Sun shirts are meant to be worn in the sun, so they need to be breathable to be comfortable. A nonbreathable sun shirt will perform like a sweat-trapping garbage bag.
For sun shirt manufacturers, it is a challenge to maximize breathability and minimize UV infiltration. Still, through a combination of high-performance materials and intentional weave/knit patterns, it can be done. The Mountain Hardwear Crater Lake is one of the most breathable sun shirts on this list. We’ve worn the Crater Lake while climbing and running in temperatures up to 100 degrees, and it always remained airy and comfortable.