Captain Paranoia, the example I quoted was not a scientific test, and for sure there may have been many other variables involved, but I doubt the hugely different outcomes could be entirely accounted for by differences in gender, nutrition or clothing between the couple. You can't enclose yourself in a polybag so it is much colder than a breathable bag, and the condensation is orders of magnitude worse. Hence the guys face was covered in ice. You say you carry a polybag because it is six times lighter - but in what conditions are you saying you will survive benightment in one? On the tops in Scottish winter? It is only a survival bag if you will actually survive in it. Otherwise it is a body bag.
What you carry is to some extent an issue of personal ethics and judgement; my own is that I don't want a report like that written about me by the Mountain Rescue guys, so I carry enough kit and food to be able to survive a night anywhere on the route I am undertaking. And last night, when I was coming off Bidean nam Bian in the dark, that included a breathable bivi-bag (a RAB survival zone). In the summer, in benign conditions, I switch to a silver survival bag, which I can supplement with my rucsac and rucsac liner. But I'm going to buy a blizzard bag because I think it gives you a much bigger safety margin.
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 Donner Party! Oh yeah - knew it rang a bell. Thanks for the answers!
(Sorry for the double question asking - think I might have resubmitted it by hitting refresh...)
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Perhaps you could make a bivi bag from the Gossomer Gear Polycryo groundsheet (they are noisy/rustley). (Winwood Outdoor sell them)
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oh - and in fact along exactly those lines Alan B 27, they also said:"You might also be interested in the Guides Siltarp2 which is a flat tarp which can velcro into an envelope to use as an emergency bivy for two people." http://www.integraldesigns.com/product_detail.cfm?id=73
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It has been said by some that as aluminium conducts heat 100 times faster than air that you shouldn,t lie on a foil blanket but wrap it around yourself instead. The idea is for it to reflect radiated body heat back to you and help reduce convected heat loss. Mine made a fab rainfly and heat deflector once when a little campfire 8ft in front of my hammock was threatened by a sudden downpour... and it neither melted , nor could i tear the mylar foil away from the parachute chord it was tied up with.  I once bought an orange "survival bag" - and its sadly in the shed somewhere, waiting for me to include it when making up my own survival tin... come the day. It is true that you would sweat in such thing as a plastic bag but the idea is to put some dry grass, leaves or whatever between yourself and the bag to give improved insulation and offer a degree of "breatheability". Still, if we,re ever going to get caught out in a life threatening situation (such as being lost on top of snowdon in thick fog when the temperature is plummetting and the daylight is slipping away) we will be glad of any kit we have. (as the aforementioned orange pouch gets dragged out of the shed) 
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 where can i find more info on this Dalton party incident? I searched google and got nowhere Mr Brittan can you help?
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 I've had one of those orange survival bags in the bottom of my bag for years now. The only time I ever used it was one summer evening when I decided on the spur of the moment to stay the night on Ingleborough to catch the sunrise. Maybe I should have a check on it. Dave Our Wainwright walks
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 John - Chris T posted stuff off Wikipedia about the Donner Party - might be the incident being referred to. Some more info here.
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 ptc* that looks like a pretty good product. Have you slept in it? Is there a UK stockist or did you order it online?
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ptc*, I agree with John, that bivvy looks pretty good. The only downside is that you won't be able to use it for sledging (which as we all know, seems to be the only thing we ever use our orange bags for!)
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 I like the idea of a mutli-functional bag... <sheesh sheesh sheesh> What's the fabric? Is is metallized TyVek or something similar?
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 Hey guys, I'll have a proper look at it over the weekend and get back to you. I'll try and get some pics as well. It's really light 
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Any update on your opinion of tyour Heatsheets bag ptc*? I came across one in Keswick this weekend and thought about it but decided not to. And ever since I got back sunday I've been looking at them and 'umming' and 'aarring' about getting one. Your impression could sway the decision. Cheers
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