How warm is a sleeping bag?
08 November 2008
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).
In the past climbers used to look at the fill-weight of the bag (the amount of down stuffed into the shell) in order to judge how warm it would be. They were aware that down bags with less than 500g of down were summer weight and for winter use they would look for a bag with 900 or more grams of down. The fill power would then be taken into account, for example, a 500g 700-fill bag would give you a ‘warm 500 gram’. This was in the days when the sleeping bag consumer was well-educated in what they wanted.
Once the outdoors became mass-market, manufacturers had to come up with ways of demonstrating how warm their bags were to customers who had only ever slept at room temperature. So, in an effort to make purchases easier, temperature ratings came into vogue, however, by simplifying the process they not only introduced manufacturer interpretation but also steered customers from understanding the mechanics of how a sleeping bag works.
Since it is the individual’s metabolism that heats a sleeping bag, it remains important for you to fully understand the functions of fill weight, loft, internal girth, etc. No amount of simplification will benefit the customer. Only by understanding how a bag works will a customer make the correct choice. To this day fill weights are still listed in manufacturers’ charts because of how useful they really are.
Judging a sleeping bag’s temperature value by looking only at its temperature rating is strongly discouraged. Temperature ratings are nothing more than a rough guide and should only be used as a starting point. Also, one company’s temperature rating has no bearing or relationship to another company’s rating system and some company’s stated rating is literally dangerous.
To give you an idea of the usage here’s a rough idea of season, temperature and usage for a well-fed sleeper wearing a thin thermal layer and sleeping on a well-insulating mat.
* 200-300g (1+ season/ +5/0°C). High quality down and construction will allow this weight of the bag to be used in mild spring and autumn conditions if used within a tent by a fit, well-fed and dressed sleeper. A common marathon weight bag. * 350-550g (2+ season/ 0/-5°C). Performing a similar role as the bag above but with a little more range for not too much more weight. A good summer weight bag for a cold sleeper. With a well-dressed and high metabolism sleeper, this weight bag can be used into the winter although don’t expect to be fully comfortable. A good summer Alpine weight bag or three-season ultra-lightweight backpacking bag. * 600-800g (3+ season -5/-15°C). A good all-round sleeping bag, suitable for four-season use within a tent. A fully dressed climber can use this bag in the open in high mountain conditions comfortably and this weight of sleeping bag is used by most expedition climbers. * 900-1,200g (4+ season -15°C to very cold indeed). Expedition weight and suitable for four-season mountain use and a popular weight for sleepers who don’t want to wear clothes in their bag or want total warmth whatever the weather (1,000g of standard quality fill).
In November of last year, I spent a cold week waiting in the Hornli Hut for the weather to improve on the Matterhorn. Although my bag was rated to -18°C and I had plenty of clothes on, I froze my ass off even though the hut temperature could only have been just below zero. We’d only had our hill food to eat, which comprised of small portions of couscous and after two nights of poor food and sleep, we decided to walk down to Zermatt and get some better ‘soul’ food. That night after slogging back up to the hut again (why doesn’t the téléphérique go all the way to the hut?), we ate a mountain of pasta, cheese and tomatoes and then crawled into bed. Wow! What a difference. My sleeping bag turned into an inferno and very soon I had all my clothes off and my feet sticking out of the bottom.
The lesson is that scrimping on fuel is a false economy if you want to stay warm. Fuel the fire and you can get away with a lighter bag. Fats are what you’re after as they produce heat due to inefficient digestion. On cold routes, I now carry a small Nalgene bottle full of olive oil and also take nuts (pine nuts are great things to add to any meal) and cheese.
If you are cold and jump into a cold bag it’s a bit like jumping into a flask, you’ll remain cold. Down is not warm in itself, it’s simply an insulator. I’ve woken up on the top of El Cap in scorching heat totally oblivious because the bag was keeping the heat out.
Firstly, you have to heat up the bag. Getting in hot is the best way. Wearing tons of clothes stops the heat from getting into the insulation. You may not be cold as your clothes will provide some insulation, but you’re not making the most of the potential warmth of the down. Heat will seep out into the down slowly if you are wearing all your clothes and you may wake up warm in the night, but it’s better to get warm from the start.
