Everybody agrees on how to dress to keep warm and most people who participate in outdoor sports understand the importance of preventing hypothermia. A layer of wicking fabric (wool or synthetic) tight against the skin pulls sweat away from your body; this prevents cooling of the body by evaporation of sweat from the skin surface. The wicking fabric insulates when wet, so evaporation of the sweat from the surface of the fabric does not cool your body. Add further insulation layers (made up of wicking fabrics) on top of this base layer and a surface wind-blocking layer on top as appropriate.
The requirements of staying cool are, obviously, exactly the opposite of the requirements for staying warm. As ambient temperatures approach body temperature, about the only way you can get rid of heat is through evaporation of sweat. You want sweat to evaporate from the surface of your skin, because this is the most effective way to dissipate heat. Garments should be loose to allow air flow against your skin. Sweat that evaporates from the surface of fabrics that do not insulate when wet (cotton, silk, linen) provide almost as much cooling as sweat evaporating from the surface of your skin.
The requirements of staying cool are exactly the opposite of those for staying warm, yet most “serious” cyclists have been convinced that a tight jersey made of wicking fabric is required for all conditions. Both synthetics and wool have been sold as “warm in winter, cool in summer”, and “cool and dry” in hot conditions. Wicking fabrics clearly can save you from hypothermia in the cold and wet, but cotton protects better from hyperthermia. Tight garments are better for aerodynamics when you are seeking top performance, but in hot conditions your performance can be limited more by overheating than by the aerodynamics of your jersey. The only modification you can make to a wicking garment to make it cool is to make it lighter or more open so that more air passes through it and there is more evaporation from your skin, but it still wicks.
Summers are pretty hot and humid here. My approach is to wear wool up to about 75 or 80F, depending on the humidity, the amount of sun, and the temperature range I will be riding in. I wear cotton at higher temperatures with a wool jersey in the handlebar bag when there is a significant chance of rain. Of course this style choice makes me look a little unconventional on summer rides, and people occasionally try tell me that what I am doing is just wrong. But no one has provided a convincing argument based on physics, either in person or in web discussions.
I don’t know of any formal studies on this subject, so I can support this logic with experience. I know I am more comfortable in cotton under hot conditions. And I also know that I passed a lot of people who were fitter, faster, and riding much lighter equipment last summer on the Cherohala Challenge century, when the temperature was in the 90’s and the dew point was in the 70’s. I rode in a Hawaiian shirt and stayed relatively comfortable.
Am I really missing something, or is this a victory of marketing over sense?