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.

On 22-Dec-2003, Peter wrote:

Heat loss is irrelevant compared to this.


Not so.[...]

Agreed. [...]

My point is that if there is no vapour barrier, then soaking your
insulation is a serious problem, whereas adding the vapour barrier
only increases the temperature rating a bit. If you want warmer,
buy a heavier bag; if you want dry insulation, get a vapour barrier.
Increased warmth is a byproduct of using a vapour barrier, not a
reason for buying one.

Mike

Michael Daly wrote:

On 22-Dec-2003, Peter wrote:


Heat loss is irrelevant compared to this.


Not so.[...]

Agreed. [...]


My point is that if there is no vapour barrier, then soaking your
insulation is a serious problem, whereas adding the vapour barrier
only increases the temperature rating a bit. If you want warmer,
buy a heavier bag; if you want dry insulation, get a vapour barrier.
Increased warmth is a byproduct of using a vapour barrier, not a
reason for buying one.

It was for me. I didn't have a problem with my insulation getting wet, but
sometimes it was getting a bit cold for the rating of my bag. Your
suggestion would have cost me a couple hundred dollars for another bag and
added another pound to my pack. A vapor barrier cost less than $10 and
added only a few ounces. Also doubles as a useful item to have for
emergencies on dayhikes just in case you need to bivouac somewhere (which
is why I got it initially). I only heard that it was called a vapor
barrier and that it solved the wet insulation problem from condensation a
few years after I started using it for the added warmth.