If you're in the market for a new sleeping bag, deciding which one best meets your needs can be a baffling business. In this two-part series scientist Matthew Fuller guides us through the minefield.
Part 1 covered the thorny issue of test standards and temperature ratings; in this follow-up our down insulation expert looks at materials, construction methods, and the many variables that influence how warm you'll actually feel in the real world.
We covered temperature ratings last time, but there’s a whole host of other things that you should consider when buying a sleeping bag. Here’s a summary:
Once you’ve bought your SlumberEezee Megadown 1000+ and you’re getting ready for your big trip, remember that it’s not just the sleeping bag that influences how warm you’ll be. There’s also the effect of you (the thing that makes the heat and the thing that you care about keeping warm), the effect of other parts of your sleep system, and then the environment itself.
Metabolism varies significantly from person to person, but in general, young males produce more heat than other demographics, and in general men produce more heat than women.
However, there are often significant variations inside populations and if you think of yourself as a ‘cold sleeper’ then you should remember this when using your sleeping bag. Conversely, if you don’t mind suffering, or are used to winter nights out on the town protected by nothing but glitter, a miniskirt, and six Jagerbombs, then you may be able to use your sleeping bag in much colder conditions than those which are recommended for the average user. Body fat, muscle mass, activity level, fitness, age, state of mind, tiredness, physical size all impact on how warm you feel.
Altitude also makes a difference to how warm you are, as your metabolism is less effective as your oxygen saturation decreases. This is particularly relevant in poorly acclimatised individuals, or in those who are sleeping at very high or extreme altitudes. People on Everest using sleeping bags rated to -40 °C aren’t generally using them because it’s as cold as that; it’s because they might be out of their tree and unable to thermoregulate properly. This is exacerbated by tiredness: anyone on a long expedition depleted day after day, waking up totally wasted, is unlikely to find their normal lightweight sleeping bag sufficient as they simply aren’t going to pump out enough heat. Don’t underestimate how cold you’ll feel if exhausted or, worse-still, get food poisoning or an infection.
Women’s sleeping bags are often provided with a lower temperature rating than their equivalent male counterparts because in general women produce less heat/ hot air than men, but women also have more inherent variability in their body temperature.
If in doubt, women (and, indeed, men) should err on the side of caution and buy a warmer sleeping bag to ensure that they do not go cold.
Warmth and comfort are influenced not just by the sleeping bag itself, but by what you lie on, what you sleep in, and what you wear.
You can lose a lot of heat through the ground, particularly if sleeping on snow, ice, or on wet ground. By using a high quality sleeping mat you will be much warmer than using a thin piece of foam or scraps of bubble-wrap. There are plenty of good mats out there and they can make a huge difference. Consider using a lighter sleeping bag with a super warm mat perhaps? Lie on your ropes, lie on your pack – anything can help if you’re desperate.
The myth that going naked inside your sleeping bag makes you warmer is entirely that: a myth. Unless the clothing that you are wearing is wet, or you are very tight inside your sleeping bag to the point where you are compressing the insulation inside it, wearing clothes makes you warmer. A sleeping bag liner (which also helps in keeping your sleeping bag clean) is also very beneficial, or laying any very thick clothing you might have – such as a down jacket – over the top of the sleeping bag.
Try and get good shelter from the wind, try to get cosy with your bivvy partner, and if you’re in a tent it’ll be warmer if you’re not rattling around inside: get more people in there, have a tent party! Remember that there is a world of difference between sleeping in a fully-featured winter tent on a still night of -10 °C and sleeping at -10 °C in a draughty featherweight tent being battered by wind. Even in the very best expedition tents, strong wind can make them feel colder. If you are sleeping somewhere very draughty then expect your sleeping bag to be less insulating than usual due to air flowing over it and perhaps compressing the face fabric and insulation underneath. Also, the psychological effect of strong winds on how warm you feel should also not be underestimated.
Damp air makes things feel colder, so if you’re sleeping in soaking Scottish air then it can feel much colder than a crisp alpine night, despite the higher air temperature. This is made worse in Scottish winter because the nights last for ages and ages and ages…
About Matthew Fuller
Matt is an experienced hiker, keen mountaineer, and terrible rock climber. Most weekends he can be found hiking, climbing, running or cycling in the hills. After 9 years of university, including the world's first PhD in down and its use in the outdoors, he decided to get a job. He is now lucky enough to work for Mountain Equipment as a Product Engineer where he works on R&D, marginal gains, number-crunching, and, obviously, down products.
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