Some time ago I did a post where I tested the insulation value of dry and wet wool. The results as I interpret them are that wool does not keep you warm when wet, and in fact loses more heat than if though you were not wearing any insulation at all. Contrary to the popular assertion that wool keeps you warm when wet, wet wool clearly underperformed when compared to dry wool in a significant way. You can see the results of that test here.
In this test I wanted to compare Merino wool and regular wool along the same lines. First, I want to see if there is any difference in terms of insulation between the two, and second, I want to do some more wool tests to confirm my initial results. Most significantly, I wanted to test if merino wool keeps you warm when wet.
I found a merino wool cap that was just about identical to the regular wool one I used in the initial experiment. I decided to compare the two head to head. Because of how similar the two caps were in design and thickness, it allowed me to judge not only the comparative rate of heat loss, but also the comparative overall insulation both when dry and wet.
Regular Wool Cap
Merino Wool Cap
Earlier I outlined a test method for measuring the heat loss through an insulating material. In it I compared dry and wet wool. You can see the procedure in the post to which I linked above. In this test I repeated the same procedure with the merino wool cap under the exact same conditions.
In the graph below you will see five lines. The middle green line shows the heat loss from the container without the use of any insulation. The top two lines show the heat loss when dry wool (purple) and dry merino wool (orange) are used as insulators. The bottom two lines show heat loss when wet wool (red) and wet merino wool (blue) are used as insulators. The wet insulation shows data when the material was continuously kept wet throughout the experiment.
The top two lines are hard to distinguish from each other because the dry wool and dry merino wool seem to offer identical insulation.
Similarly, the bottom two lines are almost identical (I’m not sure the higher temperature for merino wool is statistically significant) because wet wool and wet merino wool seem to offer similar level of insulation.
Of course, both the wool and the merino wool show significant decrease in insulation, and therefore higher rate of heat loss when wet as compared to when they were dry. Neither fabric can be said to keep you warm when wet. In fact, heat retention seems better for a dry person not wearing the clothing than for one wearing the wet wool clothing.
The results are very similar, when we repeat the experiment without continuously wetting the insulating fabric. Here both the wool and merino wool were gotten wet prior to the test, but no water was added after the test began.
Wet merino wool (blue) seems to recover its insulating ability a little faster than wet wool (red), but I am not sure that difference is statistically significant. Both materials show an increase in insulating ability as time passes and water drains/dries from the material.
Interestingly, merino wool retained a lot more water. When completely wet, the merino wool cap was several ounces heavier than the regular wool cap, even though they had the same starting weight when dry. I am not sure why.
Anyway, not a particularly interesting experiment, and the results are more or less expected, but it’s good to confirm the initial findings.
Lastly, I want to take the opportunity to address a few questions/assertions that were brought up in the comment section during the last test:
The results would be different when you use a human body because it doesn’t cool off, but rather continues to produce heat at the same level.
The results would be different only to the extent that the shape of the lines would be different. We wouldn’t see a continuous drop in temperature until the body runs out of energy, so the lines would be more horizontal. However, the result would be exactly the same with respect to the rate of heat loss. You would still see the same gap between the line showing the insulation of the dry wool and that of the wet wool. The wet wool would still lose heat at a much higher rate than dry wool.
You are using completely wet wool rather than just wet wool.
Yes, this assumes that the clothing is completely wet. It assumes a scenario where you have fallen in a river, or have been caught by rain. The less wet your clothing is, the more insulation it will provide, just like with every other insulative material.