Re: Does the vapor barrier extend over the shower pan liner?
Im confused.

Canuk; if you think about it, if the vapor barrier caught vapor as it condensed, it would run down the sheet, BEHIND the shower pan liner. That is why I am doubting this installation. If you installed the shower pan liner first, any condensing would run down the vapor barrier and drop on the shower side of the pan liner.

I believe your thinking is flawed .
There should be no condensation running down the vapour barrier. If the proper amount of insulation and installed correctly within the wall cavity will raise the temperature above the dew point. The role of the vapour barrier is to prevent warm moisture laden air ( vapour ) entering the wall cavity and condensing.
In other words -- properly insulated the temperature of the inside ( warm side ) of the vapour barrier won't allow condensation to occur.

Further to ---

Water may be present in our environment in any of its three physical states: solid ( ice ) , liquid and gas ( vapour ). In this case we are dealing with 2 states : liquid and gas ( vapour ).
Water vapour transfers through buildings by two mechanisms: air leakage
and diffusion. Air leakage is generally the major culprit in the transfer of water vapour and simply preventing air leakage takes care of that source of condensation.

Diffusion is dependant on the building materials used.
Water vapour transmission (assuming air leakage has been eliminated) is affected by the following factors:
- The chemical composition of the building materials
- The thickness of the building materials.

A sheet of rubber ( like your pan liner ) may completely stop the flow of liquid water but may permit the diffusion of water vapour -- water in the gaseous state may penetrate what appears to be a solid membrane. Whereas the 6 mil polyethylene used for vapour barriers handles both liquid water and resists diffusion far more than the rubber.
For example :

polyethylene at 4- 6 mils thickness has a perm value of around 0.08 - 0.06 respectively

chlorinated synth. rubber 30 mils thickness has a perm value of around 0.2

As you see the rubber is 5 times thicker yet is 3.3 times more vapour permeable which makes it less of a vapour barrier than the 6 mil poly.

Another example : the commonly used and popular Silicone at 20 mils thickness has a perm value of around 2.9. It's great at dealing with liquid water but has poor vapour permeability.

In other words -- unless the pan liner has an ASTM rating as a vapour barrier then it should be on the inside of the 6 mil poly.
Even if it were ASTM rated you would need to ensure an airtight and watertight seal between the two materials.

That's why it common to apply the 6 mil poly from ceiling to floor ( properly sealed ) first --- then the pan liner is applied over the vapour barrier.

Besides, if water were to penetrate the tiled surface the 6 mill poly vapour barrier will be a second barrier preventing that water entering the wall cavity.

As a side note --- when installing a steel bathtub --- the material and finish of the steel tub are one of the best vapour barriers in themselves. Hopwever, it's required the 6 mil poly vapour barrier needs to be extended beyond the tub down to the floor.
Reason being -- even though the tub is a great vapour barrier the shape and installation does not lend itself to provide an airtight seal preventing warm air reaching the wall cavity.

LIHR and jkirk -- I have gone back and forth and have settled on the 6 mil plastic since I did it in the last room. Is one way better than the other? The Schulter Kerdi membrane system looked cool -- and expensive. As I see it, there are two ways to waterproof a shower, from in front of the wall and behind the wall; why does in front of the wall seem like a better option? Can anyone convince me either way?

Personally, there is only one way to waterproof a shower -- from the front side.
A tiled shower ( in itself ) is not water proofconsidering the materials used and the installation.
Ceramic tiles aren't water proof ( the glazing is resistant to water ) , the grout is porous , the thinset is porous , the cement backer is porous.

Any cracks or unsealed grout will allow water to migrate through --- considering the multiple joints , that's a lot of potential water infiltration.
The reason for using the cement backer isn't for waterproofing. It's a good substrate for the thinset to adhere to but , more importantly when water does infiltrate it won't mould like drywall .
You water proof the front side because any water that does migrate through the finish stops at that point and doesn't penetrate any further.

Thanks everyone!
water proofing membranes and vapor barrier are two entirely different things.. teh purpose of vapor barrier is to stop warm moisture laden air from escaping the building envelope into the exterior framing where it condenses and can create mold and rot.. this is installed on every exterior wall and the ceiling of the top floor of a house it has to be continous

kerdi is strictly for showers, it keeps the water inside the shower where it belongs and directs it to the drain. you cannot connect kerdi to a vapor barrier in any way as one is in front of the wallboard hopefully your using denseshield, and the vapor barrier goes dirctly under it.. if this is to be inspected.. the inspector will be looking for vapor barrier before your board even starts to get hung
I completely agree with jkirk
The vapour barrier ( 6 mil poly ) creates a sealed envelope. Unless you're only required to use a lower type of vapour retarder -- like faced batt insulation -- then simply relying on a waterproofing system as a vapour barrier wouldn't fly in these parts.

As mentioned -- if you are required to have a type ii ( or higher ) vapour retarder then it would need to be a continiously sealed envelope -- which would be difficult to achieve with a waterproof system interuppting the barrier.

Besides -- it's far easier to simply install the 6 mil poly floor to ceiling first and then install all the other stuff.