My bivy is probably a bit on the heavy side (650 grams) - but it is heavy duty. I also complement or alternate with a sturdy trash bag, a disposable rain poncho and a AMK bivy bag.

I've read this thread with great interest and I have very little to add to a subject that I find very important.

Just one note of caution: Don't have too high expectations of breathable materials:

- Breathable materials breathe at their best when you have a temperature gradient, such as a tight fitting goretex jacket.

- Even the best breathable materials are what I call "fatigued breathers". I.e. even the best materials don't really breathe nearly ENOUGH. Particular in a bivy bag setting (where the temperature gradient is much less than in a tight fitting jacket).

My personal opinion is that condensation will form inside ANY waterproof bag that you wrap around you and sleep outside in chilly weather - no matter how breathable the material claims to be. Your body is at 100F and lets out a certain amount of water vapor all the time. The inside of the bivy bag is pretty close to the outside temperature. Condensation is inevitable.

Personally I wouldn't bother with breathable bivy bags until they have a well proven track record for keeping you dry for prolonged periods that otherwise cause massive condensation.

As always, I'm more than happy to be proven wrong