Old Man Runner wrote:
The new Shakedry stuff is, in theory, better. I have only used it once and it seemed to work okay. I was still wet on the inside from the sweat I generated (and yes, the swamp effect still happens).
I have an OutDry jacket by Columbia that I've used for running and it's okay, but not great, as far as breathability. For hiking though, it kept me absolutely 100% dry for multiple hours when at a 20-25 minute per mile pace.
I have an OutDry jacket, too. It seems to be more breathable than traditional waterproof/breathable membranes, but that isn't saying much. ("Waterproof/breathable" sounds nice, but it's kind of a myth.) I wouldn't wear it for any kind of sustained aerobic effort. Hiking, yeah.
On the other hand, despite more breathability, water resistant jackets usually rely on a DWR finish, which will wet out and become useless after about 20 mins of sustained precip. Not great for a cold, rainy marathon, either.
Mechanical venting (e.g. pit zips) might be better.
Realistically, if I had to run a marathon in 2018 Boston-like conditions, I would probably embrace the reality of getting wet and go with a thin, lightweight, water resistant/wind blocking shell like a Patagonia Houdini layered over a moisture wicking base layer. I think that would probably maximize the advantages of having a jacket (cut the wind, block some rain) while minimizing the obvious disadvantages of a beefier shell (slowly cooking yourself in your own sweat). But those conditions are going to be tough any way you slice it.