Timo Noko wrote:

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.

Agreed. I use a lightweight bag that is normally comfortable down to about
30 F. If it'll be colder than that I use a vapor barrier inside and that
way I've been comfortable down to nearly 0 F. It's a very inexpensive and
lightweight way to increase the low-temperature range of the sleeping bag.
Also cuts down on water loss overnight so I'm not as thirsty in the morning.