One County in this area won't even allow flexible duct for bath fans (which must be vented outside).
Their contention is that the convolutions create a "trap" which will catch and keep moisture and bacteria.

Scott35,
I once printed the entire '95 Title 24 Non-resi. All 600 and some odd pages.
I to this day have not had time to read it, only the 80 or so electrical pages. (and that's 2 Code cycles old) [Linked Image]...S