Is this worth it?

[Hana_Hanger]

I am thinking of a way to keep my down bag dry and also serve as a ER Bivy if ever needed.
Adventure Medical Heatsheet Bivvy

Does this work without causing the sleeping bag to get damp?
All opinions are :welcome or any suggestions.

http://www.backpackinglight.com/cgi-bin/backpackinglight/amk_heatsheets_bivvy.html

[spittinpigeon]

Moisture from your body will build up on the inside, and get your bag wet.

[dloome]

It's a vapor barrier- does not allow the transmission of moisture through the film. If you use it outside your bag like a bivy sack, any insensible perspiration your body produces (a lot) will be trapped in the insulation or on the shell of your bag.

The only time when a vapor barrier is used to keep a bag dry, is when used as a LINER in very cold conditions. In cold conditions, the dew point can occur inside your bags insulation- so used as a liner, a vapor barrier will prevent moisture from your body from condensing into liquid in your bags insulation or on it's shell.

If you want to protect your bag from external moisture, get a bag with a breathable, but more water resistant shell like E-Vent, Nextec Epic, or Pertex. You could also use a very lightweight, breathable bivy sack which will add some warmth, as well as help protect your bag. The one's made by Bozeman Mountain Works are a good example of this type of bivy.

[Hana_Hanger]

Thanks for the info on how it worked...I had an idea it would wet the bag. oh well...back to the drawing board....for the other options are very costly.

[Egads]

my experience w/ vapor barriers is they work best inside your bag, else, your bag gets wet.