Let me get the basics of what Richard is saying straight.
Hot air rises, so the hot air rises to the top of the fan cover and cannot escape, because, as hot air rises, *it will not* spill down and under the lower edges of the cover.
Cold air settles, so the cold air will settle past the bottom of the cover, and by the same forces which prevent the hot air from escaping, the cold air will not settle into the opening under the bottom of the vent cover.
Sounds like either magic is at work here, or there is a new theory for physics and the movement of air.
Also sounds like someone is going to extremes to try to justify *not* writing it up.
You can try to justify it however you want to try, for as often as you want to, in as many ways as you want to, but none of them will change the fact that *some* sewer gas *will* enter the attic, and that *is prohibited by the code* when separating the two would solve the problem to the greatest extent readily solvable. (sigh)
