Imagine a glass window in your kitchen when you have a couple pots of water boiling on the stove. The vapour you're creating goes into the air of the room and when it meets the cooler surface of the glass window ... it becomes condensation. (In colder climates this is usually a constant occurrence) The same happens with any surface or material that stops vapour. For example, the painted surface of the drywall will also prevent most moisture from passing through .... which is why you are usually told to use a higher gloss paint in kitchens and bathrooms. If the drywall were not painted, the moisture / vapour would simply absorb into the drywall material. So think of the paint as a type of moisture barrier to protect the drywall.
But vapour can occur even without obvious introduction of moisture into the air. Where ever warm air meets cold air there will be a high concentration of vapour in that area. So where ever you place the vapour barrier in a wall is where the condensation will occur.
Think of it as a cold and warm weather front meeting inside your walls .... and the forecast is rain.
The condensation held by the vapour barrier will eventually go back into the air once the volume of vapour decreases. This is allowed to happen only if there is good ventilation on both sides of the barrier.
Vapour will penetrate any material in its way until it hits a barrier, that's why it's important to position vapour barriers on the inside (warm side) of walls. Also, when warmer air meets colder air condensation will develope. Can you imagine the damage to insulation if the barrier was on the outer side of your insulation? Warm air (and/or moisture) would create condensation on the vapour barrier which would eventually absorb into the insulation causing all kinds of water damage and possible mold growth.
Rule of thumb is ... always install a vapour barrier in walls where moisture is a factor, and exterior walls (where warm air meets cold air).
So with this basic understanding, we will focus on the areas of the highest concentration of vapour and moisture which are usually showers. An area which requires a vapour barrier.
GREENBOARD or DRYWALL
If you are building a shower and you intend to use greenboard or drywall for the shower walls ... right off the bat you're in trouble if you don't understand how to use a vapour barrier. If we put a vapour barrier (6 mil polyethylene sheets) over the studs before we put the greenboard on, the vapour may eventually pass through the greenboard or drywall until it reaches the poly where it turns to condensation ..... ultimately, for any number of reasons moisture WILL form on the inside of the vapor barrier destroying the greenboard by turning it to mush.
Never use a vapour barrier behind any type of drywall product when the application is in a wet area.
||The tile was covered with a cheap vinyl tub enclosures (not shown here).
The black areas are the mold. The material in my hands is soaking wet pieces of greenboard which I pulled off the wall with my fingers. Mold is flourishing between the vapour barrier and greenboard.
The poly vapor barrier is covered with beads of moisture locked between the greenboard and poly.
But a vapour barrier is still required, so the only other option would be to but the vapour barrier over top the greenboard, but of course 6 mil polyethylene isn't going to work in this application. Instead, we find a vapour barrier which will not only prevent vapour from passing through .... but also be able to support a ceramic tile installation.
Kerdi is a waterproof membrane which does exactly that.
CEMENT BOARD UNITS (CBU)
STEAM ROOM APPLICATION:
If you're going to use cementboard over your wall studs you'll need a vapour barrier, but unlike greenboard and drywall (gypsum based products), cementboard can handle the moisture which allows you to install the poly behind the cement panels. The polyethylene will prevent vapour from passing through. The cementboard has no components which break down or deteriorate from moisture, and it is able to breathe which allows for ventilation. In other words ... it drys out.
So it's ok to use cementboard in a shower if using a properly installed vapour barrier.
Personally, I would much rather see a water-tight shower enclosure .... but that's just my opinion.
Regardless of the type of backerboard you use for the walls of a steam shower .... all surfaces MUST be completely waterproofed.
In other words ... the vapour barrier MUST be on the inside surface of the steam shower walls protecting ALL materials under the ceramic tile or stone from vapour.
If while constructing a steam room enclosure you notice there is already a vapour barrier behind the old wall board on your exterior wall .... DO NOT tear it out. Leave this barrier intact so as not to create a larger problem within a much larger area. Ideally it is wise to have only one vapour barrier on the inside surfaces of the steam room and no other properly installed vapour barrier anywhere else within any of the walls, but in this case ... leave it intact. Of course if there exists any vapour barriers in your other (inside partitions) walls ... get rid of them.
If you're building a steam room wall over a cinder block exterior wall where you are installing wooden studs to support your cement board panels, then a layer of Type 2 vapour barrier (see below) can be used directly over the block wall before attaching the wooden studs. A proper waterproof membrane must still be installed inside the steam shower walls.
So bottom line, when building a steam room use the type of waterproofing required for such an application and take no extra action in regard to the existence of a vapour barrier in your exterior wall. If you can't remove it without compromising the entire barrier in your home ... leave it intact to be the lessor evil.