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).
Synthetic sleeping bags have always been the poor relation of down, with even the highest tech fillings being far inferior to the lowest quality natural alternative (feathers). Synthetic fillings were an offshoot of the carpet industry, with hollow fibres not being developed for superior warmth, but so as not to show the dirt in carpets so readily. Companies like DuPont introduced the first commercial synthetic fillings several decades ago, but these bags have remained at the bottom end of the sleeping bag market, used by car campers, caravaners, students and general normal people. The reasons for this are price and performance. A synthetic bag, even the most state of the art, costs considerably less to manufacture than a standard down equivalent, making it the choice for those on a budget or those uninterested in high performance. This price difference is due to the high price of down and the relative simplicity of synthetic sleeping bag manufacture. A synthetic bag does not require traditional baffles, which must be carefully sewn and hand filled, allowing simpler and faster techniques to be used. The most common construction is to simply have a sheet or wad of insulation, which is held together on a light scrim, that can be cut into a mummy shape and sandwiched between the shell fabrics and simply sewn together. Other more advanced techniques include shingle construction, where overlapping strips of insulation are sewn into the inner and outer of the bag, reducing cold spots and increasing the overall integrity of the insulation. Quality of filling and construction does vary considerably between the high-end bags and the mass-produced ‘Argos’ moon bag, but all are considerably less complex than a down bag.
“I’d blagged the bag intending to use it for a couple of weeks winter camping in a portaledge on the Grandes Jorasses, unfortunately, that didn’t pan out, and so it was all I had for a super alpine assault, something that at the time I thought it was particle unsuitable for. Packing the night before we left for the mountain, I gazed down at the large bulging stuff sack beside my pack. Instead of the usual minuscule down bag, compressed to the size of a micron within the tight bind of its compression straps, I’d been lumbered with something that looked more like a medicine ball. The thing was enormous, packing down to about the same size as a small fitted kitchen - or about the same as two 1000 gram down bags. Luckily it was only about a third heavier than an equivalent down bag, even though it looked like it should weigh far more.”
One of the main drawbacks of synthetic bags over down has been the comparatively short life of the insulation, with a single down back outlasting two to four equivalent synthetic bags. What happens is the insulation value of the bag slowly decreases with use, caused by the wadding not bouncing back to 100% after repeated compression. The insulation can also be affected by the fibres being torn if handled roughly, or washed and dried incorrectly. There are also a lot of very poor fabrics on the market that although look and feel almost like down, soon collapse. This is due to their fine and delicate man-made construction not being resilient enough to cope with the general rough and tumble of outdoor use (wear and tear, washing, etc.). For many years the benchmark insulation was Polarguard, now upgraded to Polarguard HV, which although not the lightest fabric on the market is one of the most robust. If compression is applied only for transportation and not storage (put it under your bed in a pillowcase) then a well-constructed Polarguard bag’s insulation seems to outlast most other fillings (as the fibres lose their springiness and loft), this is probably due to the fibres being more robust then the Ultralite competition. Remember that no synthetic bag will lose all its insulation value, only a percentage of it, because the insulation comes from the amount of air trapped between the inner and outer fabric and there will always be some loft. If you find your old sleeping bag has cold spots then it’s probably due to the scrim being torn (hold it up to the light to check).
“Carrying a single rucksack with both our sleeping bags stuffed inside we scampered up the face. The ‘sack was of the expedition rather than the French variety, with most of its massive capacity being taken up by my pit. My partner’s bag was a custom-built expedition down design with a waterproof outer and water-resistant inner, complete with a bivi cowl. The whole thing fitted into a stuff sack the size of a small loaf of bread. Very Mark Twight. You can imagine what he said when I showed him what I’d be bringing along”
Even though synthetic bags have several drawbacks they do have one major advantage over down. The insulation is far less affected by moisture, a big advantage if you live in a maritime climate like the UK. If your down bag becomes soaked it loses virtually all its insulation, plus it’s hard to dry out properly or quickly – especially in the field. A synthetic bag on the other hand loses perhaps a third of its insulation when totally saturated, but will dry far more quickly due to the water-resistance of the fibres, and the fact that the bag will still insulate the user, whose body heat will, in turn, dry out the bag. It is perhaps for only this one reason that synthetic bags became popular with high-performance users, who needed bomb-proof dependable insulation in order to survive in ‘down hostile’ environments. Sailors, big wall climbers and cavers all accepted the drawbacks associated with these bags because in the most hostile environments nothing else would work. Himalayan climbers also began using synthetic bags as covers or ‘over bags’ to cover their down sleeping bags, finding this way the dew-point (the point at which the warm moisture vapour from the body turns back into water) formed inside the over bag, not in the down. Nevertheless, for the majority of users synthetic bags were too bulky and heavy for anything but car camping.
