I took the road bike out the other morning and wore a jersey with a full zipper. It was unzipped a good bit due to the heat. That afternoon I made a store trip and wore one of my Gildan DryBlend (50% Cotton/50% Polyester) high viz tee shirts. The afternoon temps were a good ten degrees higher but I was much more comfortable in the Dryblend shirt than I had been, even in cooler temps, with the 100% polyester jersey.