Yeah bivys were only ever meant as a unplanned or even emergency shelter for the side of a mountain. You have to be carefull that your breath vents completley out of the bag, or the inside will be soaked by morning. Shit even then your gonna get condensation from moisture coming off the body. If you have a down sleeping bag this becomes a problem.
I personally carry a ultralight bivy that is breathable on the top half. So its not gonna keep the rain out by any means. And a tarp for wet weather. Specifically a military surplus tarp that doubles as a poncho. It’s a bit annoying to set up because of the hood in the middle. But its durable, has buttons down the sides for using as a poncho AND for converting it to a quick n dirty waterproof bivy. I’ve done that on trains a couple times. This system works well, the ponchotarp is a bit heavy, but its far superior to those shitty blue tarps, and lighter silnylon tarps designed for this purpose are fucking 70$ or more now -_- And its still lighter and smaller then the lightest one man tent that isnt a piece of shit.
I would really concider investing in all 3. A bivy isnt very fun if its actually raining, and you have to remember a bivg isnt going to protect all your gear, that shit is gonna geg soaked. But a bivy and tarp works well. Shi
As far as bags go, 45L is kinda small for a winter synthegic fill bag, if you have alot of stuff you won’t be able to fit that bag in there. Down you will have no problem… but they are expensive. If properly taken care of they can last years. I paid a little under 500$ for a down bag by Western mountaineering that lasted a good 6-7 years of constant traveling, thing was surprisingly well made. I probably saved money in the long run not buying and blowing out multiple shittier bags.
For bivys, this one is, in my and many others opinion the all around best on the market.
Award-winning for its ultralight prowess with our most durable lightweight Helium fabric featuring Diamond Fuse technology. This clamshell opening bivy is ideal of fast and light missions.
www.outdoorresearch.com
The design is nearly perfect, simple, effective, minimal, durable as shit.
borahgear.com
This is the bivy I use.