“As usual, and inconsiderately, a cold day turned to a colder night far from any prime camping spots, forcing us to chisel out a bed of ice for our frozen backsides. Because of our precarious position, I had to get into the ‘behemoth’ with my boots on, carefully removing my crampons as I did so (good tip that). The first thing I noticed was how much roomier the bag was compared to my current featherweight down model, obviously accounting for some of its weight. No more agonising cramp as I removed my inner boots, and that terrible claustrophobia, caused by squeezing yourself along with several layers of bulky clothing into a narrow nylon tube filled with feathers. You see its ok designing and choosing bivi gear in the real, warm world, cutting bags thinner to save weight and reducing the insulation in order to shave those few grams, but up there you realise that some things are more important than the turn of the all-important hand on the kitchen scales. The other thing I noticed was I’d managed to bring several shovelfuls of snow into the bag with me. Usually, this would result in dread ‘damp down’ panic, but all I had to do was unzip the foot section of the bag and turn it inside out, shaking the snow into the bottom of my bivi bag. After a fine alpine meal fit for a mouse, I settled in for the night, slings wrapped around my knees and shoulders, strangely comfortable knowing I didn’t have to go to sleep with the usual down bag paranoia.”
Then a few years ago there was a sudden influx of new insulations from several companies. These fillings, unlike the first generation fibres, had been specifically designed as high-performance insulators. Companies like Ajungilak, Snugpak and Mountain Equipment had already helped elevate the synthetic bag out of the performance doldrums, and now new fillings came along like Microloft, Primaloft and Liteloft, that banished the old image of the traditional mouldy synthetic bag forever. Of these fabrics, Polaguard 3D was perhaps the best (well, most widely used anyway). At the time 3D was the most advanced insulation, giving excellent weight to loft, fair compressibility and a good insulation life span. Polaguard 3D had finally given manufacturers a toe hold in the ‘real’ sleeping bag market, with 3D offering similar performance to low quality down (450 fill power), giving all the benefits of synthetic with fewer of the associated drawbacks.
“I woke to a Niagara of snow, a storm of biblical proportions that flooded snow down onto us in waves. Seven hundred metres up the ice face it felt as if we were in the Gods, and what a show. My partner had turned into a snow-banked, frosted, blood-red slug, while I tried to remain composed in my seat under the weight of bullying snow. I shouted we’d wait two hours - unsure we could - until we abandoned our posts and tried to escape. The weather only got worse. Everything was full of snow and packing to leave was a nightmare. I stuffed my bag sans compression sack (I never did find it) under the protection of our bothy bag, yet it still went away covered and filled with snow, which I knew would soon be melted by my residual body heat inside. All I could pray for was that we wouldn’t have to bivi again.”
Once the big boys like The North Face, Marmot and Mountain Hardware had begun pushing their non-down bags as viable down alternatives, adding high-performance shells and improved quality of construction, then synthetics, finally, began to be taken more seriously. I know several climbers who tried out the new generation bags and all reported that performance was better than expected, and although one climber used a model all the way up to the South Col on Everest, most reported that because of the weight increase, these bags were best used for summer to autumn use, as winter weight bags were far too bulky and heavy. Saying that I’ve been testing a 4 season bag that weighs 2300 grams that seems to provide about the same performance as an 800 gram (fill weight) down bag (1600 grams total). The difference is that I can use a featherweight bivi bag (300 grams) rather than a fully waterproof Gore model (700 grams), which makes things a little more equal. Also, after several nights of harsh camping the down bag generally weighs more due to moisture trapped in the down.
