when insulation is required inside a bivi bag, i suggest a looking into a Blizzard Bag

http://www.botachtactical.com/blsubag.html

based on the original concept made by MPI Outdoors (which i can't find any longer), this british-version lightweight ultra-thin mylar bag is really three emergency "space blanket" bags nested inside each other and joined by internal baffles. it insulates unlike most mylar bivi bags.







i think this thermo inside a heavy-duty shell would be a perfect combo, allowing one to use the shell alone if warm or just raining, and using the two together in colder environments. its what i carry in the wilderness (but have yet to personally use them).

when these bags first came out i read one review by an ultralight hiker who used a blizzard bag as his primary sleeping bag. he claimed to be comfortable in temps below 20*F.

here's a thread about the concept:
http://www.backpackinglight.com/cgi-bin/...le_pagination=1

and here's a link to an old ETS thread asking about them:
http://forums.equipped.org/ubbthreads.php?ubb=showflat&Number=33608