of the down in a sleeping bag. The first is a general breakdown of all those little bits of down. This happens slowly with age, but it can be greatly accelerated by leaving the bag compressed for most of the year in your cupboard. Poor little bits of down: they weren't meant to take that treatment. The duck's answer is to grow new down all the time. The human equivalent is to replace the down filling when it degrades too far, but this is best done by a specialist. Unless you have tried it, you have no idea of how far a few grams of down can go in your living room! To prevent the breakdown of the down you should store your bag in a very loose cover which lets it stay ""lofted"". The other hazard for down is damp. This is discussed below.


Loft measurements

What's fill power [or loft], you ask? Well, in order for down to be effective as an insulator, it has to trap air. In a sleeping bag, it's mostly that trapped air that keeps you toasty. Sleeping bags function by minimizing the escape of the body's own heat. Fill power is a measure of the ability of down to expand after compression; the more the down expands, the higher the fill power of the down, and the more effective an insulator it is. Fill power is calculated using a gauge (and there are several types) that compresses the down by a standard amount, and then lets it expand to its full extent. The technology is (naturally) more complex then that, but in essence, that's what fill power measures. The number represents the number of cubic inches to which one ounce of down will expand after compression, under defined conditions of temperature and humidity. The bigger the number, the more trapped air, and the better the insulative qualities of the down.

However, some manufacturers of sleeping bags have used the term slightly differently. They use it to mean the height to which an uncompressed sleeping bag will expand. Generally this is measured from the ground to the high-point of the baffle. While this certainly is a very relevant use of the term, it is not subject to a Standard and does not tell you much about the actual down itself. It may tell you something about the warmth of the bag in the field. You will need to check which meaning is being used at any point.

It seemed that every manufacturer in Australia once had a slightly different way of defining loft - or so they claimed, to avoid comparisons. However, most do now use the first definition. Typical loft values (or fill power) seem to range from 600 to 850. They may not all be exactly the same, but it is possible to rely on the brand reputation here in Australia rather than to worry too much about the numbers.

The problem with this measurement system lies in the lofting process. If you let the down loft up by itself, it will of course rise so far. What happens if you fluff it up a bit? It isn't hard to imagine that the volume might increase a bit. So what is permitted? Following on from that, it isn't hard to imagine that the diameter of the cylinder may have some influence on the result. A narrow cylinder will have lots of drag between the walls and the down, while a wide cylinder will have less. Are all measurements made under exactly the same conditions? One hopes so.

That much said, we can give some rough rules. 600 loft down is fairly cheap, while 700 loft down is usually the 'good' end of the spectrum. One does not see lower loft being quoted very often. Going upwards, 800 loft down is getting quite expensive, and one has to ask how long it will stay at 800 loft. I have even seen 850 loft being quoted, but I have reservations about the significance of any difference between 800 and 850 measurements in practice.

You will also see some vendors quoting figures like '90% down, 10% feathers'. Ted mentions this above. While true, this is generally less useful. You simply can't separate all the feathers out from the down, some some little feathers do get included. Too many feathers and the insulation will go down, weight for weight, but I have also heard claims that down needs some small feathers to help it fluff up. This may be true - I don't know.

ACCC action

Fairly recently (2005?) someone bitched to the ACCC about the claim by many sleeping bag vendors in Australia that their down was 90% down or 100% down. The ACCC had some bags tested overseas, at huge expense, and found that the figures were not always exactly accurate. The bag makers replied that they conformed to the Australian Standard, which allows a 10% tolerance. Given the difficulties of measuring the fractions, this seemed a fairly tight tolerance. However, the ACCC replied that they didn't care what the Australian Standard said, if it wasn't 100% pure down the makers couldn't claim 100%. Exactly what constitutes 'down' is not that clear anyhow. A whole lot of our Australian sleeping bag manufacturers sighed and entered a consent agreement to change their ways and post a notice on their web sites explaining this. The whole things was a huge waste of time and taxpayers' money of course. Then the consenting SB makers pointed out to the ACCC that there are a huge number of doona and quilt makers in Australia, and that the ACCC now had to test items from every one of those companies as well. The cost (to the taxpayers) escalated. It should be addded that overseas manufacturers all have exactly the same problems of course. Exactly nothing was achieved.


Synthetic Fillings

Synthetic fillings try to emulate down by having a mass of very fine fibres all tangled up. The more advanced versions use hollow fibres to reduce the weight. In fact, they do work quite well, but have not caught up with down yet for warmth - until they get wet. One great advantage of the good synthetics is that they can still provide some (limited!) warmth after getting wet. You squeeze the bag out, give it a good shake, and hope to dry it out some more with your body heat. The same synthetic filling is used on ski trousers and even some padded jackets, apparently for this reason. Well, that's the commonly accepted theory, but see below for actual test results.

Synthetic bags also cantake a whole lot more abuse and washing than the down ones. The fibres are usually bonded together into some sort of mat, and can take a lot of rough treatment. Down on the other hand can be broken down by rough treatment like being crushed under shoulder straps for the day. Down also suffers when washed: the natural oils get stripped out, which accelerates its decay. I have a nice down duvet jacket, but I don't wear it while I have a pack on. I would be willing to wear a duvet with synthetic filling.

In addition, synthetics work out cheaper. The filling itself is cheaper because it is made in 'volume'. The fabric used around a synthetic fill can also be cheaper because it doesn't have to be such a tight weave to be ""down-proof"". This can makes a synthetic bag useful for very wet conditions like snow caves. If you are going trekking or travelling, a synthetic bag may be worth considering as well. However, they will be heavier and they will be bulkier. They are also good for young kids who don't look after their gear so carefully. It is interesting that a lot of the bags sold in Europe are synthetic, but the buyers are usually fairly inexperienced walkers who stay in (heated) hostels and catered refuges.

Then there are the cheap synthetic bags, sold in places like K-Mart and so on for prices like $25. They sometimes even have a brushed cotton fabric inside, in pretty printed patterns. Well, I am sure they have their place somewhere, but ygwypf and I wouldn't even bother thinking about one for bushwalking. Maybe a heavy one might be suitable for car camping in the tropics.

Down vs Synthetics when wet

I mentioned above that the popular perception is that the insulation value of down collapses when it gets wet. Well, that's the theory, but is it true, or just a myth promulgated by the manufaturers of the synthetics? This was tested at Backpacking Light, and the results were quite startling. I will quote a small bit of the report ""Drying Characteristics of Select Lightweight Down and Synthetic Insulated Tops,"" by Don Wilson of Backpacking Light.

'The initial loft after soaking showed a dramatic difference between the Flash [down] and the Micropuff [synthetic] vests. When saturated, the loft of the Flash vest dropped to 0.25 inches, losing 87 percent of its dry loft. The Micropuff maintained 0.8 inches of loft, losing only 38 percent of its dry loft. [So far, according to plan - RNC]


A surprise discovery, the down Flash vest recovered its loft quickly. After 30 minutes its loft surpassed the [synthetic] Micropuff. This challenged our perception that a fully saturated down garment is worthless and takes forever to dry. After 40 minutes, both vests had recovered to between 80 and 90 percent of their maximum dry loft. Both vests were at full loft within 80 minutes.

In short, they found that the down jackets did not get as wet, and that the down dried as quickly as the synthetic. It may well be that the down used had retained some of its natural oils, and that this prevented the down from really getting wet. This is a good reason for never dry-cleaning any down gear, by the way. But it does suggets that down gear is more robust than some people make out