I am a little confused by this answer about whether or not cotton "kills". Cotton is clearly a poor choice for an insulating layer in wet environments since its insulating properties are heavily dependent on its moisture content. The point of a base layer, however, is not to provide insulation but rather to wick the moisture away from your body. What people seem to leave out is that the moisture potentially ends up in your insulating layer (depending on the dew point and other factors) and I think takes energy to evaporate this moisture. This then leads to the argument of replacing the base layer with a vapor barrier. As I generally have no desire to hike in a plastic bag, would a cotton "base layer" serve as a reasonable middle ground between a wicking base layer and a vapor barrier?
There is a difference between just soaking up water and "wicking". Cotton is hydrophilic, which means it likes to attract and hang on to water molecules. Unlike a good wicking fabric (which relies on capillary action), cotton will not transfer that water up through your layers to the air as quickly.
Here are some reasons why a wet base layer is bad:
- Cold - If you are wearing layers at all, you're probably not in "benign" conditions. I typically don't wear more than one layer if it's not cold. If it is cold, then you don't want water directly on your skin. It transfers heat very rapidly.
- Chaffing - Wet cotton chaffes. I don't typically want my base layer chaffing me.
- Microbes - Bacteria, fungi... microbes tend to like wet conditions. Spending long periods of time wet in the wilderness is a good way to get a skin infection/condition.
The moisture ending up in your insulating layer is not necessarily a bad thing. That layer is not touching your skin, and you have an air buffer between it and your skin. That is definitely better than keeping that water against your body and since perspiration is not optional.
Regarding the risk of moisture ruining down insulation: If you're in a region where the humidity is such that your sweat will not readily evaporate through your down layer... then you probably shouldn't be using down. Down is a less ideal choice for humid/wet environments. But that's not a base layer problem and has nothing to do with why using cotton as a baselayer is generally a bad idea.
Another point about the insulation layer getting wet. If you are sweating so much that you're soaking your insulation layer faster than it can evaporate then you're not managing the layers properly. One of the key points of layering is to manage temperature and avoid sweating.
Cotton doesn't wick away any moisture, it will soak and capture it. Furthermore cotton is heavy (compared to other materials), doesn't insulate, becomes wet very quickly, and stays wet for a long time.
If you carry the right layers and adjust them well, you can often get away with cotton with no discomfort let alone danger in benign conditions.
But we shouldn't plan for only benign conditions and nothing going wrong. An example: If you're delayed and the weather changes for the worse, you could end up with cotton under waterproof layers just as the temperature drops. You're likely to get cold. If the delay means you're static outside you could get very cold. So what should have been an uncomfortable few hours caused by a fairly minor injury or a trail blockage gets dangerous.
You generally have to draw a line between a hike and a casual stroll where you would be very rare to have any difficulty in jeans, trainers and a cotton t-shirt + fleece/waterproof. This line isn't always obvious.
I understand your desire not to wear a plastic bag - I used to share some of your scepticism about synthetic base layers. But only the very worst feel like plastic against your skin - and arguably those aren't fit for use as they feel like that because they don't wick. There's always merino, but my base layers need too much washing for it to be a good option so I haven't tried it.
Another disadvantage of cotton is that, because it holds on to moisture and gets saturated quickly it can get very uncomfortable where it is between your skin and your pack, especially around the straps and back. Over a long day this can get quite unpleasant and over a few days you can get serious skin problems from salty sweat being rubbed against your skin.
You must consider here why you would be sweating in the first place. So, you must take into account that the body may produce a bit more heat from time to time and it will want to dump that excess heat into the environment using evaporative cooling. E.g.in Northern Norway I had to travel a bit to get to a place where I could take pictures, when I arrived I was a little sweaty. I opened by jacked pulled my sweater up a bit for a few seconds and closed everything, the ice cold air was then trapped inside my clothing, as this heats up it becomes bone dry. The evaporation plus the ice cold air provided me with the cooling I could still use at that moment.
Had I worn cotton clothing instead of the wool + polyamide stuff, then what would have happened is that during the time I was feeling a bit too warm, I would have produced more sweat because the cotton fabric will hold on to fair fraction of the sweat. So, the evaporative cooling would have been less with the same amount of sweat produced per until time, therefore I would have produced more sweat. All that extra sweat would have ended up in the cotton fabric.
But when I reached my destination, I needed less cooling. I would still be a little warm, but all that saved water in the cotton fabric would keep on evaporating for quite a long time. If during the walk 50% of the sweat was captured, then this means that the same amount of sweat that I lost via evaporation during the walk, would still have to be lost. This means that the energy dumped in the environment during my walk will be radiated away again, but now I'm standing still and I'm producing far less heat.
The point of a vapor barrier is that it doesn't allow any moisture to escape. A middle ground role would mean that it's not good at doing either job: cotton would soak through, and the moisture would spread to the rest of your layers.