Sleeping bag temperature ratings are about as honest as car manufacturer's gas mileage ratings. First, they are taken INSIDE a tent and usually involve a formulae involving water bottles. I don't know about you, but I am not a water bottle. IF these ratings were even remotely accurate, Every individual still has a different metabolism and body mass. The only worthwhile testing medium is to take a bag of given loft and sleep in it at various temps.Vapor barrier systems do work with the caveat of creating a swamp environment. The problem is upon leaving this micro environment your body immediately undergoes a very rapid cooling off as all that lovely sweat evaporates. In a survial situation with a body already taxed by various stresses this can prove deadly. Not to bore anyone repeating vendor references, but read the information on www.wiggys.com Jerry is VERY opinionated to the point of combatetness. He also has some pretty compelling arguments and information.