On 22-Dec-2003, Gary S. Idontwantspam@net wrote:

Good technical description at www.warmlite.com, specifically:
http://www.warmlite.com/vb.htm (Note to the prudish: close your eyes
when viewing this site)

Most of which is of secondary importance. The real reason for a vapour
barrier in a sleeping bag or winter clothing is to keep the insulation
dry (just like the vapour barrier in your house walls).

Polar adventurers have suffered from getting their down sleeping bags
soaked with water vapour that has condensed in the insulation (at the
point where the temperature in the insulation is the dew point).
Eventually, they end up with a bag that is frozen solid. This can't keep
you warm. One of the documentaries in the Banff Mountain Film Festival
showed this a few years ago.

Heat loss is irrelevant compared to this. However, it seems that the
manufacturers have picked up on it as a major selling point.

Mike

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.

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.