You may be able to get away without a vapor barrier, but I would not do it. Let me quote verse and chapter of the 2007 TCNA's Tile Handbook procedure B412-07 Bathtub/Shower Walls for Cementitious Backer Unit/Fiber Cement Underlayment: protect wall cavity, wood studs or furring from moisture with membrane (ANSI A108.02-3.8).

I'm just a DIYer with a newly built house. How do I know this stuff? Went through the same thing with my builder, except after the shower was built. Mine is a steam shower, so it's a little different from just a shower. But the tile guy said I didn't need a barrier because the cement boards are water/vapor proof. Unless a barrier, per ANSI standard, is used, nothing in a shower (tiles, grout, sealer, cement board, etc.) is water proof, let alone vapor proof. Ask you installer what the perm rating of the materials that are being used, ask to be shown where in the (industry standard) TCNA Tile Handbook that says what's being proposed is correct. Chances are if you are having this debate, installer doesn't know.

For exterior walls, the vapor barriers should be applied to allow drying of the wall cavity to at least one side. I'd have to dig deeper into online info as to what is applicable for your climate/construction method. But you have to allow for drying to either inside or outside of the house. Basically, you have to keep water out of the wall cavity first, but also allow for it to escape once it's inside. And it doesn't matter if it's inside or outside walls.

It's a long story, but I got a shower torn down to the studs, with mold growing on a wall due to steam penetration. Go to http://www.johnbridge.com/vbulletin/forumdisplay.php?f=1 and learn about all the crazy ways showers are being screwed up. Really, it's a good place to learn, really friendly pros, that know what they are doing, willing to answer questions. Much better to do it right the first time.

Any other answers?

Or is the answer, "yes it has to be used all the time every time behind a shower/tub?"

That's the answer. tufenhundel's answer is about as comprehensive as it gets. The only time you don't need a vapor barrier is if you use certain topical waterproofing membranes. I say "certain" because there are some waterproofing membranes that still require the use of a vapor barrier in conjunction with them. The ones that would not require a vapor barrier that I know of would be Custom's Redgard, Hydroment's Ultraset, and of course, Schluter's Kerdi. There may be others, but these are the ones I know of.

So if you use "hardibacker concrete or cementboard, tile, gout, caulk and sealer is a vapor barrier really necessary?" As asked by the oppoostion...

None of those items is waterproof. Well, maybe the caulk. Water and steam can penetrate cementboard, tile and grout - even if it is sealed.

So if you use "hardibacker concrete or cementboard, tile, gout, caulk and sealer is a vapor barrier really necessary?" As asked by the oppoostion...

Absolutely.

You don't seem convinced even everyone who bothered to post are on the same page. Let me provide some more details as I have a bona fide case for what is happening in most CBU built showers:

My house is built with SIPs (foam and OSB sammich, if you will). The shower was built against one inside wall (basic 2x studs), one outside wall (OSB-foam-OSB), one combi (SIP/studs) wall where the inside/outside walls meet, and glass. The cementboard was installed directly on the studs and OSB with no vapor barrier. When the shower was taken apart, there was no damage to the studs, but there was mold growing on the OSB that was attached directly to the cementboard.

When the water penetrated the cementboards, there is space in the stud-built wall cavity for water to diffuse. But the OSB trapped the water and allowed mold to grow. That's why I say you MIGHT be able to get away without a barrier, as most homes are stick built and are just letting the water pass through and diffuse throughout the house. But as homes get tighter and sealed better, it's a crap shoot with a time bomb. My home was custom built with a low volume, "luxury home" builder who was highly recommended by the architect. Even he was unaware of how a shower needs to be properly built. They were even surprised at how wet the deck mud was when they tore the shower apart (DUH! no pre-slope for the shower pan and plugged up weep holes for the drain!) They also had the notion that tile, grout, sealer and cementboards are water/vapor proof. And they never heard of the Tile Council of North America. I was probably as surprised about this as they were about the condition of my shower.

I shudder to think at what high speed/high volume track builders are throwing up, especially in the last market.

I am convinced.

It's the know-it-all hubby is being a butt. I won't ask again. He promised if I asked it "his way", he'd be satisified with the answer. Shoulda known it won't work either ;)

Thanks for taking the time and I am glad for your help.

Ask him this-- You folks are spending alot of money on this. Yes, Hardi is water resistant, and yes, a good tile installation will shed 90% of the water that hits it. But think about this. A whole roll of 8' 6 mil polyethlene costs about 40.00. It doesnt' take any time to install. Why the effort NOT to put it in, when all it can do is increase the life of your shower?

Question: So it's a vapor barrier OR Kerdi, not both?
Vapor barrier would go under cement board, Kerdi goes between cement board/tile?

So it's a vapor barrier OR Kerdi, not both?

Correct. Kerdi is not only a waterproofing membrane, it's also a vapor barrier, and as redundant as that may sound, not all waterproofing membranes ARE vapor proof. For example, both of Laticrete's waterproofing membranes, 9235 and Hydroban, both require vapor barriers, as well. With them, you WOULD need the plastic behind the cement board.

Thanks! Wow, learning so much here!

bill vincent,
where would I get Kerdi would it go floor to ceiling around
A bathtub three sides One side outside wall and one shared inside wall of another bathroom and one bedroom bouth bathrooms are down to studes.
Thank you
Annita carol dawson