The best heat insulators we ever developed were composites of fluorocarbons in ceramic matrices. We have to heat-soak them at 1,990 degrees C for at least 96 hours before they are "cooked" and all the fluorocarbons have been evaporated out. These cover the Space Shuttle. Fragile as they are, Space Shuttles batter their way out of the earth's atmosphere, attain orbital velocity/energy, and then batter their way back home.
Cotton would never stand that sort of heat or pressure.
Other heat insulation products include ablative insulation. It looks and feels like rubber but bubbles up thickly on application of heat and then they char, leaving the surface still cool enough to handle.
Some TiO/2 insulation resists heat up to 4,500 degrees C. However, being a titanium compound, it is too heavy for anything but nuclear reactors on Naval ships.
Cotton makes a very poor insulating garment for any hot service. In the presence of intense radiant heat (e.g. flash fire) it not only provides scant little insulation, but can actually smoulder and then ignite.
Cotton makes an even poorer cold-service insulator, because it lets warm air out and cold air in. Even if you put on numerous layers, cotton will not provide you with winter warmth, as it wicks in water and water saps the heat out of you. A far better choice for temperate wet climates is wool or Gore-Tex(tm).
The ONLY way a cotton garment is going to keep you warm in the winter is if you have a vinyl windbreaker over it, and even then, it's not very good as the outer garment will trap your perspiration and chill you as badly as though you had not even put it on.
The only real reason to buy cotton garments is that they look cool.
