It may be silly, but I don't want to add to the humidity inside the tent. Silly, in that the water vapor let through the tyvek may be negligible. I don't know if it is or not. If it does make a difference, it wouldn't be one of those things that you notice through experience. Unless, for example, you covered half the floor with Tyvek, and the other half with plastic, then layed a sleeping bag directly on each half, then measured the amount of water vapor in each sleeping bag (though most people use pads/mats).

I want to get Tyvek, because it is lighter. I just don't want to add the humidity in the tent by doing so. I'm hoping for the vapor impermeability of plastic, with the lightness of Tyvek.

Durability and protection are also nice. Maybe I'll just stick to the 6 mil plastic I'm currently using? I had some thorns I didn't notice make it through the 6 mil plastic, but I suppose those thorns would make it through just about anything: just means I need to be more careful clearing off the ground.

Side note: I learned that it is easy to fix big agnes air core with the provided glue: patch wasn't even necessary.

http://www.whiteblaze.net/forum/archive/index.php/t-71171.html
http://joekisner.blogspot.com/2013/09/why-i-do-not-use-polycryo.html
https://www.backpackinglight.com/cgi-bin...thread_id=92504

I'm going to try the 2 mil plastic you and a friend suggested. He said that's what his tent manufacturer suggested.