Sorry, Bill, but I'm not through yet. I've sent an e-mail to ENERGY.COM for an explanation. Here's their reply, to which I will post up the answer. And this answer will NOT be opinion. I don't think I offered an 'opinion'. A Rad Bar wouldn't be a Rad Bar if the heat DIDN'T penetrate to the barrier, right? The heat HAS to penetrate the shingles AND the sheathing to get to the barrier. So stop what? If what you describe was to occur, the barrier would be on TOP of the shingles, or be the shingles themselves.
This horse isn't dead yet. Yes, venting moisture is IN ADDITION TO venting heat. Heat will hold a lot of moisture so to get rid of it, is a good idea. As far as a house w/o any attic ventilation??! You don't know what you're missing. Just search the Forum for people asking about attics and ventilation!
As scientific an answer as I can get is coming. Here's yesterday's response to my e-mail:
J Jacobs - Thanks for visiting Energy.com and viewing our Energy University Course "How to Save Energy (And Save Money)! I have forwarded your question and comments to the author of that course, Dr. Charlotte LeGates. She will respond to you directly within the next two business days.