>>483623be careful with the bivy though. it needs enough space to not compress the loft, and also breathable enough to not trap condensation, which in the end will soak the bag. I'd recommend against a waterproof bivy in all but well below freezing temps to avoid condensation (it will hopefully freeze solid and then you can scrape it out in the morning)
if you insist on a separate bivy, I suggest one of the light "splashproof" bivys made of light nylon (pertex and others). the DWR will repel a bit of rain, while being sufficiently breathable to reduce condensation.
to be perfectly honest though
>>483629 is on point. between down treatments and DWR coatings on bag surfaces, there's little need for a bivy.
if you're concerned about the bag getting wet during travel, then pack it inside a dry bag, and open the bag only inside your shelter.
