It’s a long proffered outdoor wisdom that wool keeps you warm when wet. As I have been testing the relative insulative properties of different blanket types to determine the optimum plastic free wild camping setup I thought I would give it a test.
The theory – Why wool still works when wet
Insulation largely boils to a simple principle. Heat (energy) will always move from hot to cold. Insulation is there to slow this progress. i.e. minimising the speed of heat loss from our nice warm bodies to the cold environment.
Air is an effective insulator and most effective insultative material relies on trapping it. This is why fluffy, high loft materials, which trap the most air, such as down, are the best insulators on the planet. However the problem that a lot of insulative materials have, especially down, is that when wet they lose that ability to trap air. Down collapses so you lose the air pockets, cotton becomes so saturated with water it transmits heat readily away from the skin to evaporate rapidly sucking heat away at tremendous speed.
However wool has a few tricks up it’s sleeve which help it maintain insulative properties even when wet:
- It’s tight crimped fibre structure doen’t collapse when wet so some air pockets are maintained.
- It doesn’t fully saturate like cotton, absorbing about 30% of it’s weight in water, but maintaining some air pockets etc. in the fibres. Water held between the fibres can be easily wrung out. This wrung wool almost feels dry to the touch which is reasonably comfortable.
- During the process of adsorption of water by wool, the hydrogen bonds between water molecules break down, in an exothermic reaction, i.e. one that actually creates heat. Some tests show 1kg of merino generating as much heat energy as an electric blanket over an hour.
However wet wool is still wet, it will still lose heat to the environment through evaporation when wet. Are the above properties enough to counteract the evaporative heat loss any wet substance will experience.
The test
As my insulating tests so far have used an army surplus woollen blanket as a baseline I have chosen to use this as my first test material. The test format follows that of previous tests and is outlined in my previous blog post, wool versus synthetics, so I won’t repeat the method in detail. To summarise the test involves insulating identical cups of hot water with the test materials and measures energy loss in these over time.
This time I set the dry woollen blanket against a wet section of blanket and also an uninsulated cup. Whilst it is almost a given that the dry blanket would insulate better than wet, a key question to answer is whether you are better off using the wet blanket in preference to no blanket. The blanket was soaked to ensure proper saturation and wrung to remove any excess moisture.
The results – So would the wool keep you warm when wet
So after repeated tests to gain an average, and high hopes for the blanket, I was slightly disappointed with the result. On average the wet wool insulated cup lost 4% more heat energy than the uninsulated cup. Over 20% more than the insulated one. The implication of this is that faced with a situation where you have a soaking wet blanket and no other option for insulation you would be better off NOT using the wet blanket.
Below that surface result though, I do believe there are a few further considerations which mean all is not lost for the “warm when wet” theory.
- This test has only looked at a single type of wool blanket. A British army surplus variant. These are cheap, cheerful and popular but not the finest wool blankets on the market. I am pretty sure that that 4% deficit would be much reduced or even disappear with a superior wool type.
- Not only is the wool type not the best but the blankets are not woven to maximise air capture. They are heavy with a dense weave. Not optimal for maintaining insulative air spaces.
- As the wool drys from from fully saturated (4% worse than no insulation) to dry (20%+ better than no insulation) there will obviously be a point where mildly damp wool starts to insulate better than the no insulation. This crossover is important and emphasises the need to dry the wool as quickly as possible in a survival situation.
- Whilst I test with water close to body temperature, around 40’C, it is not constantly generating heat like we do. Thereby the blanket would almost certainly dry more quickly worn by a human than a cup of water. Additionally our movement can help dry the wool even more quickly.
A survival scenarios
So when would I choose to wear the blanket and when not? Well it all boils down to the circumstances. If i was weighing up whether to use the blanket to warm an immobile casualty I would probably not drape them in the wet blanket. I would prefer to donate them some of my dry clothing and try to dry the blanket with some activity, or use it as a wind shelter rather than in direct contact with them.
However if it is very cold, i.e. below freezing, there is also the possibility for the blanket to freeze on the surface which has a couple of benefits. First it prevents further evaporative heat loss, secondly it becomes a strongly windproof layer.
Finally thinking about that imperative to get the blanket dry as quickly as possible, I believe that whilst a saturated blanket will not keep you warm, a slightly damp one will. Techniques such as swinging the blanket with a stone in it to drive moisture out through centrifugal force could be helpful. As well as providing a warming activity!