Marketing says every rain shell is breathable. Physics disagrees. A waterproof membrane by definition blocks moisture, and that includes your sweat vapor trying to get out.
That said, some shells move vapor better than others, and a few are good enough to wear running in warm rain without feeling like you're in a trash bag.
Three that work
Patagonia Storm Racer. Lightest of the three, minimal features, best breathability. I wear this running in rain down to 50°F. Above 60°F I'll still take it off in light rain because even the best membrane cooks you eventually.
Arc'teryx Beta SL. Overkill for most, but the Gore-Tex Pro fabric genuinely breathes differently than the cheaper Gore-Tex Active. I reach for this on backpacking trips where the rain won't quit.
Outdoor Research Helium. My budget pick. Paclite, under 7 oz, fits in a cargo pocket. Not the best breathability but at $160 it's a better value than most.
What to skip
Any shell that advertises "DWR" as a feature. DWR is the coating that keeps the face fabric from absorbing water. All shells have it. It's not a differentiator.
Anything with 2.5 layer construction under $100. They ship with a printed inner layer that delaminates in 30 days.
How to care
Wash every 5 to 10 uses with Nikwax Tech Wash (not detergent). Re-apply DWR when water stops beading. Most people skip this step and then wonder why their shell "stopped working." It's the DWR, not the shell.
The honest truth
In Southeast summer, no rain shell is fun to wear. The humidity outside is already saturating the air that the shell is trying to push through. Often I just run in the rain without a shell. Lesser of two evils.
