Synethic Sleeping bags

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 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, caravanners, 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 cost 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 use a sheet or wad of insulation held together on a light scrim, cut into a mummy shape and sandwiched between the shell fabrics, then 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 varies considerably between high-end bags and mass-produced ‘Argos’ moon bags, but all are far 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 was particularly unsuitable. 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 bag outlasting two to four equivalent synthetic bags. What happens is that 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 fibres being torn if handled roughly, or washed and dried incorrectly.

There are also many poor fabrics on the market that, although they 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 rough and tumble of outdoor use. 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 slowly lose their springiness and loft. Remember that no synthetic bag will lose all its insulation value, only a percentage of it, because insulation comes from the air trapped between the inner and outer fabrics. If your old sleeping bag has cold spots 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 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 with their drawbacks, synthetic bags have one major advantage over down: their insulation is far less affected by moisture. This is a big advantage in a maritime climate like the UK. If a down bag becomes soaked it loses virtually all its insulation and is difficult to dry properly in the field. A synthetic bag, on the other hand, may lose a third of its insulation when saturated, but will dry far more quickly and continue to insulate the user while doing so.

It is largely for this reason that synthetic bags became popular with high-performance users who needed dependable insulation in down-hostile environments. Sailors, big wall climbers and cavers accepted the drawbacks because nothing else worked reliably. Himalayan climbers also began using synthetic bags as over-bags, shifting the dew point away from the down beneath.

“As usual, and inconsiderately, a cold day turned into 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. The first thing I noticed was how much roomier the bag was compared to my featherweight down model. No agonising cramp, no claustrophobia. I’d even dragged snow into the bag, but instead of panic I simply unzipped the foot, turned it inside out and shook the snow away.”

A few years later there was a sudden influx of new insulations designed specifically as high-performance fills. Companies like Ajungilak, Snugpak and Mountain Equipment helped lift synthetic bags out of the doldrums, while new fills such as Microloft, Primaloft and LiteLoft banished the mouldy-bag image. Polarguard 3D was perhaps the most widely used of these, offering performance comparable to low-quality down (around 450 fill power) with fewer drawbacks.

Once brands like The North Face, Marmot and Mountain Hardwear began pushing synthetics as real alternatives, adding high-performance shells and improved construction, these bags were finally taken seriously. Most climbers still found winter-weight synthetics too bulky, but lighter four-season models began to make sense, especially when combined with lighter bivvy systems.

“Twenty hours later we staggered into the hut. My partner’s once-proud down bag had collapsed into a soggy rag. When I pulled out mine, it was wet—but still warm. I shook it, climbed inside with my boots and slept.”

Am I advocating abandoning down altogether? Of course not. Down remains the lightest, longest-lasting and most efficient insulator known. But for those who sleep, bivvy or crash out in truly hostile conditions, modern synthetic fillings offer something down never can: reliability. I’m happy to carry the extra bulk and weight in exchange for the confidence that my sleeping bag will still work, no matter what.

“In the morning I woke to the sound of the rescue helicopter landing on the glacier outside. The inside of my bag was dry, as were my clothes. Looking at the wet pile of blankets beside me, I realised I’d learnt something important. Gear doesn’t have to be sexy or light—it has to keep you alive.”

THE SCIENCE BEHIND SYNTHETICS

Down is such a great insulator because of its extremely high surface-area-to-volume ratio. Its filaments can be as small as 7 microns, trapping huge amounts of air. Synthetic fills have a long way to go to match this.

There are two basic types of synthetic fill: short-staple and continuous filament. Short-staple fills such as Primaloft and LiteLoft are bonded into sheets using resins and scrim. Continuous filament fills such as Polarguard HV and 3D use long hollow fibres kinked to provide loft, allowing lighter construction and greater durability.

Polarguard 3D remains one of the best all-round synthetic fills available: durable, warm and relatively lightweight—though still bulky.