Choosing the right sleeping bag

By Gemma Chilton | November 20, 2014

We spend about a third of our lives sleeping, so choosing the right sleeping bag to help you drift off with a smile is one of the most important decisions you can make

YOUR MUSCLES HAVE cooled to a satisfying ache after a long day’s hike, you’ve erected your tent, ingested something warm, rehydrated and watched the first stars flicker to life in a darkening sky. It’s time for bed – well, mat – and, if you’ve done your research, a soft warm sleeping bag to cuddle those hard-working limbs until first light, when you arise ready to hit the track all over again.

Choosing the best sleeping bag for the job isn’t particularly difficult, but it’s vitally important. Here are the basics to consider, so you can make sure an otherwise great trip isn’t ruined by that most devastating of injustices – a terrible night’s sleep.

Shape-shifting sleeping bags

Sleeping bags come in three basic shapes: rectangular, semi-rectangular and the mummy. The classic rectangular is the traditional shape, and comes exactly as it says on the box – four sides and equal width from top to bottom.

The mummy, on the other hand, tapers at the top and bottom and incorporates a hood. This is the cutting-edge of sleeping bag design, offering the most efficient method of staying warm, while using less material and therefore weighing less and taking up less pack space for the same warmth rating.

However, some may find the mummy design constricting and as a result not conducive to a good night’s sleep – which is, of course, the objective. So if you’re a claustrophobic sleeper and not counting grams in your pack weight, then the rectangle might be the way to go.

Alternatively there’s the semi-rectangular, offering a middle ground with a slightly tapered foot but a rectangular top half with a choice of hood or no hood.

Down-fill sleeping bags vs synthetic sleeping bags

Down feathers are those soft fine feathers on baby birds, or the little feathers hidden under the tougher exteriors of older birds. In sleeping bags, the fine natural fibres of goose down provide millions of tiny air spaces for excellent insulating performance (see box, ‘Let’s get physical’).

Down-fill sleeping bags are lighter and can pack away more compactly for the same warmth rating compared to synthetic alternatives, and some people may prefer the idea of using a natural material.

Unfortunately, down falls short in performance when wet, becoming soggy and eventually losing its insulating properties.

Synthetic sleeping bags (made of extremely fine, intertwining plastic fibres), on the other hand, manage to retain some of their insulating performance even when wet – which could be life saving given the right situation. They are also hypo-allergenic, which is a consideration for some. Again, the important compromise, when compared to down, is size and weight relative to performance.

High-tech options are on the rise to counter down’s perceived shortcomings, including Sea to Summit’s ULTRA-DRY Down product. ULTRA-DRY Down incorporates a nanoscale water-repellent polymer treatment to reduce the down-fill’s moisture absorption by a claimed 30 per cent, while retaining over 60 per cent more loft when wet – key to its insulating performance.

Sea to Summit also offers its trademarked 2D and 3D ‘NanoShell’ technology – a treated fabric that further protects the sleeping bag filling from moisture.

As an emerging science, nanotechnology is, however, subject to some controversy. Switzerland-based Heidi Brun, head of sleeping bag product development at Exped, says she is sceptical of the environmental soundness of the increasingly popular nano-treatments for down sleeping bags, commonly dubbed DWR (Durable Water Repellent).

“We do not follow it as we think down is such an amazing natural product that it should not be treated,” she says.

What is fill power?

Put simply, fill power, which you’ll find listed as a specification of down sleeping bags, is the inverse of density – the higher the fill power, the less dense the down. More technically, it represents the relationship between the mass of the down and the space it occupies (generally represented in cubic inches per ounce).

Therefore, a sleeping bag labelled 800 fill power goose down will loft (or fluff up) to 800 cubic inches per ounce (about 28g) of down. Fill power of 400-450 is considered medium quality, 500-550 is good, 550-750 very good, and 750-plus is excellent.

Keeping warm in a sleeping bag

Introduced in 2002, the European Union’s sleeping bag warmth-rating standard, ‘EN 13537: Requirements for Sleeping Bags’, is the first and only international rating set by a standardised laboratory test.

The lab tests use a thermal manikin to mimic the average amount of heat generated by a standard man and woman inside the bag being tested, to come up with different ratings for specific ambient temperatures: Comfort, Lower Limit and Extreme.

Anyone who’s participated in a midnight doona tug-of-war knows how very individual body temperature is, based on countless variables including metabolism, weight, age and gender. In other words, one person’s Comfort sleeping bag might be another person’s sweaty nightmare. The trick with EN 13537 is that it provides an objective reference point – an alternative to what Brun dubs “marketing” warmth ratings.

Brendan Sando, a product designer at Sea to Summit, agrees. “EN 13537 finally brought a standard to the industry and an honesty to the sleeping bag’s ability to keep you warm,” he says.

Another key consideration, Sando adds, is to choose a sleeping bag warmth rating based on how you will be using the bag for the majority of the time, and not to buy for the extremes.

“People often buy a sleeping bag way warmer than they actually require for the majority of their use, so for 90 per cent of its use they are too hot, sleep with the bag open and end up carrying a heavier and bulkier bag than they require,” he says.

“I would suggest buying a sleeping bag based on the warmth required for 90 per cent of its use. If you can’t afford to buy a second bag more suited to those odd colder nights, then sleep in thermal underwear or a down jacket.”

How sleeping bags work

In understanding how sleeping bags work and which one is best for you, it’s a good idea to start with the basics, and it doesn’t get more basic than physics. A branch of physics called thermodynamics, to be specific. There’s also a bit of biology involved.

You see, warm-blooded mammals like us are basically living heaters, fuelled with energy from the food we eat. The job of a sleeping bag, like any insulator, is to prevent the flow of that heat from one place to another – in this case, from your warm body into the chilly night.

The opposite of an insulator, more or less, is a conductor. Think about it: although they feel like it, metal railings aren’t colder than wooden ones, they simply strip away your heat faster because they’re more conductive.

Air, on the other hand, is a rather poor conductor, and when trapped (for example, in air pockets in foam, or between feathers or plastic fibres) is an effective insulator. The more air pockets and the smaller they are, the more effective the insulation.