If you’re outside in cold weather, yes, you should wear a shell layer. However, it’s not just water coming in from the outside, it’s sweat from your body. You can’t avoid sweating; it’s how your body regulates temperature. But compare, say, a polypropylene sports jersey to a cotton t-shirt on a hot day; the cotton gets wet with sweat, and then stays wet longer. In really cold weather, that’s bad. (And if you’ve ever gone snowshoeing, you know that you’ll work up a sweat fast.)
There’s a lot of contradictory-seeming things happening, but it’s pretty easy to test results for yourself. Get comparable long underwear in both cotton and Merino wool, comparative outer garments in wool v. cotton (say, German or Swiss wool milsurp field pants and shirt v. heavy cotton duck pants and heavy cotton flannel shirt), go outside on a cold winter day and do the same activities wearing different fibers, and see which ends up feeling warmer.
If you’re outside in cold weather, yes, you should wear a shell layer. However, it’s not just water coming in from the outside, it’s sweat from your body. You can’t avoid sweating; it’s how your body regulates temperature. But compare, say, a polypropylene sports jersey to a cotton t-shirt on a hot day; the cotton gets wet with sweat, and then stays wet longer. In really cold weather, that’s bad. (And if you’ve ever gone snowshoeing, you know that you’ll work up a sweat fast.)
There’s a lot of contradictory-seeming things happening, but it’s pretty easy to test results for yourself. Get comparable long underwear in both cotton and Merino wool, comparative outer garments in wool v. cotton (say, German or Swiss wool milsurp field pants and shirt v. heavy cotton duck pants and heavy cotton flannel shirt), go outside on a cold winter day and do the same activities wearing different fibers, and see which ends up feeling warmer.