In the bag - clothes or not image

In the bag - clothes or not

August 30, 2014

Reading Time: 20 minutes.

Andy we’re big fans being Boy Scouts… you’re one of our heroes… I was wondering if you had found any truth to the rumors that someone could get hypothermia wearing certain clothing in their sleeping bag on extremely cold nights… The suggestion is to keep warm by wearing clothes in the sleeping bag… the counter argument is that to maximize the amount of heat wear little clothing and let the sleeping bag do it’s job because wearing too much clothing keeps in the moisture on the body and doesn’t let the sleeping bag do it’s job resulting in possible drop in core temperature… Could you explain whether such a scenario is even possible thanks

Mike Harig

I’ve had quite a lot of experience in the field of having a bag that wasn’t warm enough, while having plenty of clothes on, and so have spent many a sleepless night ruminating on this topic!

The first thing to say to novice extreme campers is that it’s vital that your base layers (well all your layers really) are as dry as possible, as you will chill with damp underwear on.  A lot of people are not great at looking after themselves in the outdoors (due to fatigue, poor self-discipline or laziness) and so just get into their sleeping bags dressed in all their wet kit (including waterproofs sometimes).  The old technique of always having dry clothes to change into in camp (even just a dry base layer) always works best (when you do kayak expededitions you really get to understand this concept as your clothes are always wet).  Just switch out of your wet clothes and put on a dry layer for hanging out in camp and sleeping, then take them off and put on damp stuff the next day (you can wear damp outer layers like fleece and use your body temperature to dry this as long as it’s not too wet).  So if it came down to sleeping naked and sleeping with damp clothes on I would go for naked, as the heat loss and potential for wetting out your bag won’t be worth it (if you have a super warm synthetic bag then you can get away with this, but there are better ways to dry your kit.

One thing I think people should never do is wear their shell in their sleeping bag (common on alpine climbs), as most fabrics need a temperature differential (warm inside, cold outside) to pump the moisture out.  Wear damp clothes inside a shell, inside a sleeping bag and the moisture will just sit there, and you will get a sort of boil in the bag effect. Worse still, when you get out of the bag in the morning you will be damp and instantly become cold (super dangerous on an alpine climb). This is one of the many reasons why I like proper softshell clothing for multi-day climbs (like Montane Extreme tops and bottoms), as you can wear them very comfortably in a sleeping bag (they also dry out in a sleeping bag).  Whatever you’re wearing under your shell, you are always best taking the shell off and either laying in it (it will often dry simply by sleeping on it under your sleeping bag, or better still laying it on top of your bag (velcro wrists together around your sleeping bag to hold it in place).  Doing this will increase the warmth of your bag by trapping heat as it leaves your bag, both making you warmer (you can feel the difference when you remove the shell in the morning), and drying out the shell.  A shell over a bivy bag will also tend to pull out the dew point (the point your body vapour turns to water, keeping your bivy bag dryer (shell must be over the top of your bivy bag).

Lastly, there is the simple question of just wearing lots of clothes in your bag.  I guess this is a yes/no question. Here are a few thoughts on it:

  • 1. If your sleeping bag is up to the job of insulating you fully for the night then this is the best insulator to use.  If you get into your sleeping bag with all your layers on (some damp, such as socks etc), then your body heat will transfer slower from your body into the bag, and so your body heat will not be radiated back equally over your whole body by the bag. Instead, you may have a warm body (say you have fleece and down jacket on), but your legs are cold (just one thin layer), and your feet are freezing.  Unless you let the heat into your bag’s insulation it will not warm up the whole bag.  In this scenario, if you already have a dry down jacket or fleece then you’d be better laying them over your feet and legs and allow your trunk to heat out into the upper area of the bag.  This inequality of heat from the head down to your toes is most often felt by using a hot water bottle (Nalgene bottle filled with boiling water), as this will speed up the warming of your lower body (you can stick it between your thighs to heat the blood), and so intern the lower half of your bag.
  • 2.  If you have an inadequate sleeping bag and wear all your clothes you need to remember that heat rises and that by wearing it you are effectively not using about 35% of the available insulation.  In these situations, you need to place all extra insulation beyond your base layer (fleece, shell, down jacket) on top of your body, not around it.  Attach your fleece to your down jacket to form a kind of blanket (you can sew in some tiny velcro tabs to make this easier if you’re doing a lot of minimal camping), and lay this over the top of you inside your bag (make sure your bag is big enough, if not stick them on the outside).  If you have a down jacket, use the hood like a scarf to further stop heat escaping out of the hood of your bag, and pull the arms inside the jacket and tuck them along the sides of your body.  In this situation, your feet tend to be forgotten, so if I have a shell I will zip it up and stick my feet/sleeping bag base inside it (if you have a large capacity rucksack you can also do stick your feet inside that too).
  • 3. Seeing as you’re scouts you should also never ignore what’s around you that can be used to boost your bag’s insulation, both in terms of insulation (like leaves), or by creating a barrier against the wind (snow, wood etc), in order to squeeze the most out of your bags insulation.
  • As I said, this is a yes and no answer. You can wear all your clothes in your sleeping bag, or you can take them all off, but most of the time it comes down to a mixture of both, as someone freezing their ass off in either situation may be missing a trick.


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