I once read that ‘bivouac’ was the native American word for ‘mistake’ … now, while that’s not actually true it did get me thinking about peoples perception of a bivvy. In the company of like-minded souls, saying you’re going to spend the night outside at this time of year is fine, it’ll usually be met with a few questions and possibly even a little envy. Mention the same thing to the non-enlightened and reactions will often be varied and wide ranging. Everything from total disbelief to flickers of interest will be present in the responses … but tell those who showed the interest that you’ll be going tonight, when temperatures may well be into minus figures and you’ll only have waterproof sheet separating you from the clear night sky and you may discover that their interest is suddenly lost … they’ve now deemed you insane.
Contrary to popular belief, choosing to spend the night outside in Winter doesn’t mean that you’ve also elected to be cold and miserable. A little planning and forethought can go a long way to helping you stay warm and cosy. Over the coming weeks we’ll take a look at the different ways of getting cold and hopefully suggest a few ways to overcome them.
|Dec, overnight temps -10 … no one died.|
There are quite a few mechanisms for heat loss but the 2 that concern us most are:
Conduction: The transfer of heat from a warmer object to a colder one.
Convection: Heat loss in response to the movement of fluid or gas.
Radiation, evaporation and respiration can result in heat loss too but in the UK it’s unlikely that any of them will play a big part in whether we sleep warmly or spend the night awake and shivering.
Conduction … something hot (you) losing heat to something cold (the ground, etc). The usual thing that springs to mind when thinking about conductive heat loss is what happens when you lie down … you’re lovely warm body will start to pass heat to the horrible cold floor immediately and unfortunately won’t stop until both objects are the same temperature. Obviously the easiest way to try and combat conduction in this situation is by using a sleeping mat, the greater the thermal barrier the less heat will get passed between the 2 objects … but it’s not quite that simple. The latest generation of air mats have made kipping on the floor a much more pleasurable pastime due to their comfort but air isn’t a great insulator UNLESS it’s somehow trapped. An mat that contains air in open tubes, where that air is free to move, will allow ‘passive’ convection to take place and the air will transfer your heat to the cold floor. However, if we can stop the air inside the mat from moving and somehow trap it, then it starts to become an insulator, a very good insulator.
So, what are our options?
Investing in an expensive mat would seem like the obvious first step but expensive doesn’t always equate to warm … big money could just be buying a reduction in weight / pack size. If you’re buying an air mat look for something that contains insulation, baffles or in preference, both. The more insulation, the less the air inside can move around and the warmer you’ll be. If you prefer a self inflating type mat, then look for something that has horizontally cored foam rather than vertically. Vertically cored foam means there’s a direct path between you and the ground … oh and the thicker the mat the better.
If buying an expensive new mat isn’t on the cards, then you can improve the efficiency of your existing mat quite easily (and cheaply).
Closed cell foam (CCF) is inexpensive, light, reasonably robust and best of all, is full of tiny sealed air pockets. The 2 main drawbacks to CCF are bulk (lots) and comfort (lack of) which is no doubt why you don’t see anyone other than DoE victims carting the things about. The good news is CCF comes in many guises, not just sleeping mats. Pop down to your local DIY store and fight your way through the isles until you reach the flooring section. Search for laminate flooring underlay and if you’re in luck you’ll should find some 3mm thick foam underlay which will hopefully be packed folded, rather than rolled. What you’ve just discovered is ultralight closed cell foam that can be easily cut, folded and if allied with your existing mat will give a real boost to how warm you’ll be. There’s no need to produce a secondary mat the exact same size as your existing one, something the length of your shoulders to your thighs and a little narrower than the top mat will work well without incurring a massive weight penalty. If you’re having trouble keeping both mats together then a small piece of self-adhesive velcro on the corner of each mat will keep them together.
|7mm CCF camping mat (blue) and 3mm underlay on top.|
Other options for increasing the thermal efficiency of what’s beneath you are, dry leaves, bracken or branches, luggage, bags, etc or if you’re going all out for the 2 wheeled tramp look … bubble wrap. Remember, you’re simply trying to put as much ‘dead’ trapped air between you and the ground as possible.
Next time we’ll have a look at dealing with heat loss through convection … bet you can’t wait.