Bivy question

If I'm fairly certain it won't rain I won't set up the tarp. much nicer to be able to see the stars and not have to bother setting it up and taking it down. I would just leave my head out of the bivy and you should be fine since that's what causes most of the issues. Just use it as kind of an overbag for your sleeping bag. I don't have a waterproof bivy though, which is why I try to be almost certain it won't rain if I am going to not set up the tarp. I am not sure how those would perform on their own.

There have also been interesting posts around here lately about how, in more humid conditions a tarp can significantly reduce the condensation that forms on your bivy by creating a different micro climate.

I often set up the tarp, but do not sleep under it.

I can go to sleep and enjoy the view of the stars, but when the stars disappear it is time to move under the tarp.

During my last trip (in Europe), several times I was absolutely convinced that there would be no rain and so I decided not to bother with the tarp due to lack of space or inability to get the stakes into the ground (solid rock under me). Big mistake! Storms can blow in from the north Atlantic without warning. From clear sky to all hell breaking loose in a matter of minutes. Luckily, I am a light sleeper and was woken by the sound of the breeze picking up rather than the rain drops which soon followed. And slow learner that I am, I made the mistake twice! Luckily, I was able to get the tarp out and cover things up so my down quilt didn't get soaked. But that is the last time I'll ever make this mistake in Northwest Europe (or the similar eastern United States).

Now in the desert or southern Europe in the summer, it might be a different story.