IMO a bivy is an absolutely TERRIBLE stand-alone shelter. I would absolutely use it in conjunction with a tarp, or just use a tarp alone. I guarantee you’ll experience bad condensation problems at some point especially in cold weather, and if you have to wait out a storm or something it’s very boring. Very little room to move around and stretch out, and if you’re hiking with others, you can’t socialize all zipped up in your bivy. It’s also extremely difficult to avoid getting your gear soaked when setting up camp in a storm. Seriously, try this some point. Turn on a sprinkler, have a friend hold a hose or something and try to do all your normal camp stuff- unpack the bivy, cook up dinner, change out of your hiking clothes, etc. all without getting everything drenched. Tricky.
Another huge disadvantage is if you’re counting on your bivy for bug protection- if it’s very warm outside, a bivy is absolutely stiflingly hot. Ugh.
I used to use bivys (a lot), in all kinds of weather and conditions, and I hate them. A simple tarp is an infinitely better shelter in my experience.
Get a tarp, throw in a mesh sleep sack for bugs as needed. Light, simple, elegant.
I’ve used a Golite Lair 1 (12 oz.) for about 3,500 miles now, including the entire AT. Nice product.
That’s my $0.02.
nuts