In article . rogers.com,
Michael Daly wrote:
Heat loss is irrelevant compared to this.

Not so. If the Sleeping Bag is rated for -10C, the Vapor Barrier
lowers the rating to -20C. But because there is no danger of water
condensation, you can use water-proof Bivouac Bag also, which further
lowers the rating to -30C.

In case of emergency it is good to know that couple of heavy duty
garbage bags can turn your 100 euro backyard bag into $400 polar bag.