Originally Posted by
vaticdart
Yes, it's a pipe dream.
No fabric exists, as of yet, that is breathable enough to be truly waterproof and provide enough ventilation for the kind of sweat one produces on a bicycle while keeping a good pace.
Actually such a fabric does exist. Paramo "directional" fabrics suck water from the wearer so effectively that you can get drenched to the bone through immersion and they will then dry you out. Or -
http://www.outdoorsmagic.com/news/article/mps/uan/3160
..very effective at handling excess moisture when you do boil over. Some mountain biking mates were amazed by the steam rising from the Velez during a rest stop on a hectic night ride. Really, clouds of steam rising from the surface of the garment.
The downsides are that Paramo stuff is on the warm side, expensive, and much harder to find than the fashion outdoor brands like North Face and Patagonia. But staying dry inside your shell on a wet autumn or winter day is not a problem with this stuff.
The next best thing - and much cheaper - would be a pertex smock that had been sprayed with Nikwax water repellent.