One trick is to do sit-ups in the bag, or jog around before you jump in. Take off clothes inside the bag so you don’t lose any of the precious heat you already have, allowing the warmth to pass from your clothing to the down. Insulation that allows convection like thermal base layers or open fleece can be worn as the heat is able to pass through it into the down. Following these rules will allow you to sleep warmer if your bag is up to the job, if it is not then you take appropriate action in order to increase your bag’s insulation or else spend a long cold night.
The snow is lying thick outside your tent, you’ve eaten all your cheese and drank all your olive oil, you’ve taken off all your clothes, yet your 200g bag just isn’t doing it for you.
Firstly, make use of any other insulation you have. Down is the best insulation to hand so this should remain your first insulation layer. Anything else should be placed on the outside of this. Are you losing heat through the ground? If so lay on your side to reduce surface contact and try to increase your under insulation. You can do this by laying on your pack, or by laying your clothing underneath you. I often suffer from a cold backside and knees so I often slip my gloves into my trousers to increase their insulation. If the top of the bag isn’t keeping you warm then drape clothing over the top of the bag. I cannot overstate how effective this can be, even if it’s just a shell jacket. Of course, the downside is that you can’t move around. Make good use of your partner, as they may well be as cold as you. Spooning together is a great way to gain heat that would otherwise be lost and a good way of getting to know your partner better.
Make sure the clothes you’re wearing are dry, especially your socks and hat. Cold feet will cool you as they won’t be able to warm the down in the bottom of the bag. If you don’t have dry socks wrap them in a spare thermal or pop your mitts on the end once you’ve warmed them up by hand. Down boots are a good investment if you suffer from cold feet. Because you lose 6% of your heat from your noggin it’s a good idea to insulate your head well. If you’ve got a down jacket with a removable hood, or can buy a down hood, you can use this to improve the rating of your bag with very little bulk or weight.
A bag may not perform due to many factors, with the two most common being either a fill weight too low to insulate the user and, secondarily, a fill that has lost some of its performance. The first one is easy to remedy before the event if experience is applied and an over-optimistic attitude is kept in check when planning your nights sitting in a warm room. The second factor can be limited by a good understanding of what can go wrong and using this to care for the bag.
There is nothing as great as a bone dry down sleeping bag. Unfortunately, there is also nothing as grim as a wet one. Saturated down loses 90% of its insulation as the delicate structure of the clusters stick together and collapse. The down absorbs a lot of water and because it loses most of its thermal performance it must be dried via an outside heat source like the sun, a tumble dryer or a body.
In many cases where there is moisture present, a down bag will lose performance each night it is used, as the down becomes slowly saturated. The rate at which this happens is dependent on the skill of the user and the conditions in which it is being used. This water contamination comes both from without and within the bag and learning to slow the speed of this performance drop is one of the skills needed to use a down bag effectively.
Down just doesn’t work in high saturated environments and best suits cold and dry conditions, or the protected sanctuary of a dry space like a tent, hut or snow hole. Open bivvies in the rain, even when using a bivvy bag are asking for trouble. When the temperature drops the down begins to really come into its own. All the nasty moisture is now frozen and so what would otherwise have wetted out your bag can be brushed off. Care must be taken to limit any frozen moisture melting on to the bag (like lying on snow that has fallen on your mat). This includes bivvy bags with hoar frost coating the inside due to you breathing into them, warm parts of the bag pressing against the snow. If boiling water within a tent or snow hole, try to keep your bag packed away so it doesn’t become damp, or if that’s not possible then limit the damage by covering the bag with clothes. The modern water-resistant shells are just that (apart from the Mountain Hardwear Banshee), as moisture can get through the untaped seams, but they still provide a very good barrier to secondary contamination (i.e. not direct rain).
Try not to get into the bag with wet clothes on and don’t breathe into it either as this will increase the amount of moisture in the bag by several thousand percent. If you must get into a down bag with frozen clothes on then try and remove any snow off your clothes and pay close attention to ice on the Velcro on your shell, removing it with a knife or peg before it can melt in your bag. If you must dry clothes then lay them on top of the bag, or lay on them, or just leave them wet and aim to dry them on the move the following day. The only exception to this rule is socks and gloves which should be placed flat against the skin (stomach, groin, armpits) under your base layer to dry. When you get up in the morning stuff the bag away straight away so the warm moisture within it is pushed out before it can cool and condense within the down. Make sure your stuff sack is waterproof and consider buying a dedicated dry bag to store it in if doing a very wet trip.