If your feet touch the outer of the bivi, then maybe it could cause some condensation . These tubs are much longer than the user (me) though, and feet hardly ever even touch the bottom. What annoys me about bivy bags/zipped sleeping bag cover types is;
Lack of airflow means moisture builds up.
Zips.
Getting in/out.
Putting your kit inside is really uncomfortable.
The top waterproof layer is wasted under a shelter
Is it half a bivy? Yes, sometimes...the best bit.
The waterproof bottom that stops your kit from getting wet from the floor and keep everything dry inside, whilst not being a pain to use and doesn't slowly dampen everything.
I suppose you could call it a bugnet hammock nappy, too.
OM'r 2.0 by
turnerminator, on Flickr