I noticed that 100% cotton shirts and clothes make me sweat and when I take them off they are soaked in sweat. This happens even when I am cold. I can only wear 50/50 cotton/polyester or (preferable) 100% polyester clothes like underarmer .
I am not a doctor, but I’m guessing you probably sweat more or less the same amount no matter what you wear…but the cotton absorbs your sweat and stays damp, while stuff like Under Armour helps wick the sweat and evaporate it.
I wouldn’t chalk it up to “100% cotton makes me sweat,” I’d chalk it up to “100% cotton is not the best type of shirt for a sweaty guy to wear.”
This is the right answer. Synthetic fabrics typically don’t actually absorb water, so it tends to go somewhere else- evaporate, soak into some other clothing, and doesn’t become sodden and stick to your skin.
That’s the big attraction of stuff like Underarmour, dry-fit, etc… and also for synthetic fabrics in long underwear. They don’t let the sweat sit on your skin- they don’t absorb it and they tend to conduct it into outer layers and/or evaporate it, keeping you subjectively cooler or warmer.
I agree with the above two posters. I do a lot of bicycling and wear technical clothing with materials like Kool Max. I tell cyclists and runners to avoid cotton, even for the socks, and go with a technical material. Heck even thin wool jerseys are better in the heat than cotton.
About the only reason cotton would make you sweat more is that a soaked cotton shirt could trap heat which would make you sweat even more.
One other point: If you use technical clothing, be very careful how you clean it. Many of the tags say to do NOT use fabric softeners or dryer sheets as they can clog the material and thereby block the wicking effect.
I machine wash on gentle cycle with cold water. I usually use a ‘free’ detergent without perfumes and such. You can also buy a detergent made especially for technical materials. I hang dry technical clothing.
But shouldn’t the cotton make me sweat less because it "breathes " stuff like polyester doesn’t “breathe”
Cotton holds onto the water - that’s why bath towels are made of cotton. You feel damp in you cotton clothes because the fabric hangs onto it very well. Most synthetic don’t hold it as well, so the water can evaporate from them faster and you don’t feel damp.
When my boys were in Scouts and prepping for winter camping trips, the most often repeated phrase was “Cotton kills”. In cold weather if you sweat in a cotton garment it stays damp and will lower you body temp significantly. Wool or synthetic clothes are much better at retaining body heat.
The moisture doesn’t come only from sweat. Your body is constantly emitting water in the form of vapor.
With cotton clothing, the water vapor condenses when it hits the cloth and the cotton gets wet and stays wet. With synthetic fabric technical clothing, the water vapor condenses when it hits the fabric and is then wicked to outside. With natural fiber merino wool clothing, the water vapor is wicked to the outside before it condenses.
A merino wool tee shirt will keep you drier than a synthetic and does not have the odor problem associated with synthetic technical fabrics. If you have never worn one, a synthetic wicking shirt will start to stink within an hour after you start exerting yourself. Some of them have some sort of antimicrobial treatment that can delay the onset of the stench for a little while, but a backpacker who has been on the trail for two weeks in a merino wool shirt will have significantly less stench about him than one who put on a clean synthetic shirt this morning … and stench is the right word. Technical fabrics work, they keep you cool, but they stink, and the smell doesn’t always come out in the wash.
Merino wool does not have the itch factor of other types of wool. Due to the wicking action, a light weight shirt is not hot. It does not cause odor, easily comes clean with gentle washing, and doesn’t need to be washed often. The big drawback of merino wool is the cost.
That was true for nasty 1970s-vintage polyester clothing. The modern “technical fabrics” that others have referred to (like UnderArmour, Nike’s DriFit, etc.) are completely different. They’re made to breathe, and wick moisture away from the body.
When I started running a few years ago, I didn’t know about tech fabrics, and I was wearing cotton t-shirts, cotton socks, cotton underwear, etc. Within a mile or two in hot weather, they were soaked. Tech fabric clothing makes a huge difference in this regard. Now, when I run, the only natural fabric which is on my body might be the cotton in my shoelaces (if they aren’t polyester, too 
).
I do tumble-dry most of my running clothes, though on the lowest setting, and only for a short time (it doesn’t take very long to get them dry). The exception is my cold-weather tights, which were particularly expensive, and I don’t want to risk messing them up.
I was a cyclist first and foremost. When I started doing half marathons, I was amazed that the runners were wearing mostly cotton t-shirts. I wore technical cycling jerseys, with the 3 pockets in the back, and people were asking where I got them. Must have been because I’m slow and all the fast tri-geeks were way up front.
Now days the shirts given at major runs are usually made out of a technical material.
So do I have a medical problem if I sweat in cotton even in the winter when I’m cold?
Maybe? What are you doing when you’re sweating during the winter? Snow shoveling leads to feeling like you’re freezing to death, but the huge amount of exertion will still make you sweat. Exercise will too. I can’t think of any other time during the winter when I’m noticeably sweaty, though.
There’s no way for us to tell. You can sweat at any temperature depending on how much you’re exerting yourself and how much insulation of any kind you are wearing.
ditto with the others who ride or run or exercise. The evaporative qualities of nike, UA, Champion and other type cool max clothing will help cool you down while cotton holds on to the moisture and in part blocks evaporation. Plus on cooler days or very cool evenings you can layer with that type of clothing to lock in heat when needed.
what about “regular” clothing (not technical) ?? because I have been avoiding poly clothes since the 1970s - believing they will make me sweaty.
This, I can’t answer from first-hand experience, since, other than tech clothing, I, too, avoid synthetic fabrics in clothing.
just sitting at my desk, not doing anything.
Are you overweight?
Right, I love my Smartwool underwear.
Note that in VERY hot weather I like loose fitting linen.