A large heavy duty garbage bag is a good thing to slip your bag into at night and also to carry your bag in during the day, roll it up in it. Done like this, you can tie it to the top of your pack or to the bottom or carry it in your pack----however you want w/o fear of it getting wet.
I did not say seal the thing super tight when you sleep in it at night. You are going to need air to breathe, so the bag and outer bag will be able to breathe too. Hence no condensation. Also if you leave a little air exchange hole open just a tiny bit, this will also stop condensation within your tent. Also will give you some fresh air to breathe.
It is also good to use a garbage bag outer shell in case your tent or tarp leaks during a heavy rain, then hopefully the outer shell garbage bay will keep your bag nice and dry.
Also using a large garbage bag as an outer shell will help keep your bag nice and unsoiled by the ground, etc.
Now there are some waterproof bags out there that you can supposely sleep in when it is raining heavily and not get wet, but don’t think most are waterproof and even if yours is, the zipper opening will/can let lots of rain in.
Whatever you do, you do not want to get your bag wet from the outside or the inside, because it will be real heavy to carry and not very nice to sleep in----a cold wet bag.
Now suppose you hike in the damp drizzle during the day. At night, you want to be able to slip on some dry clothes and jump into a dry sleeping bag-----so waterproof, and waterproof, and take extra precautions to waterproof and keep things dry some more.
Perhaps others can advise you how they managed to keep their bag nice and dry for sleeping in at night.
See you out there. :cheers
Maintain