I would try them on in the store since they come in different sizes and tightness. Some, like Specialized, are really tight like a compression shirt. I just bought some XL Pearl Izumi IQ sun sleeves because they were a little looser. I haven't had much chance to use them in the heat yet. They all seem to be a lot tighter than a long sleeve jersey. One of my favorite jerseys was the Performance UPF white jersey which was cooler in the heat than short sleeves (but isn't sold anymore.)