I think Unheard is on the mark: cabin air needs a corresponding low pressure exit, or it won't move as well.
I know basjoos recently mimicked the Aptera by adding vents through his tail to the outside. Check post #230 in his build thread for photos:
http://ecomodder.com/forum/showthrea...1-0-a-290.html
I wonder where the original cabin vents were/are on the Civic...