use of plastic or felt paper behind cbu

i've been reading here and John Bridge to learn how to add a shower to my tub.
everyone always mentions putting a waterproof layer over the studs in the walls of a tiled shower.
makes perfect sense to me, but i just talked to my local plumbing inspector (portland OR) and he's never heard of the practice. his feeling was that cbu was sufficiently water-resistant (which is contrary to what i've been reading here). oh yeah - i talked to a local licensed plumber too. same thing: he's never seen anyone do it.
so what's the deal with this?
the Wonderboard pamphlet says "in wet areas, nail or staple 15 lb roofing felt or 4 mil PE sheeting to studs".
the HardiBacker pamphlet says "If required by Local Building Codes, install a moisture barrier between studs and backerboard".
i thought Portland just used the national codes. is this moisture barrier thing only in state/local codes?
comments please
-barry

This is the type of item where codes and enforcement vary widely. Since the manufacturer recommends it, I take it to be a good idea.

i,m a gen contr.

use 30 lb. felt [tar paper] use roof cement over all holes than use 1/2" hardy board. buy a galvanized shampoo box from a tile supply store ,not home depot. the best book is setting tile by michael Burns avaible at h. depot
we go to extremes to keep water out. not many tile setters do this! We return to customers to do new projects, not to redo past ones!

good luck Tool

John Bridge forum is a good place to follow tile advise. The people on that site are serious tile people. You should use a barrier behind the cbu because the cbu is water resistant not waterproof. Over time moisture will make its way through the grout and the cbu and then into the wall cavity. You can use roofing felt or plastic sheeting, or you could install the board and then waterproof the top with a product such as redguard, or similar waterproofing membranes.