“Twenty hours later we staggered (or did we crawl?) into the hut below the mountain. The initial warmth of escaping from the vicious winter wind soon passed, proof of its lack of insulation the thick drifts of snow that carpeted its kitchen and hallway. My partner pulled out his bag and looked at it dismayed. His once huge bulging bag of red hot warmth had been reduced to a pathetic flaccid and wrinkled wet rag, its deluxe fill of feathers transformed into soggy Weetabix. A pile of soiled and musty blankets would have to do. Expecting to see something equally as grim I pulled out mine next and was astounded to find that the soaking had had virtually no effect whatsoever. Sure the bag was wet, with snow still climbing to the inside and outside. Giving it a quick shake I pulled off my boots, through my inner boots inside and crawled in after them.”
So am I advocating a complete turn-around, with down being resigned to history like moleskin trousers and fur-lined boots? Of course not. Unfortunately for the poor duck and goose their feathers are far from being copied by man; after all, it took nature several million years to develop their complex construction, and I expect we’ll see artificial intelligence cracked before the silicone feather. Down is still the lightest, most long-lasting and sexy insulator, having that ‘ooooh’ factor that material science has yet to match. For most mountain sports a quality down bag, when used in conjunction with common sense and good preventive ‘wet down’ methods, is still the top dog. But, for those who like to sleep, bivi or crash out in the worst conditions, either intentionally or otherwise, modern man-made fillings are the only way to go. In the short time that I’ve been using synthetic bags, I’ve been saved by the filling’s robustness several times. Admittedly a synthetic allows you to become lazier and less attentive, but on a few occasions, like bivying under streams of spindrift, or when you forget to close your haul-bag and a sudden freak storm fills it up with water, no amount of prevention would have allowed a down bag to survive intact. Personally, I don’t mind shouldering the slightly heavier burden or carrying a larger rucksack to accommodate its bulk, because I like the fact that I can have confidence that no matter what, my sleeping bag will not let me down.
“In the morning I woke to the sound of the rescue helicopter landing on the glacier outside, touting for business. I felt the inside of my bag and found it was totally dry, along with my clothes. I looked over to the sorry mess of blankets and the wet rag draped over it and realised I’d learnt a valuable lesson. Perhaps in the mad scramble for sexy light-weightiness, I and everyone else have forgotten the most important function of gear – not that it must weigh nothing, look good and be cheap, but that it must keep you alive and increases your survivability. After all, isn’t survival the name of the game? With that thought keeping me warm I turned over and fell back into the kind of sleep only the condemned who have received their pardon can enjoy.”
THE SCIENCE BEHIND SYNTHETICS
The reason down is such a great insulator is because of its high surface area to volume ratio, with its filaments being as small as 7 microns (a human hair is 50 to 150 microns). The air trapped on the surface of these filaments is more important than the air trapped between them (that’s why cardboard is a better insulator than steel) and so when you consider the surface area to volume ratio of such a delicate natural wonder compared to the un-supple man-made insulator we can see why synthetic fills will take a long time to catch up. There are two basic types of synthetic fill: short-staple, in which the individual fibres are a few inches long, and continuous filament, or long-staple, in which the fibres can be up to 500 miles long!
Short staple filling is made into sheets of baffling, with the fibres being bonded together using heat and resins onto a thin paper-like scrim. LiteLoft (15 microns) and Primaloft (7.5 microns) are the two main high-performance short-staple fillings. Primaloft is perhaps the best we have achieved so far in man-made down, but due to its high cost only seems to appear in clothing. LiteLoft is a cheaper version and is found in many sleeping bags. Unfortunately, it is heavier and performs poorly when compared to the other fabrics on the market.
Long-staple fillings like Polagaurd HV (23 microns) and 3D (14 microns) are continuous hollow filaments that are kinked to provide loft. They require no scrim and therefore allow for lighter construction. Polagaurd 3D is perhaps the best all-round fill at the moment - being very durable and lightweight, but bulky.