Why does a plastic bag/poncho keep me warm in the outdoors?

Plastic such as a garbage bag or poncho is a really efficient and lightweight way to stay warm and avoid hypothermia. Hence it's long been known as an easy tool in an essentials kit.

Plastic gloves (such as medical gloves) are also excellent at keeping your hands warm, which is why they are also in the EMELEE Essentials kit. 

But why does plastic work so well? and why then do we spend a lot of money on expensive Gore-Tex jackets when we could just wear plastic? 

Why does plastic work so well?

One of our body's most effective cooling systems is sweating. Sweating works because evaporation is an endothermic reaction (ie it absorbs heat) so as the sweat on the skin evaporates it absorbs heat from the skin and hence cools it. Any draft will speed up the reaction and cool the skin even faster (which is why we loosen clothes to cool down, its allowing drafts to access the sweat and increase evaporation and hence cooling). 

Plastic effectively stops any drafts from passing through it, without the draft the sweat (or water if you're wearing wet clothes) can't evaporate and hence can't cool the skin. So the plastic not only stops the rain from making you colder, it is basically trapping body heat in too. However if you wear the plastic loosely, drafts will still get in and cool you down. Make sure you wear your pack and hat on top of the poncho to trap in heat (see video below) or use the duct tape to reduce the drafts from getting in. 

Why then do we spend a lot of money on expensive Gore-Tex jackets when we could just wear plastic? 

A question I have asked myself a couple of times.

Because plastic is so efficient at stopping sweat evaporating and hence trapping in heat, and because most of the time we are not in a hypothermia situation, our bodies are working hard in the outdoors and we need to sweat to cool down, we actually don't want to trap in the heat, we want to evaporate the sweat or we over heat.

As opposed to plastic Gore-Tex is touted as a breathable yet waterproof material, which up to a certain point it is, but as we've all probably experienced, once we are working too hard, our bodies over heat past the point where the Gore-Tex can keep up with the amount of evaporation needed and we have to open the zipper or vents to allow air in and speed up evaporation.

This is also why Gore-Tex won't keep you as warm as plastic once you get too cold, the Gore-Tex is allowing some evaporation, the plastic isn't. 

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