This is better than down.
I found a product, shiver shield, online and my first thought was how it could be applied as an under quilt. This is a special type of gel that does a more effective job of retaining body heat and stoping cooler temps from getting through than any other natural or man made material. On their website they only have it for sale as clothing items but maybe instead of needing any other sort of insulation a person could just wear the same clothes they had on. Now that would be weight savings! Downside side is this product is really expensive.
The next link is to a company that makes this jell. The link is for the applications for the product.