This question seems to get asked at least once a month in just about every outdoor and bushcraft forum. The answers, based on the experience of individual people, are all over the place. Some of them are reasonable, others are based on nothing more than wishful thinking.
I tried looking for any studies which would address the issue directly, but was not able to find any. So, I decided to try to come up with my own experiment.
The experiment is fairly simple, although time consuming. I took a plastic container with a cover. I brought some water to a rolling boil (so that I can assure the same starting temperature each time). I filled the container with the hot water.
For the first part of the test, I needed to establish a control, so I measured the temperature of the water in the container without using any insulation. I took measurements at the start (0 min), and then at the 5, 10, 15, 20 ,25 and 30 min marks.
For the second part of the test, I completely enclosed the container in wool insulation. I used a 100% wool knitted watch cap. Then I repeated the above steps, measuring the temperature.
For the third part of the test, I tried to simulate a continuously wet environment, such as rain. Using the same wool cap, I soaked it in 78 degree (F) water (room temperature at the time), and let it drain for one minute. I then enclosed the hot water container with the wet wool and proceeded to record the temperatures. Each time the temperature was recorded, I poured half a cup of 78 degree (F) water on top of the insulation, to prevent drying out.
For the fourth part of the test, I simulated an emersion such as falling in a river, where there was an initial source of moisture that was then removed. I did that by just soaking the wool cap initially, and letting it drain for one minute, then repeating the above steps without adding any more water.
As a side note, the weight of the dry cap was 2.4 oz, while the weight of the cap after being saturated with water and then allowed to drain for one minute was 8.4 oz. The cap took over 16 hours to dry at room temperature.
The first run, without any insulation simply provided a baseline. As expected, the temperature decreased over time.
The second run with dry wool as insulation, demonstrated much better heat retention than without the insulation. The temperature decreased much more slowly because the heat was retained by the wool insulation.
The above graph shows the expected results. Wool insulates, so the heat producing object (the container with hot water here, or the human body under real world conditions) retains more of the heat, cooling down more slowly.
Now came the interesting part of the test. Will the wool continue to insulate in the same way when wet? As outlined above, the first part of the wet wool test provided for a continuously wetted wool insulation with room temperature water. Here are the results:
The results show a rather dramatic difference between the insulation provided by the dry and wet wool. The wet wool, represented by the red line in the graph allows for measured temperatures lower than not only the dry wool but also those without any insulation at all. The more important information however is rate of heat loss. As you can see, initially, the heat loss is higher than that without any insulation, as would be expected because during this time the water immediately surrounding the container is being warmed up, consuming more heat. After that, the rate of heat loss seems very similar to the rate of heat loss without any insulation.
Now, the results might appear more extreme than they actually are. In reality, the fact that the wet wool can provide as much insulation as no insulation at all, shows that the wool is doing quite a bit of work here. The thermal conductivity of air is 0.023. The thermal conductivity of water on the other hand is 0.6. The much higher thermal conductivity allows for much higher heat loss in water than air. Technically, an article of clothing that is saturated with water, should have thermal conductivity closer to that of water, causing much higher rate of heat loss when compared with heat loss when just exposed to air. The fact that the wet wool provides insulation similar to that of air (no insulation) shows that it is in fact insulating.
Of course, while there is a degree of insulation provided, when it comes to the ultimate question: does it keep you warm, the answer seems to be a clear “no”. It keeps you as warm as if you simply removed the wet piece of clothing.
The second part of the wet wool test seems to confirm that. Remember, here there was only initial exposure to water, without any further re-wetting. Here are the results:
As you can see, when the insulation is initially wet, it follows a similar rate of heat loss as the wet wool in the above chart. As time passes however, and the water drains /dries, the rate of heat loss decreases, and the temperatures start to increase relative to the wool that was kept wet.
Both tests however show that the heat loss from wet wool is much higher than that from dry wool. In fact the results seem to indicate that you would be as warm by removing the piece of wet clothing (assuming you can then stay dry) as you would be by keeping it on.
This shouldn’t be that surprising. The numbers that are usually tossed around on forums are that wool retains anywhere from 60-80% of its insulation when wet. While that sounds good on paper, think about the following scenario:
Imagine you are wearing sufficient clothing to thermally regulate in 20 degree (F) weather. The clothing keeps you warm, but is not overheating your body. Now let’s say that you remove a third of that clothing (the equivalent of reducing the insulation of your clothing to 70% by getting wool wet). This is a significant loss of insulation. Unless you were seriously overheating before, losing 30% of the insulation of your clothing will mean that your body is now losing much more heat than it was before, preventing thermal regulation.
The only way you can still stay warm, is if you are in a relatively warm climate, so the loss of insulation does not matter. Otherwise, wet wool clothing will not keep you warm. It may keep you warmer than some other textiles when wet, but it will not keep you warm. Make sure your clothing stays dry. Wet is wet; and wet is cold.